laboratory aerosol kinetics studies of the hydrolysis ......the hydrolysis reaction of n2o5 was...
TRANSCRIPT
Laboratory Aerosol Kinetics Studies of the Hydrolysis Reaction of N2O5 Using a Flow Tube Coupled to a New
Chemical Ionization Mass Spectrometer
by
Egda Nadyr Escorcia
A thesis submitted in conformity with the requirements for the degree of Master of Science
Department of Chemistry University of Toronto
© Copyright by Egda Nadyr Escorcia 2010
ii
Laboratory Aerosol Kinetics Studies of the Hydrolysis Reaction of
N2O5 Using a Flow Tube Coupled to a New Chemical Ionization
Mass Spectrometer
Egda Nadyr Escorcia
Master of Science
Department of Chemistry University of Toronto
2010
Abstract
The hydrolysis reaction of N2O5 was investigated at room temperature on two aerosol types
using a flow tube coupled to a newly built Chemical Ionization Mass Spectrometer (CIMS).
This instrument was fully constructed and optimized during this research period, as well as
employed to conduct one of two aerosol studies. The first examined the reaction on ammonium
bisulphate aerosols using a new ion detection method, I-•N2O5 cluster formation, which proved to
be highly advantageous over the common approach of dissociative charge transfer, yielding a
sensitivity for I-•N2O5 of 0.024 Hz/pptv. The uptake coefficients at 30% and 50% relative
humidity were 0.0067 ± 0.0002 and 0.0120 ±0.0014, respectively. The second study was
performed using a different CIMS previously assembled in the laboratory. In this case, the
reaction was investigated on secondary organic aerosols produced through the ozonolysis of α-
pinene, and resulted in an uptake coefficient of 8.5x10-5 ± 7x10-6 at 0% relative humidity.
iii
Acknowledgments I would like to thank my supervisor, Professor Jonathan Abbatt for his support and guidance
these past two years. Jon has always been very confident in my work and abilities, and has
constantly encouraged my academic decisions.
Thank you to my committee members, Professor Jennifer Murphy and Professor Jamie
Donaldson for their constructive feedback and suggestions regarding my research.
I owe a great deal of my ‘CIMS knowledge’ to Sasha Vlasenko and Steve Sjostedt, who were
always available to answer my questions regarding theory and experimental techniques.
I would also like to thank all current members of the Abbatt group, most especially Rachel
Chang and Rob McWhinney, who have constantly assisted me in my work. I wish you both the
best in your academic pursuits.
My academic and personal achievements would not be possible without the constant support of
my friends and family. Thank you to my best friend Sri Jamalapur for his continuing
encouragement during my undergraduate and graduate years. Most especially, thank you to my
mother, Clarisa Escorcia and my sister, Alioska Escorcia for their enduring confidence and faith.
Gracias Mami. Gracias Ali.
Special thanks to Gregory Huey and David J. Tanner from the Georgia Institute of Technology
for their designs for the CIMS, and Glenn Wolfe from the University of Washington for the
LabVIEW program used for this research. Funding for this project was provided by the Ministry
of the Environment and the Natural Sciences and Engineering Research Council.
iv
Table of Contents
Abstract .......................................................................................................................................... ii
Acknowledgments ........................................................................................................................ iii
List of Tables ............................................................................................................................... vii
List of Figures ............................................................................................................................. viii
CHAPTER 1 : INTRODUCTION AND BACKGROUND ....................................................... 1
1 Heterogeneous Chemistry of Dinitrogen Pentoxide (N2O5) ...................................................... 1
1.1 Research Incentives ............................................................................................................ 1
1.2 NOx Chemistry .................................................................................................................... 2
1.2.1 Daytime Reactions .................................................................................................. 3
1.2.2 Nighttime Reactions ................................................................................................ 4
1.2.3 Uptake Coefficient of N2O5 Hydrolysis .................................................................. 5
1.3 Experimental Approaches and Observed Dependences of γN2O5 ........................................ 7
1.3.1 Experimental Techniques and Detection Methods ................................................. 8
1.3.2 Field Studies .......................................................................................................... 11
1.3.3 Modeling Studies .................................................................................................. 12
1.3.4 A Laboratory Study: Mozurkewich and Calvert, 1988 ........................................ 14
1.3.5 Organic and Inorganic Aerosol Studies ................................................................ 16
1.3.6 Secondary Organic Aerosol Studies ..................................................................... 17
1.4 Research Objectives .......................................................................................................... 20
CHAPTER 2 : INSTRUMENTATION ..................................................................................... 21
2 Characterization and Optimization of the New Chemical Ionization Mass Spectrometer (CIMS) ..................................................................................................................................... 21
2.1 CIMS Detection Methods ................................................................................................. 21
2.2 Mobile CIMS System ....................................................................................................... 22
2.2.1 CIMS Overview .................................................................................................... 23
v
2.2.2 Ion Molecule Region (IMR) ................................................................................. 24
2.2.3 Collisional Dissociation Chamber (CDC) ............................................................ 26
2.2.4 Intermediate Chamber (IC) ................................................................................... 28
2.2.5 Multiplier Chamber (MC) ..................................................................................... 30
2.3 Integration of CIMS System, Electronics, and Software .................................................. 33
2.4 CIMS Optimization ........................................................................................................... 35
2.4.1 Calibration of N2O5 Concentrations under Two Pressure Regimes ...................... 35
2.4.2 Statistical Measurements ...................................................................................... 41
2.4.3 Ion Cluster Formation as a Function of Relative Humidity .................................. 44
2.4.4 Mass Spectra of Room and Ambient Air .............................................................. 46
CHAPTER 3 : AEROSOL KINETICS EXPERIMENTS ...................................................... 50
3 Two Laboratory Studies ........................................................................................................... 50
3.1 Motivation for Ammonium Bisulphate Study .................................................................. 50
3.2 Ammonium Bisulphate Study: Experimental Approach and Results .............................. 51
3.2.1 N2O5 Flow Tube .................................................................................................... 52
3.2.2 Kinetics Flow Tube ............................................................................................... 54
3.2.3 Experiments Performed at 30% RH ...................................................................... 55
3.2.4 Experiments Performed at 50% RH ...................................................................... 58
3.2.5 Results and Discussion for Ammonium Bisulphate Study ................................... 60
3.3 Motivation for Secondary Organic Aerosol Study ........................................................... 68
3.4 Secondary Organic Aerosol Study: Experimental Approach and Results ....................... 69
3.4.1 Aerosol Formation ................................................................................................ 70
3.4.2 CIMS Flow Tube .................................................................................................. 71
3.4.3 Results and Discussion for SOA Study ................................................................ 72
CHAPTER 4 : CONCLUSIONS ................................................................................................ 79
4 Future Studies of the Heterogeneous Chemistry of N2O5 ........................................................ 79
vi
4.1 Summary ........................................................................................................................... 79
4.2 Future Research ................................................................................................................ 80
References .................................................................................................................................... 82
vii
List of Tables Table 1.1: Summary of reported γN2O5 values for organic and inorganic aerosol studies. ......... 16
Table 1.2: Experimental results reported by Mentel et al. regarding the nitrate effect of sodium salts on the uptake coefficient of N2O5 [9, 10]. ............................................. 17
Table 1.3: Summary of reported γN2O5 values for secondary organic aerosol and organic monolayer studies. ..................................................................................................... 19
Table 2.1: Statistical data acquired for the I-•N2O5 ion cluster and NO3- anion. ........................ 43
Table 3.1: Summary of N2O5 uptake coefficients obtained under aqueous conditions. .............. 64
viii
List of Figures Figure 1.1: Mechanism for NOx daytime chemistry in the troposphere. ................................... 3
Figure 1.2: Mechanism depicting the four stages of a hydrolysis uptake reaction as adapted from Ravishankara [4]. ............................................................................. 6
Figure 1.3: An adapted schematic of the rectangular channel flow reactor as designed by Cosman and Bertram [17]. ..................................................................................... 9
Figure 1.4: Adapted schematic of the dark aerosol chamber used at Forschungszentrum Jülich [9]. ............................................................................................................... 10
Figure 2.1: Overall schematic of the new Chemical Ionization Mass Spectrometer. .............. 24
Figure 2.2: Schematic of the Ion Molecule Region (IMR) and methyl iodide permeation tube. ...................................................................................................................... 26
Figure 2.3: Schematic of the Collisional Dissociation Chamber (CDC). ................................ 28
Figure 2.4: Schematic of the Intermediate Chamber (IC). ...................................................... 29
Figure 2.5: Schematic of the Multiplier Chamber (MC). . ...................................................... 31
Figure 2.6: Schematic of the electronics situated on the back flange of the Multiplier Chamber (MC). ..................................................................................................... 32
Figure 2.7: Front view schematic of the electronics and accessory components of the CIMS system on a mobile aluminum boxed frame. .............................................. 34
Figure 2.8: Signal (cps) of I-•N2O5 vs. temperature for two separate calibrations performed at 25 and 45 Torr. ............................................................................... 37
Figure 2.9: Representation of data collected by D. R. Stull relating the vapor pressure of N2O5 to temperature [39]. The equation of the line of best fit is employed in further calculations to determine N2O5 concentrations. ....................................... 38
Figure 2.10: I-•N2O5 signal as a function of 1/Temperature for calibrations at two pressure regimes. The slopes of the lines of best fit are representative of the enthalpies of vaporization of N2O5. ....................................................................................... 39
Figure 2.11: Calibration graph of N2O5 using two detection methods: I-•N2O5 and NO3-.
The slopes are representative of the sensitivity of each ion at 45 Torr. ................ 42
Figure 2.12: I-•N2O5 cluster as a function of relative humidity. The N2O5 cluster mirrors the response of the water cluster, I-•H2O. .............................................................. 46
ix
Figure 2.13: Twenty minute averaged mass spectra for nitrogen and room air sample flows. . ................................................................................................................... 48
Figure 2.14: Twenty minute averaged mass spectra for daytime and nighttime ambient air sample flows. ......................................................................................................... 48
Figure 2.15: Spectra of nighttime and daytime ambient air. Nighttime spectra sampled at 9:00 pm. I-•N2O5 detected at 64 cps (2.7 ppb N2O5) amidst a maximum background count rate of 30 cps. .......................................................................... 49
Figure 2.16: Spectra of control (nitrogen) and room air sample flows. The signal of the reagent ion (I-) peaks at 100,000 cps. Resolution taken as the full width at half maximum is reported as 1.0 a.m.u. ............................................................... 49
Figure 3.1: Schematic depicting the overall experimental setup for the ammonium
bisulphate study. .................................................................................................... 52
Figure 3.2: Schematic of the N2O5 flow tube. ........................................................................ 53
Figure 3.3: Schematic of the kinetics flow tube. .................................................................... 54
Figure 3.4: Original data of the I-•N2O5 signal vs. injector position for kinetic runs performed under varying aerosol surface area concentrations at 30% relative humidity. .............................................................................................................. 57
Figure 3.5: Corrected decay plots of the I-•N2O5 signal vs. injector position at 30% RH. The decays were adjusted to account for the inherent background signal. .......... 58
Figure 3.6: Corrected decay plots of the I-•N2O5 signal vs. injector position at 50% RH. The decays were adjusted to account for the inherent background signal. .......... 60
Figure 3.7: Pseudo first order rate constant vs. aerosol surface area for the hydrolysis of N2O5 on ammonium bisulphate aerosols at 30% and 50% relative humidity. ...... 62
Figure 3.8: N2O5 uptake coefficients plotted as a function of relative humidity for various literature values (see Table 3.1). The black data points correspond to results obtained during this research work at 30% and 50% RH, whereas the red and blue data points correspond to literature values obtained at 30% and 50% RH, respectively. ........................................................................................................... 67
Figure 3.9: Schematic depicting the overall experimental setup of the secondary organic aerosol study. ......................................................................................................... 70
Figure 3.10: Schematic depicting the formation of secondary organic aerosols. ..................... 71
Figure 3.11: Schematic of the CIMS flow tube. ....................................................................... 72
x
Figure 3.12: Original data of the NO3- signal vs. injector position (cm) for kinetic runs
performed under varying aerosol surface area concentrations. ............................. 74
Figure 3.13: Corrected decay plots of the NO3- signal vs. injector position. The decays
were adjusted to account for the HNO3- chemical and systemic backgrounds. ... 75
Figure 3.14: Pseudo first order rate constant vs. aerosol surface area for the hydrolysis of N2O5 on secondary organic aerosols produced through the reaction of ozone and α-pinene. ........................................................................................................ 76
1
CHAPTER 1 : INTRODUCTION AND BACKGROUND
1 Heterogeneous Chemistry of Dinitrogen Pentoxide (N2O5)
1.1 Research Incentives The atmosphere is an inherently dynamic system, constantly affected and regulated by natural
cycles on Earth, which can include terrestrial and oceanic cycles. Changing gas concentrations,
aerosol formation and emissions, and temporal and seasonal variations are all factors that
influence the composition of the atmosphere. Gas concentrations, in fact, are constantly in a
state of flux as a result of natural atmospheric processes and increasing emissions from
anthropogenic sources. High levels of nitrogen oxide (NOx) emissions in industrialized areas
are a prime environmental and health concern as they lead to increased ozone (O3)
concentrations in the troposphere [1]. Ozone is harmful in three ways: as a greenhouse gas, as
an oxidant, and as a molecule that reacts with the surfaces of ambient aerosols to produce
dangerous substances that can be inhaled and lead to respiratory problems [1]. Many of the
present day sources of nitrogen oxides in the troposphere are a result of fossil fuel combustion
and high levels of nitric oxide emissions from automobiles and power generation.
Although ozone production is also dependent on other factors, such as volatile organic
compounds (VOC), sunlight, and oxidants present in the atmosphere, more information is
required to determine the degree to which it is affected by nocturnal NOx fluctuations and
chemical processes [1, 2]. In fact, a great deal of research has been performed in establishing
the ambient aerosol composition in a given area, as well as determining the relationship
between the hydrolysis reaction of dinitrogen pentoxide, N2O5, a nocturnal NOx reservoir
species, and aerosol composition. The number of possible nighttime reactions is vast, due to the
large variability of aerosol types present in the atmosphere. Tropospheric photochemical
models are therefore lacking precise values for kinetic reaction rates and species concentrations,
resulting in inaccurate estimates of NOx and O3 budgets [3]. In addition to analyzing the NOx
budget as a function of aerosol phase, studies have also examined the dependence of ozone
production on ambient aerosol size, relative humidity, and temperature [3].
2
Motivations for this research can therefore be summarized as follows. First, further
investigations are required to identify key influences on this heterogeneous chemistry,
particularly factors such as aerosol composition and relative humidity to determine whether
they are of concern to the nocturnal NOx budget, and consequently the O3 budget. Second,
additional laboratory and field experiments involving the hydrolysis of N2O5 on aerosols are
required to confirm previous findings, as well as examine reactions which have not yet been
studied. Third, the reaction mechanisms and relationships between the various parameters
involved in these heterogeneous reactions must be addressed to develop methods and
implement necessary modifications to control gas pollutant emissions, aerosol production, and
global air quality.
1.2 NOx Chemistry Nitrogen oxides, nitric oxide and nitrogen dioxide, NO and NO2, respectively, are the primary
reactants for the production of ozone. Collectively referred to as NOx, these species are
continuously recycled when ozone is produced during the day, but may ultimately be lost at
night-time if they enter a different pathway to form dinitrogen pentoxide, N2O5. N2O5 can
subsequently react with water on aerosol surfaces to produce nitric acid, HNO3 [1]. Several
chemical processes, including the increasing concentrations of ozone in the troposphere are
influenced by the overall production, depletion and chemistry of N2O5.
Reactions leading to the formation and depletion of a gas are oftentimes controlled by several
mechanisms. Gas phase species can react either homogeneously (gas-gas phase interactions), or
heterogeneously (gas-particle interactions) [4]. N2O5, in particular, is a very reactive molecule
that participates in several heterogeneous reactions both in the troposphere and stratosphere [5].
Although previous N2O5 studies have focused primarily on gas phase reactions, research
concerning gas-aerosol interactions has been gaining impetus over the past few decades. As
such, it is necessary to understand both the daytime and nighttime reactions influencing the NOx
budget in the troposphere.
3
1.2.1 Daytime Reactions
As has been stated, NO and NO2 are responsible for the production of ozone during daylight
hours, as they function as catalysts to produce O3 in cycles that continue until sunset [1]. Figure
1.1 illustrates the key reactions that occur in the presence of sunlight. NO is primarily produced
through the photolysis of NO2 via oxygen, O2. This reaction also leads to the formation of O3.
NO then reacts with O3, HO2, or RO2, where R is an alkyl group to a peroxy radical, to produce
NO2 along with O2, OH, or RO, another radical, respectively. The methylperoxy radical,
CH3O2, for example, reacts with NO to produce a methoxy radical, CH3O and NO2 [6].
Another reaction not shown in Figure 1.1, as it is not of much consequence in the troposphere
due to really low concentrations of O in this region, is that of NO2 reacting with O atoms to
produce NO and O2. It is thus mentioned solely for the purpose of summarizing all
mechanisms. The last reaction depicted in the figure identifies the main sink of NOx in the
daytime. As can be seen, NOx is oxidized by OH to yield nitric acid, HNO3. Being water-
soluble, HNO3 is either scavenged by precipitation or lost to the troposphere by dry deposition,
the latter of which occurs within two days in the boundary layer [6].
Apart from its conversion to nitric acid, there is therefore no net loss of NOx, as NO and NO2
are alternatively regenerated within each cycle. In fact, most of the NO to NO2 conversion in a
city during the summer actually happens via HO2 and RO2. Likewise, O3 is also recycled,
resulting in no net loss of this molecule from the troposphere during the day.
Figure 1.1: Mechanism for NOx daytime chemistry in the troposphere.
OH
(O2)
hvNO2
NO
O2, OH, RO
O3, HO2, RO2
O3
HNO3
Emissions
4
1.2.2 Nighttime Reactions
At nighttime, photochemical production shuts down and NO cannot be regenerated through the
photolysis of NO2. Rather, NO and NO2 enter a different pathway to form dinitrogen
pentoxide, N2O5. Reactions (R1.1) to (R1.4) below illustrate this nocturnal mechanism.
NO + O3 → NO2 + O2 (R1.1)
NO2 + O3 → NO3 + O2 (R1.2)
NO3 + NO2 ↔ N2O5* + M ↔ N2O5 + M* (R1.3)
N2O5 + H2Oaerosols → 2HNO3 (R1.4)
Although reaction (R1.2) happens in the daytime as well, it becomes more important at
nighttime, as NO3 cannot be subsequently rapidly photolyzed. NO2 reacts with O3 to yield the
nitrate radical, NO3, which continues forward to react with NO2 to produce N2O5. Reaction
(R1.3) is reversible, as N2O5 can easily dissociate back into NO2 and NO3. In fact, reaction
(R1.3) undergoes an intermediate step wherein a highly energetic complex N2O5* forms, is
quenched by another molecule, M, and then loses its excess energy to yield gaseous N2O5 [7].
The formation of N2O5 does not proceed during the day since NO3 is quickly photolyzed to O
and NO2. Additionally, photolysis of N2O5 in the early morning hours that had been produced
at nighttime can be a sink of N2O5 that is competitive with hydrolysis.
Reaction (R1.4) can, in fact, proceed in the atmosphere in one of two ways: either as a
homogeneous reaction between N2O5(g) and H2O(g), or as a heterogeneous reaction between
N2O5(g) and water found on the surfaces of aqueous aerosols [1, 8]. As stated in the paper by
Mozurkewich and Calvert, an upper limit to the rate constant of the homogeneous reaction with
H2O has been estimated as 1.3 x 10-21 cm3 molecule-1 s-1 [8]. Thus, if the heterogeneous
reaction is faster, it can compete with the homogeneous reaction as an important source of
HNO3 in the troposphere, depending on the aerosol substrate and surface area [8]. Once
formed, nitric acid can then partition from the aerosol surface to the atmosphere where it is
quickly lost to the surface by wet or dry deposition, or it can remain in the aqueous particle as
nitrate [5, 8]. Reactions (R1.1) to (R1.4) above demonstrate a net loss of NOx that is not
replenished during nighttime, and thus holds great consequences on O3 production.
5
1.2.3 Uptake Coefficient of N2O5 Hydrolysis
Upon closer inspection, reaction (R1.4) above can be broken down into reactions (R1.5) to
(R1.7) as shown below. The addition of reactions (R1.5) and (R1.7) yields reaction (R1.4)
listed above. Consequently, the reaction rate of (R1.4) is limited by the individual rates of
reactions (R1.5) to (R1.7) [9, 10].
N2O5(g) → N2O5(aq) (R1.5)
N2O5(aq) ↔ NO2+
(aq) + NO3-(aq)
(R1.6)
NO2+
(aq) + H2O → 2H+(aq) + NO3
-(aq) (R1.7)
This gas-to-particle uptake, as shown in Figure 1.2, actually occurs in four key stages: (i)
N2O5(g) first diffuses to the surface of an aerosol, at a rate governed by its gas-phase diffusion
coefficient, (ii) it then settles into the liquid water layer and undergoes a phase change from gas
to liquid, as shown in reaction (R1.5), (iii) it diffuses further in the liquid phase along a radial
concentration gradient, and dissociates into nitrogen dioxide and nitrate ions, NO2+ and NO3
-,
respectively, and (iv) NO2+ ultimately completes the mechanism by reacting with water within
the bulk volume of the aerosol to generate nitric acid [4]. This hydrolysis reaction therefore
consists of gas-phase diffusion, mass accommodation (as stated in step (ii)), liquid-phase
diffusion, and reaction completion. As stated by A. R. Ravishankara, the mass accommodation
coefficient, denoted as α, is solely the probability that a molecule is seized by the liquid phase
once it has diffused to the surface [4]. The uptake coefficient, however, is the likelihood that
the overall reaction occurs [4, 9].
6
Figure 1.2: Mechanism depicting the four stages of a hydrolysis uptake reaction as adapted from Ravishankara [4].
Ravishankara also distinguishes between reactions which occur at the surface of a solid, and
those which occur in the bulk volume of a particle; he refers to the former as heterogeneous
reactions and the latter as multiphase reactions [4]. Naturally, the distinction is a function of
aerosol phase, composition, mass accommodation, and diffusion coefficients. Although it is
uncertain which of the two mechanisms is pertinent to the hydrolysis reaction of N2O5, both
cases have been considered by various researchers [4, 8, 11, 12].
The principle concern of any N2O5 hydrolysis reaction experiments is to determine the uptake
coefficient, γ, of N2O5 on various aerosol surfaces. γN2O5 is defined as the probability that a
reaction will occur between N2O5 and an aerosol surface given these two species collide [9]. As
derived by the kinetic theory of gases, γN2O5 is defined as shown in equation (E1.1), where is
the pseudo first order rate constant, is the total aerosol surface area, and is the mean
molecular speed of N2O5 [11]:
(E1.1)
1.45 10 (E1.2)
As expressed in equation (E1.2), is the temperature of N2O5 gas and is the molecular
weight of N2O5. Laboratory experiments often determine for N2O5 at varying temperatures
and relative humidities from the slopes of graphs depicting the N2O5 signal (cps) versus
Aerosol
Liquid-phase diffusion
Mass accommodation
Atmosphere
Gas-phase diffusion
Reaction completion
7
distance (cm), where distance is a proxy for the time duration of the reaction [7]. In addition to
these calculations, the total surface area is also measured, and along with , permits the
calculation of γN2O5 according to equations (E1.1) and (E1.2). A subsequent graph of versus
total surface area yields a slope equivalent to the second order rate constant, , defined as 4γω
[1, 11]. Further, detailed descriptions of common experimental techniques can be found in the
following section.
1.3 Experimental Approaches and Observed Dependences of γN2O5
To date, studies have confirmed the dependence of heterogeneous reaction kinetics of aqueous
aerosols on aerosol size, phase, relative humidity (RH), and temperature [3, 12, 13]. These
relationships have been gathered over many years of study by various researchers employing
several distinct experimental techniques. In fact, the first breakthrough study in the area
concerning the heterogeneous chemistry of N2O5 is attributed to Mozurkewich and Calvert for
their work performed on ammonium bisulphate and sulphuric acid aqueous aerosols [8]. As
they were successful in demonstrating that γN2O5 is faster with heterogeneous reactions than
with homogeneous reactions under atmospheric conditions, much progress was soon made in
this field over the years that followed with a wide variety of aerosols. Research has been
conducted, both in the laboratory and in the field, on organic, inorganic, and secondary organic
aerosols, ice, nitric acid trihydrate (NAT), nitric acid dehydrate (NAD), organic monolayers,
organic acids, soot, sea salt, and mineral dust [5, 8, 9, 12-19]. In addition, studies performed
have utilized a wide array of experimental methods and approaches, including aerosol flow tube
techniques, chamber experiments, and field studies to monitor ambient aerosols. Instrumental
analyses and detection schemes include Chemical Ionization Mass Spectrometry (CIMS), cavity
ringdown spectroscopy (CaRDS), FTIR spectroscopy, UV absorption, and chamber
experiments [1-3, 9-17, 20]. Following is a more detailed examination of some of the above
research methods, as well as studies that have been conducted thus far.
8
1.3.1 Experimental Techniques and Detection Methods
Several experimental techniques and detection methods have been used by researchers over the
years in numerous combinations, and for a variety of purposes. The most common
experimental approach for measuring kinetics is the flow tube reactor technique, which assumes
laminar flow. A large flow tube, typically 5 to 10 cm in diameter and 80 to 150 cm in length is
fitted with a moveable glass, or stainless steel injector that allows the reaction time to be
altered. It is mounted vertically and the interior is often coated with halocarbon wax to
minimize the reaction of N2O5 with the water on the walls of the flow tube. The aerosols,
which are either monodisperse or polydisperse, are usually produced by a collision atomizer and
enter the flow tube through a separate inlet than the N2O5 gas molecules. The flow tube may
also be fitted with a jacket to control the temperature. Pseudo first order rate constants are then
retrieved from the slopes of graphs depicting the molecular signal of interest versus distance (or
reaction time) [3, 8, 12, 14, 20].
A variation on the flow tube approach is that of Cosman and Bertram, who employ a
rectangular channel flow reactor to measure the uptake coefficient of N2O5 on aqueous solutions
coated with organic monolayers [17]. Figure 1.2 depicts this modified entrained flow tube.
Composed entirely from aluminum, it is situated horizontally, coated on the interior with
halocarbon wax to minimize wall losses, and temperature-controlled by coolant running the
length of the flow reactor. A glass trough fitted in the bottom of the flow tube is filled with an
aqueous solution that is coated with an organic monolayer. As the stainless steel injector slides
over the aqueous solution, N2O5 exits through a series of holes such that it can react evenly with
the coated solution in question. The pressure measured at the air-liquid interface corresponds to
a given molecular surface area, and thus also to the packing density of the molecular monolayer.
In this case, the N2O5 signal is monitored as a function of distance (or reaction time) for various
coated solutions at a given temperature and RH by chemical ionization mass spectrometry
(CIMS) [17]. The major advantage of this technique is the ability to relate γN2O5 to the
molecular surface area of the surfactant [17].
9
Figure 1.3: An adapted schematic of the rectangular channel flow reactor as designed by Cosman and Bertram [17].
A completely distinctive approach from the aerosol flow tube technique is a laboratory chamber
study, which employs a large aerosol chamber to monitor the gas phase species partaking in the
kinetic reactions. Such aerosol chambers are usually composed of an aluminum outer wall and
a Teflon-FEP bag acting as the inner wall. The volume and inner wall surface of laboratory
chambers are approximately 250 to 260 m3, and 240 to 250 m2, respectively, resulting in a
surface to volume ratio greater than 1 m-1 [9-11]. The aerosol chamber and double wall system
are flushed with a clean air supply prior to each experimental trial, and kept at a high pressure to
prevent contamination from ambient air. The aerosols are generated in a deliquescent state in a
small pre-chamber and then introduced into the main aerosol chamber via a production air
stream; conductive mixing is maintained by a floor heater. Figure 1.3 is a schematic of the
aerosol chamber at Forschungszentrum Jülich [9]. Gas phase species concentrations are
monitored either through UV absorption (O3) or FTIR absorption spectroscopy (NO, N2O5,
HNO3). All experiments are performed under dark conditions [9-11].
to CIMS
Quartz Trough Coated Aqueous Solution Cooling Coil
Head Space
N2O5/He
Pressure Moveable Injector
10
Figure 1.4: Adapted schematic of the dark aerosol chamber used at Forschungszentrum Jülich [9].
Detection methods employed for observing such kinetic experiments include tunable diode laser
absorption spectroscopy (TDLAS), chemical ionization mass spectrometry (CIMS), and cavity
ringdown spectroscopy (CaRDS) [2, 3, 12, 15, 21, 22]. TDLAS, an older approach, is
performed by adding an additional flow of NO to the flow exiting the flow reactor, and then
passing this total flow through a hot quartz converter where N2O5 thermally decomposes; three
NO2 molecules are produced in this manner. Either NO or NO2 concentrations can then be
measured by TDLAS [22]. CIMS, perhaps the most common detection method employed by
investigators, is the instrument utilized for the purposes of this research, thus it is discussed
thoroughly in Chapter 2. Please refer to this section for a detailed analysis of this detection
method. CaRDS, also a frequently used method, is well-suited for both laboratory and in-situ
measurements. It utilizes a pulsed Nd:YAG laser at 662 nm to simultaneously detect the
concentrations of NO3 and N2O5 (via thermal decomposition) in two separate optical cavities.
Filters are positioned at the inlet to minimize false readings in NO3 and N2O5 concentrations as
a result of aerosols entering the cavity. Calibrations are not necessary, as it is considered an
absolute concentration measurement. In fact, Brown et al. have repeatedly demonstrated the
success of this instrument in field experiments, such as those described below [23].
O3 UV Absorption
NO2, N2O5, HNO3 FTIR Spectroscopy
Clean Air
Aerosol Pre-Chamber
Floor Heater
Volume = 250-260 m3 Inner Surface = 240-250 m2
Aluminum
Teflon FEP Bag
11
1.3.2 Field Studies
Field studies are necessary not only for validating laboratory work, but also for testing new
experimental techniques and instrumentation. Laboratory aerosol studies, in particular, are
often limited in scope in that they cannot account for true atmospheric aerosol compositions.
As such, innovative, reliable, and simple sampling methods are developed for the purposes of
field work, such that ambient aerosol composition and N2O5 hydrolysis kinetics can be
measured at a specific site. Field studies may be conducted over a given distance or area while
aboard any number of vessels, including aircrafts, ships, or vehicles, or they may be situated in
one location for the duration of the campaign. In any case, field work offers invaluable data,
which cannot be obtained in a laboratory setting.
Brown and colleagues performed aircraft and ship measurements of NO3 and N2O5 in the areas
of Pennsylvania, New Jersey, New York, and the East Coast of the United States as part of the
Near England Air Quality Study (NEAQS) in 2002 and 2004 [1, 23, 24]. The flight study
conducted in 2004 employed CaRDS for in-situ measurements of NO3 and N2O5. Data taken
aboard the aircraft were obtained at night during an eight hour flight over three regions with
distinct mass loadings: (I) Ohio and Western Pennsylvania with high sulphate and low organic
mass loadings, (II) Eastern Pennsylvania with moderate sulphate and low organic mass
loadings, and (III) New Jersey and New York with low sulphate and organic mass loadings.
Brown et al. reported a strong dependence of γN2O5 on the mass loading of sulphate aerosols,
which were influenced by anthropogenic SO2 sources [1]. Region I with high concentrations of
sulphate resulted in a high value of γN2O5 of 0.017, whereas Region II with lower amounts of
sulphate, yet higher amounts of nitrate and water-soluble organic carbon yielded a low γN2O5
value of 0.001. Region III with low amounts of sulphate, nitrate, and water-soluble organic
carbon yielded a low γN2O5 value of 0.0016. This study therefore demonstrated that γN2O5 seems
to be dependent on the sulphate mass loading for a sampling area [1].
The field work conducted by Bertram, Thornton, and Riedel from the University of Washington
in Boulder, Colorado and Seattle, Washington during the summer of 2008 exemplifies various
difficulties that can be encountered on field studies [20]. Researchers employed an entrained
aerosol flow reactor adapted to a home-built CIMS to measure N2O5 reactivity on ambient
aerosols. N2O5 was created in-situ through the reaction of NO2 and O3; subsequent
12
measurements of its hydrolysis reaction on ambient air that had been pulled into the flow
reactor were then performed. Unlike laboratory experiments, conditions such as temperature,
RH, and trace gas concentrations could not be controlled during field measurements. In fact,
changes in RH that occurred during sampling sessions affected the background measurements
that were used as controls for wall losses observed in the flow tube in the absence of aerosols.
Additionally, temperature oscillations above 25°C influenced the equilibrium between NO3 and
N2O5 as depicted in reaction (R1.3) above [20]. Higher temperatures led to lower N2O5 levels
since thermal decomposition to NO2 and NO3 occurs more easily at higher temperatures [25].
Also, NO3 levels produced in-situ for the formation of N2O5 were affected due to changing NO
concentrations within the sampling area. Local vehicular emissions of NO led to quick
titrations of NO3 and thus affected the γN2O5 values measured. As such, deviations from
average, expected kinetics due to any of the above factors were not included in the analyzed
data set. Bertram et al. reported that only 20% of the measurements taken were actually
included in the final analysis, and that most of the reliable data were acquired at night when
temperature and RH remained fairly constant and NO emissions were small [20].
This study, similar to the field work of Brown et al. mentioned above, indicated that γN2O5
depended on the organic and sulphate mass ratios within the sampling area. The conditions at
Boulder, CO were not very humid during sampling times, as the ambient RH was low at 30%.
Conversely, in Seattle, WA, the ambient RH was high at 74%. Both regions had bulk particle
chemical compositions with similar mean organic-to-sulphate mass ratios. As such, the uptake
coefficients measured for Boulder and Seattle, 0.003 and 0.009, respectively, seemed to be a
result of relative humidity, and other chemical factors. Upon further modeling, using the
aerosol inorganics model (AIM Model II), Bertram et al. determined that the increasing organic-
to-sulphate ratios indirectly affected γN2O5 values by decreasing the ability of (the already low)
sulphate and/or inorganics from taking up water. Thus, this water limitation led to lower γN2O5
values [14].
1.3.3 Modeling Studies
In addition to laboratory and field studies, researchers often employ computer models to better
understand atmospheric reactions and kinetics. Modeling studies rely on data acquired from
laboratory experiments, such as reaction rates, temperature and RH dependences, relative
13
branching ratios between side reactions, species concentrations, and local aerosol composition.
These data are then used in model simulations to investigate large-scale, complex, and
interdependent chemistries in the atmosphere. The resulting estimates are used for future
laboratory, field, and modeling studies, or as standards against which policy strategies can be
developed. Tropospheric models are concerned with understanding NOx, NOy (NOx + HNO3 +
N2O5), HOx (H + OH + HO2), VOC, and O3 chemistry. Specifically with regards to NOy and O3
chemistry, these models attempt to evaluate how variations in γN2O5 values for the hydrolysis of
N2O5 with various types of aerosols affect the O3 budget.
In an initial study, Dentener and Crutzen from the Max-Planck Institute in Mainz, Germany
employed a three-dimensional global model of the troposphere as developed by P. H.
Zimmerman (1984, 1988) to analyze the hydrolysis reactions of NO3 and N2O5 on aerosols and
their effects on NOx and O3 levels [26, 27]. Based on 10º latitude by 10º longitude resolution
and 10 vertical layers, each of which increases in pressure by 100 hPa, this model expresses gas
transport as defined by monthly averaged winds and parameterized eddy diffusion [26, 27]. It
also takes into account a convection scheme that simulates fast, vertical air mass fluxes in deep
cumulus clouds using long-term averaged meteorological data [27]. As described by Dentener
and Crutzen, the model (i) employed reactions that address the CH4-CO-NOx-HOx
photochemistry in the troposphere, (ii) utilized aerosol distributions as determined by Langner
and Rodhe (1991), and (iii) performed calculations assuming a γN2O5 value of 0.1. All
parametric values used were obtained from various laboratory sources [26]. Following the
inclusion of N2O5 heterogeneous loss, the researchers reported a substantial drop in global NOx,
and northern hemisphere O3 concentrations on the order of 80% in the winter and 20% in the
summer, and 25% in the springtime and 10% in the summer, respectively [26]. These seasonal
declines corresponded to averaged global concentration decreases of 50% and 9% for NOx and
O3, respectively. The resulting distributions of O3 and NOx showed better agreement with
previously reported data compared to estimates from models that had not incorporated
heterogeneous aerosol chemistry in their calculations [26].
The global chemical transport model GEOS-CHEM, accessible at http://www-
as.harvard.edu/chemistry/trop/geos, has been used by many researchers, including Evans and
Jacob to investigate the O3-OH-NOx chemistry and its dependence on aerosol composition [28].
Although previous simulations using this model had used a general value for γN2O5 for all
14
calculations, Evans and Jacob specified different γN2O5 values for each of the five different
aerosols included in the model: dust, sulphate, organic carbon, black carbon, and sea salt. The
γN2O5 values were obtained from recent laboratory work on each of the specific aerosols [28].
As a comparison study, they conducted two simulations: the first assumed a uniform γN2O5
value of 0.1 for all aerosols, and the second designated different γN2O5 values for each of the
above aerosols. The global mean value for the latter was 0.02. Results indicated that the
second simulation performed by Evans and Jacob yielded more realistic values for the NOx and
O3 burdens than the study by Dentener and Crutzen, or any other modeling studies who had
assumed a standard γN2O5 for all aerosol types [28]. By employing variable uptake coefficients
for each aerosol substrate, Evans and Jacob reported an overall global decrease of 53.5% and
9.36% in the NOx and O3 budgets, respectively, as a result of heterogeneous N2O5 reactions.
These values, being 7% and 4% lower than those of Dentener and Crutzen, underestimated NOx
and O3 reductions to a lesser degree, as they took into account aerosol composition and its
diverse effects on N2O5 hydrolysis reactions [28]. The variability in the results, however, could
simply be a result of deviations in the chemical transport models employed. Nonetheless, the
work of Dentener and Crutzen illustrated the importance of including heterogeneous reactions
in tropospheric models, while Evans and Jacob demonstrated that further laboratory and field
work is required to properly characterize the relationship between γN2O5 and aerosol
composition, given that variations do exist [26, 28].
The next three sections (1.3.4 to 1.3.6) offer an overview of laboratory studies that have been
performed in order to better understand the heterogeneous chemistry of N2O5 and its
dependence on aerosol phase, temperature, and relative humidity. As such, organic, inorganic
and secondary organic aerosol studies have been summarized below.
1.3.4 A Laboratory Study: Mozurkewich and Calvert, 1988
The 1988 study of Mozurkewich and Calvert was instrumental in demonstrating the viability of
a laboratory heterogeneous N2O5 experiment, and examining the kinetics of the reaction on
aqueous particles. Although researchers were aware of a potential nighttime sink for NO3 and
N2O5, the nocturnal reactions of nitrogen oxides, NO and NO2, and N2O5 had not yet been
confirmed. As such, the mechanism by which this occurred remained unidentified [8]. Two
years previous, Russell et al. had concluded that if the rate constant for the homogeneous
15
reaction of N2O5 with H2O was 21103.1 −× cm3 molecule-1 s-1, it would translate to a lifetime of
N2O5 of approximately 1 hour [8, 29]. Mozurkewich and Calvert therefore reasoned that if a
heterogeneous reaction existed, and it was much faster, it could also be more influential on the
production of HNO3 in the troposphere. As such, they studied the reactions of N2O5 with
ammonium bisulphate particles (NH4HSO4) and sulphuric acid (H2SO4) aerosols, which form
following the oxidation of sulphur dioxide, SO2, emissions, and varied the relative humidity and
temperatures from 1-76%, and 274-293 K, respectively [1, 8]. Their aim was to determine
γN2O5 on aqueous aerosols at high humidities, as well as uncover the mechanism by which NO
and NO2 are transformed into HNO3 and then eliminated by either wet or dry deposition [8].
Monodisperse aerosols were created from a solution of NH4HSO4 and water in a collision
atomizer, size selected with a differential mobility analyzer (DMA), and counted with a
condensation nuclei (CN) counter. They used an entrained flow tube with a movable injector to
measure the N2O5 concentration as a function of injector position and plotted the results to
obtain the pseudo first order rate constant from the slope of this graph. vs. aerosol surface
area concentration then yielded the second order rate constant and γN2O5. Mozurkewich and
Calvert verified the following trends for NH4HSO4 aerosols: (a) increasing wall losses of N2O5
with increasing RH, (b) small γN2O5 values at 293 K for dry air and increasing values up to 40%
RH, the deliquescent point of NH4HSO4, and (c) larger γN2O5 values for lower temperature
(γN2O5 = 0.05 at 293 K/60% RH, 0.09 at 274 K/66% RH) [8]. γN2O5 is small at low RH because
below the deliquescent point there exists only a thin aqueous layer on the surface of the
particles, permitting an equal rate of diffusion and evaporation of N2O5 from the surface layer.
Moreover, increasing temperatures yield lower γN2O5 values as they increase the rate of
evaporation of N2O5, rather than its rate of dissociation as shown in reaction (R1.6) [8]. The
reaction probabilities on H2SO4 particles were calculated as being 0.108 at 293K/1% RH, and
0.093 at 293K/10% RH. There was little dependence on RH for these aerosols and a negative
dependence on temperature for similar reasons to those of NH4HSO4 particles. Mozurkewich
and Calvert therefore concluded that aqueous aerosols in the atmosphere could be the
unidentified scavengers that were removing NO3 and N2O5 at night-time [8].
16
1.3.5 Organic and Inorganic Aerosol Studies
A diverse range of studies has been performed on organic and inorganic aqueous aerosols over
the years. Table 1.1 (below) displays several γN2O5 values acquired using different aerosol
compositions. As stated earlier, the general pattern is that γN2O5 is much smaller for low RH and
increases with increasing RH. In the aerosols studied by Thornton et al., malonic and azelaic
acid were chosen since they have been identified as composing a fraction of organic
components in atmospheric aerosols [12]. As can been seen, there is a clear dependence on
aerosol phase: higher γN2O5 values are reported for the aqueous aerosols and lower values for
the solid aerosols. In addition, there is a strong correlation with relative humidity, as the uptake
coefficients increase with high RH [12]. This relative humidity dependence is also evident in
the study of Mentel et al., as γN2O5 increases by two orders of magnitude from 2% RH to 56%
RH for sodium bisulphate (NaHSO4) aerosols [9]. Additionally, the values of Mozurkewich and
Calvert demonstrate the negative dependence on temperature for both aerosol types, as well as a
small dependence on relative humidity for the sulphuric acid particles [8].
Table 1.1: Summary of reported γN2O5 values for organic and inorganic aerosol studies.
STUDY T (K) RH (%)
AEROSOL γN2O5
Thornton et al., 2003 300 10-50
10-50
10
50-70
solid malonic acid
solid azelaic acid
aq. malonic acid
aq. malonic acid
<0.0010
0.0005
0.0020
0.0300
Mentel, Sohn, Wahner, 1999
298 2
56
71
sodium bisulphate
sodium bisulphate
sodium sulphate
0.0002
0.0180
0.0370
Mozurkewich and Calvert, 1988
274
293
274
293
1
66
60
1/10
1
10
ammonium bisulphate
ammonium bisulphate
ammonium bisulphate
sulphuric acid
sulphuric acid
sulphuric acid
0.0060
0.0090
0.0050
0.1390
0.1080
0.0930
17
Water soluble inorganic salts constitute a large fraction of tropospheric aerosols, especially
along the coastline [8-10]. Sea salt aerosols, in general, may also contain large amounts of
sulphate and nitrate [9]. Research pertaining to the kinetics of N2O5 on sodium salts will aid in
further understanding the heterogeneous reactions on more complex salts. Mentel et al.
launched a new study by examining the ionic mechanism of N2O5 hydrolysis, and thus relating
the uptake coefficient to a ‘nitrate effect’ [9, 10]. Essentially, aerosols that contain large
amounts of NO3- in solution can potentially lead to smaller values of γN2O5. This nitrate effect is
valid when water is scarce, as it shifts the equilibrium of reaction (R1.6) and (R1.7) to the left,
thus increasing the concentration of N2O5(aq) on aerosol surfaces [9, 10]. The results of the
Mentel et al. study conducted in 1999 are summarized in Table 2; this report on the nitrate
effect corroborated results previously presented by the same group of researchers in 1998 [9,
10]. As is evident in the table, there exists a direct relationship between relative humidity
concentrations and γN2O5 values, as well as an inverse relationship between γN2O5 and the nitrate
content found in aerosols.
Table 1.2: Experimental results reported by Mentel et al. regarding the nitrate effect of
sodium salts on the uptake coefficient of N2O5 [9, 10].
STUDY NO3- MOLALITY
(mol/kg)
RH (%) AEROSOL γN2O5
Mentel, Sohn, Wahner, 1999
158.0
13.4
6.8
27.5
4.5
2
56
71
48
88
NaHSO4
NaHSO4
Na2SO4
NaNO3
NaNO3
0.0002
0.0180
0.0370
0.0018
0.0230
1.3.6 Secondary Organic Aerosol Studies
We classify primary organic aerosols (POA) as those directly emitted into the atmosphere from
biogenic and anthropogenic sources. Secondary organic aerosols (SOA), however, are formed
by chemical transformation and condensation of semi volatile organic species. An additional
factor that has been under investigation, particularly with secondary organic aerosols, is the
degree to which organic monolayers or coatings present on aerosol surfaces affect γN2O5 [11, 30,
18
31]. The mechanism by which organic coatings decrease the uptake coefficient is not well-
understood, but it is generally believed that they inhibit the mass accommodation of N2O5
within the outer layers of the aerosol [30, 31].
Thornton and Abbatt were the first to address the effects of organic monolayer aerosol coatings
on the uptake coefficient of N2O5 hydrolysis reactions [32]. Employing an aerosol flow tube
coupled to a CIMS, they investigated these effects by coating sea salt aerosols (SSA),
specifically artificial seawater (ASW). Heterogeneous N2O5 reactions can proceed on the
surface of SSA particles or other chloride-containing aerosols to release chlorine radicals that
then enhance the oxidation of VOCs and accelerate O3 production in the troposphere [32, 33].
As displayed in Table 3, their results demonstrate that the organic coating suppressed γN2O5 by a
factor of three. It was undetermined whether the surfactant monolayer had the greatest impact
on the likelihood of accommodation at the interface, the mass accommodation coefficient, or
the rate of reaction at the interface; it might have been all three [32].
As can be seen in the other four studies, the uptake coefficients decreased with the addition of
an organic coating onto the aerosol surface [11, 30, 31]. Also, the decreases were greater for
coatings composed of large organic molecules than for small molecules, such as 1-hexanol
versus 1-butanol [31]. The results reported by Folkers et al. also demonstrated that higher
organic coating concentrations onto SOA surfaces cause a larger decrease in γN2O5 than smaller
concentrations [11]. A separate study performed by Cosman and Bertram investigated the
difference between a one-component surfactant, 1-octadecanol, and a two-component
surfactant, 1-octadecanol and phytanic acid on aqueous sulphuric acid aerosols [17]. They
performed this work using the rectangular channel flow reactor described above and discovered
that although the straight chain surfactant (1-octadecanol) decreased the uptake coefficient to a
significant degree, branched monolayers (phytanic acid) affected γN2O5 only slightly [17].
Branched monolayers do not pack efficiently on the aerosol surface, and thus do not decrease
the uptake coefficient as much as straight chain monolayers because the spaces present in
between branched monolayers allow N2O5 to diffuse more quickly, both into and out from the
aerosols [17].
19
Table 1.3: Summary of reported γN2O5 values for secondary organic aerosol and organic monolayer studies.
STUDY AEROSOL MONOLAYER γN2O5
Thornton and Abbatt, 2005
artificial seawater (NaCl)
artificial seawater (NaCl)
---
hexanoic acid
0.024
0.008
McNeill, Wolfe, Thornton, 2007
sodium sulphate
sodium sulphate
---
3.2 wt % oleate
0.020
0.001
Park, Burden, Nathanson, 2007
72 wt% sulphuric acid
72 wt% sulphuric acid
72 wt% sulphuric acid
---
1-butanol
1-hexanol
0.150
0.100
0.060
Folkers, Mentel, Wahner, 2003
ammonium bisulphate
ammonium bisulphate
---
---
11.4 ppb α-pinene
1.22 ppm α-pinene
0.01870
0.00340
0.00045
Cosman and Bertram, 2008
60 wt% aq. sulphuric acid
60 wt% aq. sulphuric acid
1-octadecanol
1-octadecanol + 0.2 mole fraction of phytanic acid
0.0013
0.0220
It is known that tropospheric aerosols can encompass organic surfactants, thus if present at the
air-aqueous interface they will reduce the rate of transfer of N2O5 molecules across it [17].
Since monolayers formed in the atmosphere will most likely consist of non-uniform, multi-
component coatings, the dependence of γN2O5 on the orientation and composition of the
surfactants must be further addressed.
A recent paper by Bertram and Thornton offers a thorough summary to the above studies [14].
The researchers report a systematic parameterization of the influences of H2O, nitrate and
chloride concentrations, in internally mixed inorganic and organic particles, on the uptake
coefficient for N2O5 [14]. They assume the formation of a protonated nitric acid intermediate
(H2ONO2+), which can then react with H2O, nitrate, or chloride to either promote or suppress
N2O5 reactivity. Both ammonium bisulphate and malonic acid particles were used to test the
relative importance of each of the above factors [14]. Bertram and Thornton demonstrated that
although γN2O5 showed varying dependences on relative humidity below 50% for each aerosol
type, the dependence on H2O concentration up to 15 M was equivalent for both aerosols. As
20
such, they concluded that either a water limitation exists in the reaction of H2ONO2+ with water
or nitrate, or that mass accommodation of N2O5 is water dependent [14]. In addition, they first
illustrated that increasing nitrate concentrations in the particles led to a decrease in the uptake
coefficients measured, and second that low water and high nitrate concentrations worked in
synergy to produce lower γN2O5 values. Moreover, Bertram and Thornton demonstrated that
chloride concentrations ≥ 1M negated the nitrate effect, resulting in higher uptake coefficients
than would be possible without a chloride presence [14]. Essentially, they derived an
expression for γN2O5 that is dependent on H2O, nitrate, and chloride concentrations, where the
relative importance of each is regulated by the rate coefficients of the reaction of H2ONO2+ with
each species [14]. The effects of organic films, however, are yet to be parameterized.
1.4 Research Objectives The primary goal of this research has been to investigate the hydrolysis of N2O5 on aerosols
produced in the laboratory. We performed the experiments using an entrained aerosol flow tube
coupled to a CIMS. Two heterogeneous reaction studies were conducted with aerosols of
distinct composition. The first study investigated the reaction of N2O5 with ammonium
bisulphate aerosols as a function of relative humidity, as it has been studied extensively in the
past by various investigators and therefore can easily be corroborated. The second study
examined the reaction of N2O5 with SOA particles produced through the reaction of ozone and
α-pinene. Of particular importance was the SOA study since not many similar examinations
have been conducted or reported in literature, to date. Both studies, however, have confirmed
previously reported values, as they are in good agreement with past findings.
The secondary objective, to which a significant portion of the research study period was
allocated, has been the construction, optimization and characterization of a mobile CIMS. As a
means to further support the hydrolysis experiments mentioned above, as well as assist in the
development of kinetic experiment techniques, this newly built instrument was used in the first
study. It has improved the manner in which these heterogeneous reactions can be monitored in
several ways, as will be discussed below. Additionally, it offers the opportunity to conduct
future field studies in locations with diverse ambient aerosol compositions, such that hydrolysis
reactions may be monitored on site.
21
CHAPTER 2 : INSTRUMENTATION
2 Characterization and Optimization of the New Chemical Ionization Mass Spectrometer (CIMS)
2.1 CIMS Detection Methods Chemical Ionization Mass Spectrometry is commonly used to detect ions such as halogens,
radicals, nitrate, ClNO2, and N2O5 [21, 34]. The instrument typically consists of four separate
chambers, each with a distinct function, and each of which will be discussed in detail in the
proceeding sections. All species, however, enter the CIMS through the first chamber, called the
Ion Molecule Region (IMR), where they are ionized via one of two ion chemical pathways:
dissociative charge transfer or ion cluster formation [21]. The reagent ion generally employed
in this work, I-, is produced from methyl iodide, CH3I, following exposure to a polonium-210
radioactive source, [21, 34]. Reactions (R2.1) to (R2.3) illustrate this reaction scheme for N2O5:
CH3I + Po-210 → I- (R2.1)
I- + N2O5 → NO3- + INO2 (R2.2)
I- + N2O5 → I-•N2O5 (R2.3)
As can be seen, reaction (R2.2) denotes the dissociative charge transfer, whereas reaction (R2.3)
expresses the mechanism for ion cluster formation. Although dissociative charge transfer is the
common method of detecting N2O5, chemical interference is an issue with this approach. N2O5
is indirectly measured by detecting the NO3- ion (62 a.m.u.) that forms from its dissociation,
rather than N2O5 itself. Unfortunately, this is problematic, as I- can also react with nitric acid,
HNO3, to yield an equivalent NO3- ion. As such, this approach requires that a chemical
background be subtracted from the total NO3- signal measured to account for the portion of the
signal corresponding to the HNO3 component and not N2O5. (This correction will be further
discussed in Chapter 3.) Ion cluster formation, however, requires no such correction.
Reaction scheme (R2.3) was first demonstrated by Kercher et al. from the University of
Washington in 2009 [21]. They demonstrated the direct approach of this method by optimizing
22
a home-built CIMS they had recently constructed for ion cluster formation by adding water
vapor to the IMR and increasing the pressure in the second chamber. Since the reagent ion
clusters with N2O5 rather than causes its dissociation, the chances of chemical interference are
much reduced through this channel. The cluster is detected intact at the multiplier at a mass
equivalent to the composite mass of I- and N2O5, 235 a.m.u [21]. Due to this direct and simple
method, N2O5 was detected via ion cluster formation for the ammonium bisulphate study
performed during this research work.
Kercher et al. demonstrated that the NO3- and I-•N2O5 anions were detected in a 3:1 ratio under
dry sampling conditions and low pressures, thus proving that dissociative charge transfer was
the dominant pathway under these conditions [21]. In addition, the researchers were also able
to monitor nitryl chloride, ClNO2, via both channels, detecting ICl-, as well as the cluster,
I-•ClNO2. The sensitivities of this instrument to both I-•N2O5 and I-•ClNO2 was >1 Hz/pptv,
with a background signal of <10 Hz. Limits of detection for both clusters were ~5 pptv [21].
An alternative detection method called Thermal Dissociation Chemical Ionization Mass
Spectrometry (TD-CIMS) developed by Slusher et al. at the Georgia Institute of Technology
was employed to detect peroxyacyl nitrates (PAN), peroxypropionly nitrate (PPN), as well as
NO3 and N2O5. The sample flow was heated to temperatures between 160 to 180 degrees
Celsius, which caused the molecules to thermally dissociate, producing radicals that were then
detected by I-. Similar to the Kercher study, Slusher et al. demonstrated detection limits of 7, 4,
and 12 pptv for PAN, PPN, and N2O5, respectively [34].
2.2 Mobile CIMS System The Chemical Ionization Mass Spectrometer built during this research work in the winter and
spring of 2009 was characterized and optimized for the purposes of performing and developing
N2O5 hydrolysis experiments. It is a home-built instrument constructed according to the
designs of Gregory Huey and David J. Tanner from the Georgia Institute of Technology. The
instrument, along with all essential electronics, pressure control boxes, external pumps, reagent
ion source, laptop computer, and data acquisition (DAQ) board are mounted on a wheeled
41" x 24″ x 45″ aluminum boxed frame, fitted with wire rope isolators for shock and vibration
23
absorbance. The aluminum frame pieces (Faztek parts) were purchased from Motion Canada,
and the wire rope isolators from D & D Hydraulic Components, Inc. The entire system weighs
approximately 300 lbs, and is built for easy shipment and transport for field deployment. Refer
to Figure 2.7 and Section 2.3 for a schematic depicting the placement of the various components
within the boxed frame, as well as a description of the integration of the CIMS instrument, the
electronics and the software, respectively. Following is a thorough discussion of the theoretical
background and operational mode of the CIMS instrument.
2.2.1 CIMS Overview
The CIMS is divided into four differentially pumped chambers, the Ion Molecule Region
(IMR), the Collisional Dissociation Chamber (CDC), the Intermediate Chamber (IC) and the
Multiplier Chamber (MC). Ions entering the CIMS follow a trajectory from the inlet into the
IMR, CDC, IC and lastly MC, where they are detected [35]. The IC and MC are each pumped
by a V81 Turbo Pump (Varian Inc.), whereas the CDC is directly pumped by a MDP5011 Drag
Pump (Alcatel Vacuum Technology), which in turn serves to back up the turbo pumps. The
drag pump (and therefore turbo pumps) is further backed up by a TriScroll 300 Vacuum Pump
(Varian Inc.), which also directly pumps the IMR (see Figure 2.1). This instrument contains
four critical orifices, two octupole ion guides, one quadrupole, and an on-axis electron
multiplier detector. The octupoles and CDC were purchased from G. Huey (THS Instruments,
LLC), whereas the quadrupole was purchased from Extrel Core Mass Spectrometers (Extrel).
The dual-cavity comprising both the IC and MC was built in the Machine Shop located in the
basement of the Department of Chemistry (Lash Miller Laboratories) at the University of
Toronto.
The CIMS was characterized under two different pressure regimes. The low pressure regime
maintained the IMR, CDC, IC, and MC at 25, 0.1, 1x10-4, and 1x10-6 Torr, respectively. The
high pressure regime was achieved by placing a ball valve (Swagelok, part number SS-63TF8)
in the bellows line leading from the scroll pump to the IMR, thus allowing the pressure in this
chamber to be increased to 45 Torr. When working in the high pressure regime, the pressures in
the subsequent three chambers were maintained as before by adjusting the pinhole sizes at the
entrance of these chambers. Figure 2.1 illustrates the overall configuration of this CIMS.
24
Following is a detailed discussion of each individual chamber with respect to its function,
components, and configuration.
Figure 2.1: Overall schematic of the new Chemical Ionization Mass Spectrometer.
2.2.2 Ion Molecule Region (IMR)
Ions entering the CIMS do so via the first chamber, the Ion Molecule Region (IMR). It is a 13.5
cm long electrically isolated (-7.0V) stainless steel tube. The IMR is pumped directly by the
scroll pump, which is fitted with a ball valve along its line, such that the pressure can be
preferentially increased in this region. As can be seen in Figure 2.2, the incoming sample flow
(approximately 4 standard liters per minute (slpm)) enters the inlet and travels through the first
critical orifice, 0.031″ in diameter, which drops the pressure from atmosphere to 25 Torr when
the ball valve is completely open. If this valve is partially closed, however, the pressure in this
Sample Flow (~4 slpm)
Scroll Pump
Octupole
CDC
4 slpm N2
Po-210 Source
IMR IC MC
Turbo Pump
Turbo Pump
Ball Valve
Drag Pump
Quadrupole Detector
25
region can be increased up to 45 Torr, the empirically determined optimal pressure for ion
cluster formation. As has been previously stated, the IMR serves as the site of ion chemistry,
and therefore has been optimized for the detection of the I-•N2O5 cluster (see Section 2.4)
[21, 35].
Methyl iodide (Sigma Aldrich, 99%) is delivered from a permeation tube (2.5″ long) that is
composed of Teflon tubing (PFA, 3 mm ID, 5 mm OD, 1 mm wall thickness, #5733K73),
plugged at the ends with a 1cm Teflon rod (PTFE 1/8” diameter, #8546K31), and crimped shut
with stainless steel tubing (0.219” OD, 0.205” ID, 0.007” wall thickness, #5560K413). All
parts were purchased from McMaster-Carr Supply Co. The permeation tube is placed within an
aluminum block, which is heated to ~70 degrees Celsius by a temperature controller (Omega
Engineering Inc., CN1A Series). As the permeation tube is heated, CH3I diffuses through the
Teflon walls and is carried in a 4 slpm N2 flow towards the Po-210 source, such that it passes
through the interior of the device. In addition to also encompassing the radioactive source (20
mCi, leased from NRD LLC.), the IMR is also fitted with a pressure gauge (MKS Baratron
Capacitance Manometer, Type 626 Pressure Transducer) at the juncture immediately preceding
the second critical orifice (0.020″ diameter), the entrance to the second chamber.
26
Figure 2.2: Schematic of the Ion Molecule Region (IMR) and methyl iodide permeation tube.
2.2.3 Collisional Dissociation Chamber (CDC)
The critical orifice leading into the Collisional Dissociation Chamber (CDC) is, in fact, a 0.02″
diameter aperture of a pinhole plate held in place by a centering ring and biased to -7.0 V.
Temperature Controller
Permeation Tube
Aluminum Block
4 slpm N2 from cylinder
IMR at -7.0V
Scroll Pump
Ball Valve
4 slpm N2
Po-210 Source
Critical Orifice 0.020″ diameter
To CDC Sample Flow (~4 slpm)
Critical Orifice 0.031″ diameter
Pressure Gauge
27
Since the voltages on the IMR flow tube and pinhole plate are equivalent, the ions travel
inwards solely as a result of the pressure differential [35]. The CDC housing itself, however, is
grounded. The pressure drops from 25 (or 45) Torr to approximately 0.25 (or 0.4) Torr when in
operational mode. The CDC is directly pumped by the drag pump, which is backed by the
scroll pump. The bellows leading from the drag pump to the CDC is also fitted with a ball
valve, which is maintained fully open during experimentation. It has been added only as a
means to further control the pressure in the CDC, if ever needed. In fact, although in most
instruments the CDC typically serves as the site of dissociation of weakly bound cluster ions via
energetic collisions, the CDC in question is not being employed in this manner [35]. Rather
than increasing the pressure in this region to promote ion cluster dissociation, the CDC is used
solely as the housing for an octupole guide and as a means to sequentially drop the pressure.
The pressure in this region is monitored via a gauge (HPS Instruments, MicroPirani Transducer,
Series 925C), situated on the underside of the CDC (see Figure 2.3) [35].
The CDC encompasses the first octupole ion guide, consisting of eight stainless steel rods that
require both a direct current (DC) bias (-1.3 V) and alternating current (AC) voltage (1.5 V) to
operate. The frequencies of the AC voltage lie within the radio frequency (RF) range of the
electromagnetic spectrum [36]. The AC voltage applied to the octupole, regulated by the
octupole boards located inside Power Supply 2, causes the polarities of the rods to alternate
between positive and negative potentials, such that every other rod has the same polarity at any
given time [36]. This constantly changing polarity collimates the charged ions as they travel
down the middle of the octupole, and thus they are not pumped away by the drag pump, as are
the neutral molecules. As the pressure drops across the aperture, the gases expand and diffuse
outward; consequently, the octupole is required to re-focus the ions into a narrow beam before
they move onto the next chamber [36]. Focusing power is determined by manipulating the
amplitude on the AC voltage, and thus changing the AC to DC ratio, rather than the frequency
of the AC voltage, as is done in the quadrupole for mass selection. Octupoles are optimal for
beam collimation rather than mass selection as a result of the electric field created from their
geometry and configuration [36]. Both octupoles were acquired fully constructed and
electrically wired from Huey and Tanner (THS Instruments, LLC) at the Georgia Institute of
Technology. The CDC octupole is followed by another critical orifice (0.040″ diameter)
leading into the third chamber.
28
Figure 2.3: Schematic of the Collisional Dissociation Chamber (CDC).
2.2.4 Intermediate Chamber (IC)
The aperture leading into the third chamber, the Intermediate Chamber (IC) is 0.040″ in
diameter and sits at the same potential as the CDC, 0.0 V. The IC is pumped to 3-7x10-4 Torr
by a turbo pump, which is sequentially backed up by the drag and scroll pumps, respectively.
The pressure in this region is monitored via a MicroPirani Transducer identical to the one
located in the CDC; it is also situated on the underside of the chamber. The IC contains the
second octupole guide found in the CIMS. As stated in Section 2.2.3, the octupole focuses the
CDC grounded
AC Voltage (1.5 V) DC Bias (-1.3 V)
Pinhole Plate at -7.0 V
From IMR
Drag Pump
Pinhole Plate
Ball Valve MicroPriani Transducer
Critical Orifice 0.020″ diameter
Octupole
To IC
Critical Orifice 0.040″ diameter
29
charged ions into the next chamber, and minimizes losses to the turbo pump or walls of the
chamber [36]. This second octupole is identical to the first in geometry and function, except for
the applied voltages, which are optimized empirically. Both the AC voltage (2.0 V) and DC
bias (-0.178 V) for this octupole are supplied from the octupole board located inside Power
Supply 2. The fourth orifice, also called the octoplate (0.100″ diameter), is electrically
separated from the octupole by a plastic disk. The octoplate (7.0 V) also acts as the lid to the
stainless steel can, which encompasses the quadrupole in the multiplier chamber (MC). As
such, along with the can, it serves as a barrier between the IC and the MC. The input signals for
the octupole, which apply the DC and AC voltages, also enter the chamber from beneath. A
schematic of the IC is illustrated in Figure 2.4.
Figure 2.4: Schematic of the Intermediate Chamber (IC).
To MC From CDC
MicroPriani Transducer
7.0 V
AC Voltage 2.0 V DC Bias -0.178 V
0.0 V
Critical Orifice 0.040″ diameter
Critical Orifice 0.100″ diameter Octoplate
Insulating Plastic Disk
Octupole
Input Signal for Octupole
Can for Quadrupole
Drag Pump
Turbo Pump
30
2.2.5 Multiplier Chamber (MC)
The last and fourth chamber, the multiplier chamber (MC) houses both the quadrupole and
detector (multiplier). The pressure in this chamber, 2x10-6 Torr, is lower than any of the
preceding chambers, and similarly to the MC, it is pumped by a turbo pump, which is
sequentially backed up by the drag and scroll pumps, respectively [35]. Unlike the MC,
however, the pressure is monitored via a Cold Cathode gauge (Varian Inc.), as the MicroPirani
Transducers cannot read pressures lower than 1x10-5 Torr. All input signals required for the
quadrupole, except the AC voltage and DC bias enter the chamber through a cavity situated at
the bottom of the chamber. The changing RF for the AC voltage and DC bias are supplied
through two large feedthroughs directly from the Extrel RF/DC Power Supply unit. The
quadrupole purchased for this instrument has 3/8″ (9.5 mm) diameter rods, an upper limit mass
range of 500 m/z, and an oscillator frequency of 2.1 MHz.
Although the quadrupole operates on the same principles as the octupole, it functions to mass
select rather than collimate ions [36]. It consists of four identical stainless steel cylindrical rods,
each of which is electrically connected to the rod opposing it. As with the octupole, the
quadrupole requires an AC voltage whose frequency is varied by a radio frequency generator, as
well as a DC bias (Extrel RF/DC Unit). Unlike the octupole, the AC to DC ratio is kept
constant, while the frequency of the AC is varied [37]. Thus, frequency oscillation is virtually
synonymous with mass selection. As a result of the constantly changing polarity of oppositely
paired poles, the AC voltage applied to the quadrupole guides the charged ions down the length
of the mass filter. The ions therefore travel in a spiral trajectory as they are attracted to and
repelled by the potentials on the poles. The changing RF of the AC voltage coupled with the
DC bias that is applied along the z-axis, the length of the quadrupole, allows ions of different
masses to be selected [37].
The DC bias, in actuality, determines the speed of the ions as they travel into and through the
mass filter, and is hence synonymous with resolution. The ratio of the DC bias to the RF
voltage is roughly equivalent to resolution, where a higher DC bias implies a higher resolution
and a lower bias entails a lower resolution (David Tanner, personal communication). Although
a higher DC bias allows the ions to move more quickly down the filter, it also triggers a greater
number of ions to collide with the rods. This in turn leads to a decrease in signal.
31
Consequently, there exists a balance between setting the DC bias to a value that will ensure a
high enough signal and a high enough resolution [37]. In addition to the changing RF applied
to the AC voltage and the DC bias, the quadrupole possess pre- and post-filters on the end of the
rods that also help fine tune the trajectories of the ions as they enter and leave the quadrupole.
The pre-filters have two voltages (29.4 V, 49.2 V) for each set of rods, whereas all four post-
filters are set to the same voltage (88.5 V). Figure 2.5 is a schematic of the complete
configuration of the MC.
Figure 2.5: Schematic of the Multiplier Chamber (MC).
The MC also houses the detector (7550M Channel Electron Multiplier, ITT Power Solutions,
Inc.), which is situated at the very end of the chamber on the back flange. Upon exiting the
quadrupole, ions reach the detector by passing through a rather large aperture in the center of
the detector shield, aligned with the entrance of the multiplier. Unlike in other instruments, the
detector for this CIMS is on-axis, rather than off-axis to the quadrupole in order to minimize the
size of the MC, thus no focusing lenses are required to guide the ions to the detector (see Figure
2.6) [21]. The electron multiplier is cone-shaped and has a voltage difference across the front
Post-Filters (4)
RF/AC Voltage (variable) 29.4 V
49.2 V 7.0 V
Critical Orifice 0.100″ diameter
Input Signal for AC Voltage
Pre-Filters (4)
Can for Quadrupole
Input Signal for Pre- and Post-Filters, DC Bias
Turbo Pump
Drag Pump
Back Flange
Detector
Cold Cathode
Quadrupole
Octoplate
88.5 V
From IC
32
and back ends. Regardless of the charge of the ions, the back end of the multiplier can be set to
a maximum of 6.0 kV, whereas the front end is set to either 4.0 kV for negative ions, or -4.0 kV
for positive ions. As such, the ions naturally move through the multiplier down their potential
gradient. As the ions enter the multiplier, electrons are ejected from the interior walls of the
detector, causing an electron cascade to travel down towards the more positive end of the
multiplier [37]. The signal from these electrons then reaches the high voltage (HV) decoupling
capacitor (7.5 kV, Part #HT50, High Energy Corp.), which removes the portion of the signal
corresponding to the high voltages applied to the detector. This new signal is subsequently
cleaned by the pre-amplifier discriminator (A101, AMPTEK, Inc.), as it removes any
background peaks below a set threshold voltage. It is this final version of the signal that is
counted by the DAQ Board and converted into a signal measured in counts per second (cps).
Figure 2.6: Schematic of the electronics situated on the back flange of Multiplier Chamber (MC).
Ions entering the CIMS thus encounter the following components in sequence: (i) critical
orifice in IMR, (ii) pinhole plate/critical orifice leading to the CDC, (iii) CDC octupole guide,
(iv) critical orifice leading to the IC, (v) IC octupole guide, (vi) octoplate/critical orifice, (vii)
pre-filters, quadrupole, post-filters (viii) detector shield, (ix) detector (electron multiplier).
Preamplifier Discriminator
Electrical Feedthrough for Pre-Amp
High Voltages for Multiplier
±4.0 kV
6.0 kV
Electron Multiplier
HV Decoupling Capacitor
Signal out to DAQ Board
33
2.3 Integration of CIMS System, Electronics, and Software As can be seen in Figure 2.7, the CIMS instrument, along with its pumps and permeation source
is situated in one half of the aluminum frame; the other half encompasses all the electronics.
The electronics consist of several power supply units, the first of which functions as an interface
box that can control eight mass flow controllers (or pressure gauges), four temperature zones,
and four valves. The second unit contains high voltage (HV) power supplies for the detector,
and power supplies to run the DC voltages for the two turbo pumps. It also encompasses a
voltage board to bias various components of the system (i.e., voltages to the pinhole plate,
octoplate, pre- and post-filters), two octupole boards to control the two octupole guides, and a
built-in safety circuit. These power supply units were acquired, fully assembled, from Huey
and Tanner (THS Instruments, LLC).
In addition, the following power supply units were purchased from Extrel Core Mass
Spectrometers (Extrel): (i) 300 Watt Raw DC Pole Supply (not shown in Figure 2.7, but situated
behind component (8) displayed below, (ii) Model 150QC RF/DC Power Supply, and (iii) Main
DC Power Supply. Mass selection, performed by the quadrupole, is controlled by the
integration of these three power supplies, as well as other voltages supplied from Power Supply
2. Both the Main DC Power Supply and the DC Pole Supply connect to the RF/DC Power
Supply. The former provides most of the power and lower end voltages to the RF/DC unit, as
well as a vacuum interlock connection (safety-circuit), whereas the latter is a voltage converter
that supplies the higher voltages (with little current). The DC Pole Supply is responsible for
resolving mass at the higher end of the mass spectrum. The RF/DC unit supplies the
quadrupoles directly with the changing frequency to the AC voltage and the DC bias for mass
selection.
The safety circuit, regulated by the pressure sensed by the Cold Cathode, shuts down the high
voltages of the multiplier, the RF/AC voltage to the quadrupole, all Extrel units, and additional
voltages if the pressure in the multiplier chamber is too high. It recognizes any pressure above
the set point as a liability to the electronics in the instrument, most especially the multiplier, and
thus deactivates the system. The pressures of the IMR, CDC, IC, and MC are monitored from
the front panel of Power Supply 2, and an external Cold Cathode control box, respectively. The
pumps themselves are controlled via their respective control boxes.
34
The DAQ Board (USB-6251 M Series, National Instruments) has a dual function: it sends out
the voltage to the mass command input on the RF/DC Power Supply, and it counts the signal
coming in directly from the preamplifier discriminator. The board is connected to and
controlled by the laptop computer, which is installed with LabVIEW software (National
Instruments). The LabVIEW program, “MapleCAKE”, written by Glenn Wolfe from the
University of Washington is the interface that allows for ion monitoring through the DAQ
Board, Extrel units, and the CIMS. For example, if a particular ion (or mass range) is selected
on MapleCAKE, it signals the DAQ Board to send out the voltage associated with this mass to
the RF/DC Extrel unit, which then supplies these voltages to the quadrupole. The mass
command ratio for this particular quadrupole is 20 mV/a.m.u. All other voltages associated
with the different components in the CIMS are determined empirically and controlled via the
switches on the front panel of Power Supply 2.
Figure 2.7: Front view schematic of the electronics and accessory components of the CIMS system on a mobile aluminum boxed frame.
1 2
3
4
5 6 6
7
8
9 9
10 10
11
12
14
13
1 – DAQ Board 2 – Laptop Computer 3 – Power Supply 1 (Interface Box) 4 – Power Supply 2 (Control Box) 5 – Drag Pump Control Box 6 – Turbo Pump Control Box 7 – Extrel Main DC Power Supply Unit 8 – Extrel Model 150QC RF/DC Power Supply Unit 9 – Compression Coil 10 – Wheel 11 – Scroll Pump 12 – Permeation Tube and Thermostat Controller 13 – CIMS (back flange) 14 – Turbo Pump (2 mounted, rear one shown) 15 – Drag Pump
15
35
2.4 CIMS Optimization Following the assembly and integration of all components, the Chemical Ionization Mass
Spectrometer was optimized and characterized at two different pressure regimes in the ion
molecule region, 25 and 45 Torr, respectively. Pressures in subsequent chambers varied only
slightly for both cases. The instrument was optimized with the intent of performing N2O5
kinetics experiments through the detection of the I-•N2O5 ion cluster, as seen in the Kercher et
al. study [21]. Calibrations were thus performed at both high and low pressure conditions to
determine the N2O5 concentrations being used. As such, the sensitivities and detection limits
were obtained for the I-•N2O5 cluster and the NO3- anion by means of the calibration
measurements [37]. In addition, the I-•N2O5 cluster was examined as a function of relative
humidity, as will be shown in Section 2.4.3. Mass spectra of a control scan (N2 gas only), room
air, daytime ambient air, and nighttime ambient air will also be compared in Section 2.4.4.
2.4.1 Calibration of N2O5 Concentrations under Two Pressure Regimes
The calibrations at both 25 and 45 Torr were performed following identical procedures. N2O5
was generated in the fume hood and the calibration was performed on the bench next to the
CIMS [12]:
1. N2O5 gas was produced in situ in a 60 cm long flow tube through the reaction of NO2
and O3. Pure NO2 gas (from an N2O4 cylinder) was added in excess to the flow tube.
A flow of 300 standard cubic centimeters per minute (sccm) O2 was passed over a 22.9
cm long mercury penray lamp (UVP, LLC). Ozone is produced through the photolysis
of O2 at a wavelength of 185 nm.
2. An ethanol/liquid nitrogen (liq. N2) slush bath was prepared to a temperature of 163
Kelvin (-110 Celsius).
3. N2O5 gas flowed into a 20 cm long 2-way valve Pyrex flask. The pathway
circumventing the trap was closed such that the flow was forced to pass down and
through the trap.
4. The flask was immersed into the slush bath and white solid N2O5 crystals were
produced and trapped for approximately 2 hours.
36
5. The CIMS, including all Teflon lines, was purged with a flow of 1.0 slpm N2 for the
duration of the trap preparation.
6. N2O5 generation was discontinued and the trap and ethanol/liq. N2 bath was brought
over to the bench to perform the calibration. The I-•N2O5 signal was monitored as a
function of temperature.
N2O5 gradually leaves the trap when the crystals in the flask first condense and then evaporate
from the coldest regions of the trap as the surrounding temperature increases. It is carried
towards the CIMS in a small helium (He) flow, which passes through the trap, and a larger
dilution (1.0 slpm) N2 flow, which joins the main line after the trap. Additionally, a wet flow of
N2 (~3.0 slpm) from a water bubbler also enters the IMR to facilitate ion cluster formation (see
Section 2.4.3 and Figure 3.1). Figure 2.8 illustrates the signal of I-•N2O5 vs. temperature for
both pressure regimes. As can be seen, the signal increased as the temperature of the slush bath
increased. The points acquired after the signal peaked are not shown in Figure 2.8, as they do
not correspond to calibration points where the vapor pressure was at equilibrium, and thus are
not reliable. The signal acquired at 25 Torr reached a maximum value at 8000 cps, whereas at
45 Torr the signal peaked at approximately 4000 cps (as reference, the reagent ion signal was
approximately 20,000 cps at the times the calibrations peaked). This difference can be
attributed to the carrier flows that were set during each calibration: 50 sccm He at 25 Torr, and
24 sccm He at 45 Torr. As such, a higher flow yielded a higher signal. These different dilution
flows are accounted for in all calculations.
The labels on Figure 2.8 for the high pressure calibration correspond to times when the carrier
flow was either decreased or increased. Points labeled A and C denote the start of a decrease in
He flow (from 24 to 15 sccm) and points B and D denote an increase in He flow (from 15 to 24
sccm). The signal responded as expected: it decreased with lower flows and increased with
higher flows. Although the drop in the signal should have been proportional to the change in
dilution, this was not observed since the signal kept increasing quickly as the temperature
increased. That is, the signal did not have an opportunity to settle because the temperature was
not constant. Observations performed during experimental trials, however, do yield
proportional changes in the signal as a result of changes in dilution flows.
37
Point E corresponds to manually changing the pathway of the He flow from passing through the
trap to passing around it. As can be seen, the signal dropped to 50 cps when bypassing the trap
and only recovered when the flow was again routed through the trap (points F and G). Both
calibrations were conducted over a span of approximately two hours, and each peaked at a
temperature of 213 Kelvin (-60 Celsius). The temperature was monitored through a
thermocouple placed in the slush bath at the same height as the bottom of the flask.
Figure 2.8: Signal (cps) of I-•N2O5 vs. temperature for two separate calibrations performed at 25 and 45 Torr.
The relationship between the vapor pressure and temperature of a substance at the liquid-vapor
phase boundary can be examined using the Clausius-Clapeyron equation shown below (E2.1),
where and are the temperature and pressure, respectively, ∆ is the enthalpy of
vaporization, ∆ is the entropy of vaporization, and is the universal gas constant
(8.314 · · ) [38].
ln ∆ ∆ (E2.1)
Graphically, a plot of vapor pressure vs. temperature yields a slope equivalent to the enthalpy of
vaporization. As such, graphing data collected by D. R. Stull, which relates the vapor pressure
and temperature of N2O5 over a range of values, yields an equation that can be employed to
relate any given temperature of N2O5 to its corresponding vapor pressure [38]. Figure 2.9
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
190 195 200 205 210 215
I‐ •N2O
5Signal (cps)
Temperature (Kelvin)
Low Pressure (25 Torr)
High Pressure (45 Torr)
B
CA
D
E
F
G
38
displays the data collected by Stull and the equation employed
ln 5 10 exp 6892 in subsequent calculations for relating the temperature
(and signal) obtained during the calibration to its corresponding vapor pressure [38].
Figure 2.9: Representation of data collected by D. R. Stull relating the vapor pressure
of N2O5 to temperature [39]. The equation of the line of best fit is employed in further calculations to determine N2O5 concentrations.
The data presented in the signal vs. temperature graph (Figure 2.8) can be plotted in a similar
fashion to the Clausius-Clapeyron graph (Figure 2.9). The linear regions of the resulting graph
(Figure 2.10) thus illustrate the rise in the count rate as the surrounding temperature increased
for both calibrations; the slopes are negative because the signal is a function of 1/Temperature.
The first point (at 191 K) and the last three points (when the signal peaked) of the high pressure
calibration are not included in the data shown in Figure 2.10, as these represent times during the
calibration when the vapor pressure was not at equilibrium. Since the signal is ultimately
related to a vapor pressure, the slopes of the lines shown in Figure 2.10 can be compared to the
slope of the equation obtained for the Stull data shown in Figure 2.9. That is, they both
correspond to the enthalpy of vaporization of N2O5 [38]. As can be seen, the slope of the
calibration for the high pressure regime (45 Torr) is -5198 J/mol, whereas the slope of the
calibration performed at 25 Torr is -11529 J/mol. The slope of the Stull data (Figure 2.9)
however is -6892 J/mol [38].
1
10
100
1000
0.0032 0.0034 0.0036 0.0038 0.004 0.0042 0.0044
Vapo
r Pressure (Torr)
1/Temperature (1/Kelvin)
ln(P) = 5x1012{exp[‐6892(1/T)]}
39
An interpretation of this data suggests that the high pressure calibration, which has an enthalpy
of vaporization very similar to that of the Stull data, is the more accurate calibration. Given that
the flow of He through the trap for the low pressure calibration was 50 sccm, it is believed that
the vapor pressure was not at equilibrium at these high flows and thus the signal measured was
not an accurate representation of the true N2O5 concentration. This finding is supported by the
value of the slope of the low pressure calibration; it suggests that the signal increased too
quickly during the calibration and did not have time to equilibrate. Alternatively, the low He
flow (24 sccm) supplied during the high pressure calibration, as well as the responses observed
in the signal when the flow was adjusted, further support the conclusion that the vapor pressure
was at equilibrium during this run and therefore is the better calibration. Subsequent
calculations to determine N2O5 concentrations were therefore performed only using the data
corresponding to the high pressure (low flow) calibration from 198 to 207 K.
Figure 2.10: I-•N2O5 signal as a function of 1/Temperature for calibrations at two
pressure regimes. The slopes of the lines of best fit are representative of the enthalpies of vaporization of N2O5.
100
1000
10000
0.00466 0.00471 0.00476 0.00481 0.00486 0.00491 0.00496 0.00501 0.00506
I‐ •N2O
5Signal (cps)
1/Temperature (1/Kelvin)
Low Pressure (25 Torr)
High Pressure (45 Torr)
ln(S)=4x1026{exp[‐11529(1/T)]}
ln(S)=3x1014{exp[‐5198(1/T)]}
40
The N2O5 concentrations were therefore calculated as in the following example:
HIGH PRESSURE (45 Torr)
Take T = 205.2 K (-67.8 Celsius) to determine the vapor pressure of N2O5 in the trap:
5.00 10 exp 68921
5.00 10 exp 68921
205.2
0.0129
Account for the total He and N2 dilution flow entering the CIMS:
0.0129 24
4000 7.74 10
Convert the pressure of N2O5 to a concentration based on the number of molecules/cm3 in one
Torr at room temperature:
7.74 10 · 3.2 10 ·
2.48 10
Correlate the N2O5 concentration to the signal obtained for I-•N2O5 at 205.2 K:
3200 · 2.48 10
Thus, 1 cps corresponds to the following N2O5 concentration:
1 · 7.75 10
Or equivalently, the sensitivity to the I-•N2O5 cluster following this method can be reported as:
0.031 / (at standard temperature and pressure)
41
These systematic equations can therefore be used to determine the concentration of N2O5 at
every temperature measured between 198 and 207 K. The resulting signal is then plotted vs. the
concentration to yield a graph whose slope is equivalent to the sensitivity.
2.4.2 Statistical Measurements
The sensitivities of the instrument for both the ion cluster and nitrate ion were acquired by
graphing the N2O5 signal obtained by detecting either the I-•N2O5 cluster or the nitrate ion vs.
N2O5 concentration for measurements conducted between 198 and 207 K. The slopes of the
lines of best fit in Figure 2.11correspond to the sensitivity of I-•N2O5 and NO3- at 45 Torr [37].
As illustrated in Figure 2.11, the sensitivity of the ion cluster is 1x10-9 cps molecule-1 cm-3, or
0.024 Hz/pptv and the sensitivity of the nitrate ion is 3x10-9 cps molecule-1 cm-3, or 0.072
Hz/pptv. The sensitivity for I-•N2O5 obtained through the calculations above (0.031 Hz/pptv) is
relatively close to the one obtained from Figure 2.11 (0.024 Hz/pptv), however, as the graph
depicted in Figure 2.11 covers more than one calibration point, it is the more accurate result.
Comparing the sensitivity of the I-•N2O5 cluster to that cited in the Kercher et al. study for the
same ion cluster (> 1Hz/pptv), it is evident that this CIMS system is not yet as sensitive as the
CIMS at the University of Washington [21]. Changing the pinhole in the intermediate chamber
would increase the pressure in the multiplier chamber, thus permitting more ions to pass
through into the quadrupole in the multiplier chamber, rather than having most of them pumped
away by the scroll pump. Increasing the pressure ten-fold in the multiplier chamber (from 10-6
to 10-5 Torr) would presumably increase the sensitivity of the CIMS by an equivalent amount.
These modifications will be implemented once fans are placed on the turbo pump, which is
situated on the IC to prevent it from overheating due to its increased activity as a result of the
increased pressure.
42
Figure 2.11: Calibration graph of N2O5 using two detection methods: I-•N2O5 and NO3-.
The slopes are representative of the sensitivity of each ion at 45 Torr.
The detection limit is minimum signal that can be detected at a known confidence level [37].
As this limit depends on the signal to noise (S/N) ratio, it is common to quote a detection limit
to a known confidence level. The minimum distinguishable signal is defined as the sum of
the mean blank signal plus a multiple of the standard error of the blank , as shown in
equation (E2.2). The standard error is the standard deviation divided by the square root of N
(number of data points). Using the sensitivity obtained from the slope in Figure 2.11, the
minimum distinguishable signal can be converted to a detection limit as shown in equation
(E2.3) [37].
(E2.2)
(E2.3)
The blanks for these signals correspond to time intervals of ion monitoring when N2O5 was not
present in the system. That is, the mean of the blank is calculated by averaging over a time
period equivalent to the duration of a sample run, which in this case is 3 minutes (64 data
points). Table 2.1 displays the statistical data obtained for both I-•N2O5 and NO3-.
S = 1x10‐9[N2O5] + 347.8
S = 3x10‐9[N2O5] + 164.9
0
2000
4000
6000
8000
10000
12000
0 1E+12 2E+12 3E+12 4E+12
N2O
5Signal (cps)
[N2O5] (molecules/cm3)
I‐•N2O5
NO3‐
43
Table 2.1: Statistical data acquired for the I-•N2O5 ion cluster and NO3- anion.
Ion Sensitivity ( (Hz/pptv)
Mean Blank (cps)
Std. Error Blank (cps)
Min. Dist. Signal (cps)
Detection Limit
(pptv)
I-•N2O5 0.024 15.3 1.3 1
2
3
16.5
17.8
19.1
53.3
106.5
159.8
NO3- 0.072 252.5 8.0 1
2
3
260.5
268.5
276.5
111.3
222.6
333.9
As stated in Skoog et al., a value of 3 usually corresponds to a confidence level (CL) of
95%, and is oftentimes the recommended value for [37]. Table 2.1 thus shows that at a 95%
CL, the CIMS can detect the I-•N2O5 ion cluster to 159.8 pptv. If 2, however, the cluster is
detected to 106.5 pptv, and comparing this value to the one obtained by Kercher et al. (11.0
pptv), also at 2, it is evident that this CIMS is not yet comparable to other instruments cited
in literature. Both the sensitivity and detection limit obtained for the cluster thus indicate that
further work is required to improve the performance of the instrument. As noted earlier,
increasing the pressure in the multiplier chamber would increase the sensitivity of the
instrument, but would also possibly increase the noise as well by an equivalent factor.
Additional empirical data at higher pressures would confirm whether or not the S/N ratio at
higher pressures does in fact increase the sensitivity of the CIMS.
With regards to the NO3- ion, the limit of detection at 3 is 333.9 pptv, suggesting that a
rather large signal is required of the NO3- anion before it becomes statistically significant and
reliable. The value obtained for the mean of the blank for NO3- also reveals the presence of a
large background noise for this ion. This background signal may be due to HNO3 present in the
N2 cylinder, which can also complex with I-, indicating that an ultrapure N2 cylinder might
solve the issue of noise for the NO3- ion. Consequently, in addition to the advantage of
avoiding chemical interference from nitric acid, the statistical data shown in Table 2.1, and most
especially the detection limits, confirm that the method of ion cluster detection (I-•N2O5) is more
reliable than the dissociative charge transfer detection scheme (NO3-) [21].
44
2.4.3 Ion Cluster Formation as a Function of Relative Humidity
Higher pressures imply more collisions, which in turn suggest more ion cluster formation. In
addition to the N2O5 cluster, the presence of other complexes have also been observed,
including I-•HNO3 and I-•H2O. Most notably, the I-•N2O5 complex has been observed to
shadow the response of the I-•H2O cluster, indicating a relationship between relative humidity
and N2O5 concentration. As was stated in the introductory chapter (Section 1.2.2), the
formation of N2O5 occurs through a transient, highly energetic complex, N2O5* [7]. Likewise,
the I-•N2O5 cluster forms via a highly energetic intermediate. In effect, as can be seen in
reactions (R2.1) to (R2.3), both the water cluster and the N2O5 cluster form via a termolecular
reaction and therefore require a third body (N/M) to assist in dissipating the excess energy.
Kercher et al. have in fact demonstrated that the water cluster functions as the third body (M) in
the formation of the I-•N2O5 cluster as shown in reaction (R2.3) [21]. The researchers
confirmed an increase in the cluster signal at low water concentrations up to a water vapor
pressure of 0.3 Torr, where no further correlation was detected [21]. Reaction (R2.3) was thus
verified in the Kercher et al. study.
I- + H2O + N → I-•H2O + N (R2.1)
I- + N2O5 + M → I-•N2O5 + M (R2.2)
I-•H2O + N2O5 → I-•N2O5 + H2O (R2.3)
Furthermore, qualitative observations performed during experimental trials conducted during
this research period have also confirmed the dependence of complex formation on RH. Figure
2.12 illustrates this relationship (signals have been normalized to the reagent ion signal recorded
during the trial). A small flow of nitrogen through a water bubbler was added to the sample
flow immediately preceding its entry into the CIMS. As can be seen in Figure 2.12, the N2O5
cluster mirrored the response of the water cluster. For example, a rapid decline in the N2O5
cluster signal accompanied a rapid fall in the water cluster signal when the wet flow was
removed at time t = 3:32 pm. In addition, no change in the water signal was observed at
3:43 pm when N2O5 was not being generated, confirming that the N2O5 cluster is dependent on
the water cluster, and not vice versa. This relationship was further confirmed at 4:24 pm when
the wet flow was once more removed and the N2O5 signal decreased. Interestingly, it was noted
45
that only a small flow (50 sccm N2) through the water bubbler (see Figure 3.1) was required to
boost and maintain the I-•N2O5 signal, as further increases in this flow were not accompanied by
scaled increased in the N2O5 cluster. Complete removal of the wet flow, however, did result in
drastic changes in the I-•N2O5 signal, as is evident at 3:32 pm and 4:24 pm.
Although the CDC in this CIMS is not being employed at the moment for ion cluster formation,
an additional factor which should be considered is the actual cluster formation which could be
occurring in the CDCs of other systems. It is possible that larger water-N2O5 clusters could
form (I-•N2O5•H2O) and then dissociate in the CDC, resulting in either I-•N2O5 and H2O, or
I-•H2O and N2O5. Though confirmation of this cluster formation and its dissociation is not
possible, it should nonetheless be considered as a potential pathway.
These data further suggest that the I-•N2O5 signal could be normalized to both the reagent ion
signal (I-) and the water cluster (I-•H2O) signal. If conditions inside the flow tube were
constantly changing, that is if the relative humidity was fluctuating, the better approach would
be to normalize to the water cluster signal. Such would be the case for ambient measurements.
Contrarily, if the relative humidity was held constant within the system, specifically in the
kinetics flow tube, then the I-•N2O5 signal could simply be normalized to the reagent ion signal.
In any case, whether cluster formation occurs as depicted in reactions (R2.1) to (R2.3), or via
dissociation of a larger cluster, water does participate in the formation of the N2O5 cluster.
46
Figure 2.12: I-•N2O5 cluster as a function of relative humidity. The N2O5 cluster mirrors
the response of the water cluster, I-•H2O.
2.4.4 Mass Spectra of Room and Ambient Air
Mass spectra were obtained under four separate conditions with only the following sample
flows (3-4 slpm) entering the IMR: (i) nitrogen gas (control); (ii) room air; (iii) daytime ambient
air; and (iv) nighttime ambient air. Each of the above sample flows was scanned continuously
for twenty minutes, amounting to five scans each from 50 a.m.u. to 250 a.m.u.; the results of the
five spectra were then averaged for each of the four flows. Figure 2.13 and Figure 2.14
illustrate the resulting spectra for nitrogen and room air, and for daytime and nighttime ambient
air, respectively.
The observed peaks did not vary greatly for each of the four sample flows. As can be seen on
Figures 2.13 and 2.14, major peaks were present at 127 a.m.u. (I-), 145 a.m.u. (I-•H2O), 173
a.m.u. (I-•NO2-), 190 a.m.u. (I-•HNO3), 217 a.m.u. (unidentified), and 242 a.m.u. (unidentified).
The largest peaks correspond to the reagent ion (I-) at approximately 100 kcps, the water cluster
ranging from 32 to 73 kcps, and the nitric acid cluster at 10 to 40 kcps. Although large signals
were expected for both the reagent ion and the water cluster, the intensity of the nitric acid
cluster was surprising. As this signal was also large in the control scan, it can be safely stated
0.00001
0.0001
0.001
0.01
0.1
1
10
100
3:21:36 PM 3:36:00 PM 3:50:24 PM 4:04:48 PM 4:19:12 PM 4:33:36 PM 4:48:00 PM
Signal (cps)
Time
I‐•N2O5
I‐•H2O
N2O5 generation off
wet flow removed
47
that the nitric acid cluster was due to an impure N2 flow, and since 4 slpm of N2 was always
flowing through the permeation tube and radioactive source to yield the reagent ion, the nitric
acid cluster was present in all scans.
Upon closer inspection, peaks are observed at masses corresponding to multiples of 14, which is
consistent with the addition/removal of –CH2 groups to long chain molecules. These peaks are
observed at 78, 80, 92 (78+14), and 94 (80+14) a.m.u., and could potentially correspond to
organic acids. As they are present in all the mass spectra, under all sample flows and with
varying degrees of intensity, they suggest a pattern that cannot be attributed to an inherent noise
in the multiplier or electronics. Further investigation is required to verify this assumption.
As is illustrated in Figure 2.13, although most of the signals of the room spectrum are higher
than those of the control spectrum by approximately 50 cps (A), a few are substantially higher
by 300 to 1000 cps (B). Similarly, Figure 2.14 demonstrates that the spectrum corresponding to
the daytime ambient air registers higher signals than those of the nighttime scan by 20 cps (C),
and some peaks are higher by roughly 300-1000 cps (D). The background levels (where no
large peaks are present), however, are equivalent for all spectra.
Additionally, it should be noted that the I-•N2O5 cluster was observed in the nighttime ambient
air spectra (E), which was scanned at 9:00 pm (see Figure 2.15). Although the signal is quite
small at 64 cps, it is definitely above the minimum distinguishable signal and detection limit for
this ion cluster, as well as significantly higher than the background signals in the vicinity (30
cps). This corresponds to approximately 2.7 ppb of N2O5. Thus, it can be confidently stated
that this peak is indicative of N2O5 present in the ambient air for this particular night, but could
be confirmed by scanning for longer periods of time during the night, or at specific intervals
throughout the night.
Examination of the reagent ion peak (100,000 cps) reveals that the resolution is about 1-2 a.m.u.
at the base of the peak, and therefore 1 a.m.u. if taken as the full width at half maximum (see
Figure 2.16). The background signals are 100 cps prior to the peak and 10 cps after the peak.
The noise leading up to the peak is probably an indication that the resolution needs
improvement; it can theoretically be enhanced by adjusting the DC bias and altering the
voltages applied to the quadrupole.
48
Figure 2.13: Twenty minute averaged mass spectra for nitrogen and room air sample flows.
Figure 2.14: Twenty minute averaged mass spectra for daytime and nighttime ambient air sample flows.
1
10
100
1000
10000
100000
1000000
50 70 90 110 130 150 170 190 210 230 250
Signal (cps)
Mass (a.m.u.)
Room AirControl (Nitrogen)
B
1
10
100
1000
10000
100000
1000000
50 70 90 110 130 150 170 190 210 230 250
Signal (cps)
Mass (a.m.u.)
Daytime Ambient Air
Nighttime Ambient Air
ED
A
C
49
Figure 2.15: Spectra of nighttime and daytime ambient air. Nighttime spectra sampled at
9:00 pm. I-•N2O5 detected at 64 cps (2.7 ppb N2O5) amidst a maximum background count rate of 30 cps.
Figure 2.16: Spectra of control (nitrogen) and room air sample flows. The signal of the
reagent ion (I-) peaks at 100,000 cps. Resolution taken as the full width at half maximum is reported as 1.0 a.m.u.
0
10
20
30
40
50
60
70
80
90
100
233 233.5 234 234.5 235 235.5 236 236.5 237
Signal (cps)
Mass (a.m.u.)
Daytime Ambient Air
Nighttime Ambient Air
1
10
100
1000
10000
100000
1000000
120 121 122 123 124 125 126 127 128 129
Signal (cps)
Mass (a.m.u.)
Room Air
Control (Nitrogen)
50
CHAPTER 3 : AEROSOL KINETICS EXPERIMENTS
3 Two Laboratory Studies
3.1 Motivation for Ammonium Bisulphate Study The heterogeneous reaction of N2O5 has been studied extensively on ammonium bisulphate
(NH4HSO4) aerosols. These particles have been investigated under varying relative humidity
conditions and temperatures, as well as in modified forms, such as with organic coatings or
monolayers [8, 9, 11]. As reported by Zhang et al., sulphate constitutes up to 32% of the
submicron particle mass in northern midlatitudes, and is largely formed as a result of
atmospheric processing of anthropogenic SO2 emissions [1, 39]. The first laboratory experiment
to characterize the uptake coefficient of N2O5 on NH4HSO4 aerosols under RH conditions
varying from 1 to 76%, and temperatures ranging from 274 to 293 K was performed by
Mozurkewich and Calvert [8]. Values reported in their 1988 paper are at the upper limit of most
uptake coefficients that have since then been reported by other researchers. Consequently, they
are used as upper threshold references for similar aqueous N2O5 hydrolysis studies, including
kinetics performed on artificial seawater, malonic acid, ammonium sulphate, and nitrate aerosols
[3, 8, 9, 11, 12, 32]. As such, this study was chosen because results could be easily compared to
a variety of literature values obtained under similar relative humidity conditions. It was also
selected as an initial experiment using the new mobile CIMS described in Chapter 2 due to its
easy implementation and execution. Moreover, this study both investigated and validated the
aerosol flow tube kinetics approach, as well as examined the new detection method of ion cluster
formation [21].
51
3.2 Ammonium Bisulphate Study: Experimental Approach and Results
Ammonium bisulphate aerosols were produced from a 20 wt% (200 g/1L water) solution of
ammonium bisulphate particles (Sigma-Aldrich, #307602) using a constant output atomizer (TSI,
#3076). The aerosols passed through a silica gel dryer to remove excess moisture from the
system and reduce the size of the particles before flowing into a continuous flow reactor. A
dump line was also added from the exit of the atomizer to vent excess aerosols. Specific ratios of
a wet and dry flow of nitrogen were also added to the flow reactor to regulate the relative
humidity in the system, specifically in the kinetics flow tube (see Section 3.2.3 and 3.2.4). One
third of the flow exiting this flow reactor was directed towards a scanning mobility particle sizer
(SMPS) system in order to monitor the aerosol surface areas, and the remainder entered the
kinetics flow tube through a side inlet at the top. N2O5 was produced under conditions outlined
in Section 3.2.1, and entered the flow tube through an injector positioned at the top of the
kinetics flow tube. Experiments were performed in the kinetics flow tube; the resulting products
exited from the bottom of the flow tube and entered the ion molecule region (IMR) of the mobile
CIMS.
The flow supplied to the kinetics flow tube was modified depending on the sample flow the
CIMS was pulling on that particular day, such that the flow emerging from the atomizer
accounted for the difference. The total flow during any given set of experiments, however, was
regulated by a critical orifice placed in the Ultra Torr (Swagelok) union at the bottom of the flow
tube. An additional wet flow from a water bubbler was also added to the sample flow
immediately preceding its entry into the IMR in order to facilitate the formation of the I-•N2O5
cluster, as shown in Section 2.4.3. As the CIMS naturally pulls 3-4 slpm through the inlet, the
total flow entering the IMR consisted of the sample flow from the kinetics flow tube and the flow
through the water bubbler. Figure 3.1 below illustrates the overall configuration for these
experiments.
52
Figure 3.1: Schematic depicting the overall experimental setup for the ammonium bisulphate study.
The generation of N2O5, as well as the kinetics approach was equivalent for all experiments
performed during this research work, including the SOA study outlined in Section 3.3. Below
are detailed descriptions of both processes.
3.2.1 N2O5 Flow Tube N2O5 was produced in situ in a 50 cm long glass flow tube through the dark reaction of O3 and
NO2. Ozone was produced through the photolysis of O2 flowing in at 3-8 sccm over a (small)
11 cm long Hg penray lamp (UVP, LLC). NO2 was supplied from an NO2 cylinder custom-
ordered from Linde BOC Gases consisting of a 2000 ppm concentration of NO2 in nitrogen; the
NO2 flow was constant at 15 sccm. N2O5 was carried from this flow tube towards the kinetics
SMPS
Dry N2 Flow
Dump
Dryer
Wet N2 Flow (through water bubbler)
N2O5 Flow Tube
To IMR of CIMS
Kinetics Flow Tube
Mixing Flow Tube
Aperture
Injector Constant Output
Atomizer Aerosols
Water Bubbler
Room Air
53
flow tube in a 50-60 sccm flow of helium (He). The pressure, maintained at atmosphere, was
read by an MKS Baratron pressure gauge. N2O5 was generated within this flow tube through
reactions (R1.2) and (R1.3). NO2 was added in excess to ensure that the reaction proceeded to
the right, that is, to guarantee that the equilibrium shifted towards the products. An approximate
calculation of the N2O5 to NO3 ratio in this flow tube may be calculated using the equilibrium
constant for reaction (R1.3), 2.9x10-11 molecule-1 cm3, the known NO2 concentration (5.4x1016
molecule cm-3) from the cylinder, and the total dilution flow in the kinetics flow tube:
(E3.1)
This results in a [N2O5]/[NO3] ratio of 1.6x104, thus indicating that N2O5 concentrations are
much larger than NO3 concentrations. Although N2O5 can dissociate back into NO2 and NO3, the
forward rate constant is much faster than the reverse [25]. Thus, along with the supply of excess
NO2, the reactions in the flow tube proceeded towards the formation of N2O5. Figure 3.2
illustrates the configuration of the N2O5 flow tube.
Figure 3.2: Schematic of N2O5 flow tube.
Hg Lamp (photolyzing O2 to O3 at 185 nm) O2
O3 NO2 + He
Glass Flow Tube (50 cm) (Covered with aluminum foil)
Baratron P = 760 Torr
N2O5 to Kinetics Flow Tube
NO2 + O3 → NO3 + O2
NO3 + NO2 ↔ N2O5
54
3.2.2 Kinetics Flow Tube
The kinetics flow tube, situated vertically, is 90 cm long with an inner radius of 3.0 cm. Wall
losses were minimized by maintaining the flow tube at atmospheric pressure and coating the
interior walls with halocarbon wax. It was also covered with aluminum foil to eliminate the
possibility of N2O5 photolysis, and fitted with a glass injector, 120 cm long. The injector
distance inside the flow tube was varied during kinetic runs. All aerosols produced, either for
this study or the next, were pulled in through a side inlet at the top of the flow tube, whereas
N2O5 entered the flow tube directly through the injector. The following reactions occurred
within the flow tube: either N2O5 reacted with the water on the walls of the flow tube, or it
reacted with the water present in/on the aerosols. In either case HNO3 was produced. Figure 3.3
summarizes these details.
Figure 3.3: Schematic of the kinetics flow tube.
N2O5
Aerosols
Injector (120cm)
Glass Flow Tube Coating = Halocarbon wax ID = 3.0 cm L = 90 cm (Covered with aluminum foil)
To CIMS
N2O5 + H2Oaerosols → 2HNO3 N2O5 + H2Owall → 2HNO3 15 cm
30 cm
45 cm
60 cm
75 cm
86cm
55
Kinetics measurements were performed by varying the position of the injector inside the flow
tube. It was pulled out systematically in 15 cm increments every 3 minutes, while the ion signal
and aerosol surface area were monitored. The N2O5 signal was highest when the injector was
completely inside the flow tube, as there was virtually no time or space available for the
molecules to react with the incoming aerosol particles. However, as the injector was steadily
pulled out, the signal decreased because the reaction time increased. Naturally, this also resulted
in greater wall losses, as N2O5 more easily encountered the walls of the flow tube upon exiting
the injector. Background controls runs were therefore required to determine the losses due solely
to the reaction of N2O5 with the water on the walls of the flow tube. These background runs
were conducted in the absence of aerosols, that is, aerosols were not produced at these times.
The hydrolysis reaction of N2O5 on ammonium bisulphate aerosols was investigated under two
separate conditions: 30% RH and 50% RH. The particulars of each subset of experiments are
discussed in the proceeding sections.
3.2.3 Experiments Performed at 30% RH
N2O5 was monitored via the ion cluster detection method described in Section 2.1 [21]. The
reagent ion I-, produced when CH3I (from a permeation tube heated to ~70 Celsius) passes
through the Po-210 radioactive source, clusters with N2O5 and is detected along with this
molecule at 235 a.m.u. As higher pressures in the ion molecule region promote ion cluster
formation, the pressure in this chamber was increased to 45 Torr by partially closing the ball
valve situated in the bellows line from the scroll pump to the IMR. The first set of experiments
was performed under 30% relative humidity by supplying a wet to dry N2 flow in a 1 to 2 ratio.
Although the flows changed daily, this ratio was maintained for all trials: (i) Day 1 – 500 sccm
wet N2 flow and 1000 sccm dry N2 flow, (ii) Days 2 and 3 – 400 sccm wet N2 flow and 800 sccm
dry N2 flow. The RH (30.22%) was measured with a hygrometer (VWR, #35519-041) placed at
the top of the kinetics flow tube, thus the relative humidity reported corresponds to the
conditions inside the flow tube where the hydrolysis reactions occurred. Although an additional
wet flow was added to the sample flow prior to its entrance into the IMR, it did not contribute to
the RH in the kinetics flow tube and thus did not affect the reactions in this region. The flows
were varied on a daily basis to ensure that the flow entering the kinetics flow tube was less than
the flow the CIMS was pulling at that time. This guaranteed that the remainder of the flow
56
required by the CIMS was being pulled from the atomizer. Aerosols were thus drawn into the
system when required.
The experimental procedure was as follows: (1) begin N2O5 generation, (2) increase the pressure
in the IMR to 45 Torr, (3) add wet N2 flow to increase the RH, (4) add aerosols, (5) perform
kinetics. The signal was allowed to equilibrate after each modification to ensure a maximum
count rate. In order to maintain constant flows throughout the system, the aerosol surface areas
were varied by increasing/decreasing the pressure of the regulator on the N2 cylinder. Combined
with the N2 flow of the reagent ion source (4 slpm), the total flow entering the IMR at any given
time was 7-8 slpm. Background control runs were conducted under the same conditions (45 Torr
and 30% RH) in the absence of aerosols. At this point a filter was placed on the line leading
from the atomizer to the dryer, such that the incoming flow from the room was free of particles.
Figure 3.4 displays the original data of the decays obtained at 30% RH. Kinetics were performed
under aerosol surface area concentrations ranging from 9.0x10-5 to 2.26x10-3 cm2/cm3. The
original concentrations were underestimated because the SMPS system could not scan past
750 nm; thus a significant portion of the tail end of the distributions was not measured.
However, given that these were log normal distributions, the concentrations were easily
corrected by doubling the concentrations pertaining to the first half of the distributions. The
concentrations reported here have therefore been corrected, resulting in a 30% increase from the
original concentrations measured by the SMPS system.
As can be seen in all kinetic runs obtained in the presence of aerosols, the signals decline
uniformly until the 60 cm mark, upon which time they plateau. Although the signal present at
this time corresponds to a minimum N2O5 concentration, it is mostly due to the inherent
background noise of the electronics. The detection method employed for these experiments
ensured that the N2O5 signal was not affected by chemical interference resulting from high nitric
acid levels in the flow tube. That is, the signals did not require a correction to account for the
nitrate ion, as with the dissociative charge transfer detection scheme, but they did require an
adjustment (subtraction) corresponding to the signal of the background noise present during the
sampling times [21]. This value ranged from 7 to 25 cps, or 1.2 to 4.2 ppb N2O5, respectively,
for any given kinetic run.
57
Figure 3.4: Original data of the I-•N2O5 signal vs. injector position for kinetic runs
performed under varying aerosol surface area concentrations at 30% relative humidity.
Under 30% RH, the signal of the ion cluster usually dropped by a factor of 2 with the addition of
the wet flow, and then by another factor of 3-4 with the addition of aerosols, such that the signal
decreased from approximately 8000 cps (1300 ppb N2O5) to 1000 cps (170 ppb N2O5) at the
onset of the kinetic run. The reagent ion signal would register at 100,000 cps and then decrease
by a factor of 10 upon the addition of a large concentration of species to the flow tube during
N2O5 generation, in particular HNO3. This is verified by large increases in the nitrate, the nitric
acid cluster (I-•HNO3), and the nitrate-nitric acid cluster (NO3-•HNO3), the latter two reaching
levels of up to 100,000 cps during maximum N2O5 production times.
Figure 3.5 illustrates the corrected decays obtained from Figure 3.4. The last two points at 75
and 86 cm are omitted from Figure 3.5 because they are highly uncertain with the background
subtraction. Close inspection of Figure 3.5 reveals that the ion cluster signal at 30% RH varied
from 360 cps (60 ppb N2O5) to 2.7 cps (0.45 ppb N2O5) for runs conducted with high aerosol
surface area concentrations (0.00226 cm2/cm3). As such, N2O5 levels decreased by as much as a
factor of 130 in the presence of high aerosol concentrations under these conditions. In the
absence of aerosols, however (data shown in black), the signal was relatively constant at 5000
1
10
100
1000
10000
15 25 35 45 55 65 75 85
I‐ •N2O
5Signal (cps)
Distance (cm)
0.00226 cm2/cm3
0.00118 cm2/cm3
0.00112 cm2/cm3
0.00073 cm2/cm3
0.00066 cm2/cm3
0.00059 cm2/cm3
0.00053 cm2/cm3
0.00009 cm2/cm3
no aerosol
58
cps (833 ppb N2O5). It is also evident from Figure 3.5 that the slopes of the lines of best fit of
the decays corresponding to higher aerosol surface areas (orange data points) are steeper than
those obtained for low aerosol surface areas (blue data points). Consequently, these slopes
translated into larger pseudo first order rate constants.
Figure 3.7 in Section 3.2.5 illustrates the resulting graph of vs. aerosol surface area for both
30% RH and 50% RH. Discussion of the uptake coefficients corresponding to both these data
sets is deferred to Section 3.2.5.
Figure 3.5: Corrected decay plots of the I-•N2O5 signal vs. injector position at 30% RH.
The decays were adjusted to account for the inherent background signal.
3.2.4 Experiments Performed at 50% RH
The second set of experiments was initially designed to produce a relative humidity inside the
kinetics flow tube of 60% RH, and thus the wet to dry N2 flows were supplied in a 2 to 1 ratio as
follows: (i) Day 1 – 800 sccm wet N2 flow and 400 sccm dry N2 flow, (ii) Day 2 – 767 sccm wet
N2 flow and 378 sccm dry N2 flow, (iii) Day 3 – 600 sccm wet N2 flow and 300 sccm dry N2
flow. However, upon measuring the RH at the top of the kinetics flow tube it was discovered
that the RH was in fact 50.72% in the flow tube. This discrepancy can be attributed to greater
water losses along the interior of the flow reactor and the walls of the Teflon tubing leading to
1
10
100
1000
10000
15 20 25 30 35 40 45 50 55 60 65
I‐ •N2O
5Signal (cps)
Distance (cm)
0.00226 cm2/cm3
0.00118 cm2/cm3
0.00112 cm2/cm3
0.00073 cm2/cm3
0.00066 cm2/cm3
0.00059 cm2/cm3
0.00053 cm2/cm3
0.00009 cm2/cm3
no aerosol
59
the kinetics flow tube, or incomplete saturation of the bubbler flow. As with the experiments
performed at 30% RH, the flows were varied on a daily basis to ensure that the flow entering the
kinetics flow tube was less than the flow required by the CIMS in order to draw aerosols into the
system. The pressure in the IMR was maintained at 45 Torr, and the same procedure as that
outlined for the trials conducted at 30% RH was also followed for these experiments.
Figure 3.6 displays the corrected decays obtained for the kinetic runs performed at 50% RH. As
with the experiments conducted at 30% RH, the surface areas for these kinetic runs were also
corrected for, resulting in a 30% increase from the original concentrations measured by the
SMPS system. As such, the surface areas ranged from 0.00015 to 0.00126 cm2/cm3. The
original data is not shown as in Figure 3.4, but results at 30% and 50% yielded similar patterns.
The background signal subtracted from the 50% RH data was approximately 7 cps (1.2 ppb
N2O5) for each run. Since these decays were steeper than those obtained at 30% RH, there were
several points, at 75 and 86 cm, as well as at 60 cm for the runs at higher aerosol surface areas
(orange and black data sets), which no longer fell within range and thus could not be plotted on
the graph.
The signal of the ion cluster was affected to a greater degree at these relative humidity conditions
than at 30% RH. It dropped by a factor of 5-10 upon the addition of the wet flow, and then by
another factor of 3-4 with the addition of aerosols. Thus, a signal at 10,000 cps (1670 ppb N2O5)
decreased to approximately 2000 cps (330 ppb N2O5) and then to 700 cps (117 ppb N2O5) at the
onset of the kinetic run, with variability existing in between runs. The larger drop therefore
resulted from the addition of the wet flow and not the addition of aerosols, suggesting that the
wall losses were most influenced by the relative humidity component of these experiments. In
fact, the decrease corresponding to the introduction of aerosols to the system was equivalent, in
relative terms, at both 30% and 50% RH. The reagent ion signal also showed a similar response
under both conditions dropping from 100,000 cps to approximately 10,000 cps, upon the
introduction of several species into the flow tube during N2O5 production, particularly nitric acid.
Figure 3.6 also demonstrates lower signals at the beginning of the kinetic run for all trials
compared to those conducted at 30% RH. The background signals at 50% RH (1700 cps, or 280
ppb N2O5), for example, are a factor of 3 lower than the backgrounds obtained at 30% RH (5000
cps, or 830 ppb N2O5). This pattern is also visible when comparing the kinetic runs of both
60
subsets of data. The orange data points in Figure 3.6, corresponding to an aerosol surface area
concentration of 0.00126 cm2/cm3 illustrates a decline in the N2O5 signal from 90 cps (15 ppb
N2O5) to 2.7 cps (0.45 ppb N2O5), that is it decreases by a factor of 33. Roughly the same
surface area concentration (0.00118 cm2/cm3, shown in teal) in Figure 3.5, however,
demonstrates that the signal declines only by a factor of 13 (22 to 1.7 ppb N2O5). Since the
slopes of the decays displayed in Figure 3.6 also steeper than those shown in Figure 3.4, they
yield larger pseudo first order rate constants. These trends are better illustrated in Figure 3.7
included in Section 3.2.5.
Figure 3.6: Corrected decay plots of the I-•N2O5 signal vs. injector position at 50% RH. The decays were adjusted to account for the inherent background signal.
3.2.5 Results and Discussion for Ammonium Bisulphate Study
Figure 3.7 displays the pseudo first order rate constants as a function of aerosol surface area
concentration for experiments conducted both at 30% and 50% RH. Equation (E3.2) below
denotes the velocity of the flow in the kinetics flow tube, where is the velocity, is the inner
radius (3.0 cm), is the pressure (760 Torr), is the temperature (296 K), and is the flow
1
10
100
1000
10000
15 20 25 30 35 40 45 50 55 60 65
I‐ •N2O
5Signal (cps)
Distance (cm)
0.00126 cm2/cm3
0.00109 cm2/cm3
0.00084 cm2/cm3
0.00071 cm2/cm3
0.00057 cm2/cm3
0.00036 cm2/cm3
0.00015 cm2/cm3
no aerosols
61
(variable for these trials) in the flow tube; the slopes of the decays are converted into pseudo first
order rate constants using the bulk flow velocity as follows [12]:
(E3.2)
In addition, since the flow moves down the flow tube in a parabolic form with the middle portion
resulting in higher velocities than the flows moving down alongside the walls of the flow tube,
the resulting pseudo first order rate constants were corrected for non-uniform flow velocities [12,
40]. The method, as described by R. L. Brown, employs a repetitive, incremental software
routine that accounts for corresponding to wall loss only (in the absence of aerosols),
corresponding to the decays in the presence of aerosols, the gas-phase diffusion constant for
N2O5 (0.1 cm2 s-1), and the respective velocity of the flow in the kinetics flow tube [12, 40].
Due to these corrections, the rate constants changed by approximately 5 to 30%, with variability
depending on the velocity flows. The corrected pseudo first order rate constants for each of the
decays shown in Figures 3.5 and 3.6 are then plotted against the aerosol surface area, such that
the uptake coefficient can be calculated for this particular reaction (see Figure 3.7). The second
order rate constant corresponds to the slope of the graph shown in Figure 3.7. Typically the
y-intercept denotes the kinetics attributed to the losses along the walls of the flow tube, but since
corrections have been made to the pseudo first order rate constants, such that they represent the
losses due only to aerosols, these intercepts are 0 in these data [12, 40].
62
Figure 3.7: Pseudo first order rate constant vs. aerosol surface area for the hydrolysis of N2O5 on ammonium bisulphate aerosols at 30% and 50% relative humidity.
The points encircled in Figure 3.7 from the data set corresponding to 30% RH correspond to
kinetic runs performed under virtually the same aerosol surface area concentrations but different
N2O5 concentrations. The N2O5 concentrations were modified by changing the O2 and NO2
flows; lower flows produced lower N2O5 concentrations. These data points are shown in teal
(0.00118 cm2/cm3) and purple (0.00112 cm2/cm3) in Figure 3.5 above. As expected, lower N2O5
concentrations resulted in lower I-•N2O5 signals, but the slopes of the three runs are almost
identical, and run virtually parallel to each other. The purple data sets (2) and teal data set
correspond to signals at 850 cps (142 ppb N2O5), 450 cps (75 ppb N2O5), and 132 cps (22 ppb
N2O5) at the start of each of the kinetics run, respectively. As such, the latter two data sets
demonstrate a decrease in the N2O5 signal by a factor of 2 and 6.5, respectively, compared to the
original (highest) concentration. First order kinetics were therefore confirmed with these three
trials, as the pseudo first order rate constants were dependent only on the aerosol surface area,
not the N2O5 concentrations.
γN2O5 for each set of experiments can be obtained using the slopes from Figure 3.7, which are the
second order rate constants , and equation (E1.1) introduced in Section 1.2.3. Rearranging to
solve for and defining as shown below leads to a value for γN2O5 as follows [8]:
kI = (91.8 ± 10.9)x(SA)
kI = (50.1 ± 1.6)x(SA)
‐0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.00E+00 5.00E‐04 1.00E‐03 1.50E‐03 2.00E‐03
kI(1/s)
Aerosol Surface Area (cm2/cm3)
50% RH
30% RH
63
4
4
Such that at 30% RH:
4 4 50.1 1.6
3.0 106.7 10 2.0 10
And at 50% RH:
4 4 91.8 10.9
3.0 101.2 10 1.4 10
The uncertainties in the resulting uptake coefficients are attributed to the standard errors from the
least squares fit of each respective data set. Alternatively, the uptake coefficient can be
calculated using the Fuchs and Sutugin correction, which accounts for the effects of gas-phase
diffusion limitations using equations (E3.3) and (E3.4) below [3, 12, 41]:
. (E3.3)
Given that and , equation (E3.3) becomes:
.
(E3.4)
, the Knudsen number is defined as 6 / , where is the diffusion coefficient for N2O5
(0.1 cm2/s) and is the mode of the aerosol diameter. Given that was 262 nm and 164 nm,
and that was 50.1 cm/s and 91.8 cm/s for experiments conducted at 30% and 50% RH,
respectively, equation (E3.4) above can be used to calculate new uptake coefficients for this
study. The resulting values are 6.71x10-3 and 1.23x10-2 for 30% and 50% RH, respectively. As
64
such, it is evident that this correction is negligible (<1%) for the aerosol sizes observed during
these experiments, implying that gas-phase diffusion did not occur, and confirming that the first
method to calculating γN2O5 is a valid approach [3, 12, 41]. It is also a testament as to why sub-
micron particles are best for such experiments.
The uptake coefficients obtained from Figure 3.7 can now be compared to those obtained by
other researchers. Table 3.1 below displays the values reported here along with a summary of
studies performed on aqueous N2O5 systems under various relative humidity conditions, and with
different aerosol substrates [8].
Table 3.1: Summary of N2O5 uptake coefficients obtained under aqueous conditions.
STUDY AEROSOL RH (%) γN2O5
Escorcia and Abbatt ammonium bisulphate
ammonium bisulphate
30
50
0.0067
0.0120
Thornton and Abbatt, 2005
artificial seawater (ASW)
30
50
0.005
0.018
Mentel, Sohn, Wahner, 1999
sodium bisulphate
sodium nitrate
sodium nitrate
58
62
48
≥0.018
0.0032
0.0018
Thornton, Braban, Abbatt, 2003
malonic acid
malonic acid
30
50
0.014
0.025
Folkers et al., 2003 ammonium sulphate
ammonium bisulphate
62.1
60.1
0.0182
0.0187
Hu and Abbatt, 1997 ammonium sulphate 50 0.044
Mozurkewich and Calvert, 1988
ammonium bisulphate
ammonium bisulphate
30
50
0.03
0.05
As shown in Table 3.1, the hydrolysis reaction of N2O5 has been researched extensively under
wet conditions. Given that the results presented above indicate a large dependence on the
relative humidity, the values reported in this document are best analyzed by comparing them to
other aqueous systems. The uptake coefficients summarized above can also be plotted as a
65
function of relative humidity (see Figure 3.8). In fact, Figure 3.8 is both a better representation
of the trends observed for these N2O5 hydrolysis experiments, as well as a better indication of
how the results reported here compare to literature values. Data points shown in black
correspond to the values obtained above at 30% and 50% RH, 0.0067 ±0.0002, and 0.0120 ±
0.0014, respectively. The data points shown in red and blue, however, correspond to literature
values obtained at 30% and 50% RH, respectively.
It is evident that the values reported by Mozurkewich and Calvert lie at the upper limit of other
literature values reported under the same conditions, and therefore are used solely as references
of an upper threshold [8]. Additionally, their study reports that N2O5 concentrations were
determined by first heating N2O5 in quartz tubing (to 375 K) to cause its dissociation, and then
measuring the amount of NO remaining that did not react with the NO3 formed by means of a
standard chemiluminescent reaction with ozone [8]. As such, unlike the experiments outlined in
this report, the researchers detected N2O5 indirectly. Also, the aerosols used in the Mozurkewich
and Calvert study were produced by an atomizer containing equivalent ratios of NH3 and H2SO4
in solution, whereas the experiments conducted for this research work employed a uniform
solution of ammonium bisulphate particles [8]. The more direct method employed here therefore
probably generated aerosols of more uniform composition, thereby affecting the uptake
coefficient measured. Thus, these differences naturally lead to discrepancies in the results
reported in this document and in literature.
The uptake coefficient at 30% RH (0.0067 ±0.0002) is within range of the values corresponding
to malonic acid and ASW [12, 32]. ASW, in fact, has a lower uptake coefficient than the
ammonium bisulphate value reported here; it is most likely a result of higher aerosol surface area
concentrations used in that study. Thornton and Abbatt report that the ASW particles at 30% RH
were partially crystallized, thus implying that insufficient water might have been present in/on
the aerosols, resulting in a lower uptake coefficient [32]. Additionally, the higher N2O5
concentrations used in this study (60-100 ppb for 30% RH) compared to the ASW study (8-25
ppb) might also have resulted in higher nitric acid concentrations, causing experimental artifacts
to arise, such as a larger nitrate effect and hence a lower γN2O5. Malonic acid, however, is
slightly higher. Though this discrepancy cannot be attributed to N2O5 or aerosol surface areas
concentrations, as they were equivalent in both studies, it may be due to systematic errors in the
corrections of non-uniform velocities or surface areas measured [3, 12].
66
The same conclusions may be drawn when comparing the uptake coefficient reported here at
50% (0.0120 ± 0.0014) to those shown for ASW and malonic acid. In this case the value
obtained for ammonium bisulphate is lower than both ASW (0.018) and malonic acid (0.025),
but only to a small degree [12, 32]. This difference may again be attributed to systematic errors,
or imprecision of the least squares fit for this data. The value reported by Hu and Abbatt for
ammonium sulphate aerosols (0.044), however, lies well above the value cited here (0.012) [3].
This variance can possibly be attributed to high nitric acid concentrations in this study, leading to
a lower uptake coefficient, or perhaps additional factors, such as different aerosol concentrations
or further systematic errors [3].
The values reported by Folkers et al. for ammonium bisulphate and ammonium sulphate at 60%
and 62% RH, respectively, seem to illustrate that, for the former aerosols, the relative humidity
dependence does not persist at higher RH values [11]. Since the uptake coefficients for
ammonium bisulphate (0.0187) and ammonium sulphate (0.0182) are very similar to each other,
the trend depicted in Figure 3.8 suggests that ammonium bisulphate no longer responds to RH
changes upon reaching its deliquescence point (40% RH), thus the uptake coefficient plateaus at
this point. As such, the value reported here for ammonium bisulphate (0.012) at 50% is in good
agreement with that reported by Folkers et al. at 60% [11]. Ammonium sulphate, however, has a
deliquescence point at 80% RH, and therefore the value reported by the researchers is probably
low due to the fact that it has not yet reached this point.
67
Figure 3.8: N2O5 uptake coefficients plotted as a function of relative humidity for various
literature values (see Table 3.1). The black data points correspond to results obtained during this research work at 30% and 50% RH, whereas the red and blue data points correspond to literature values obtained at 30% and 50% RH, respectively.
The last study outlined in Table 3.1 and Figure 3.8 is that of Mentel, Sohn, and Wahner [9].
They demonstrated the effect of nitrate concentrations on the suppression of the uptake
coefficient by examining the hydrolysis reaction on sodium bisulphate and sodium nitrate
aerosols. As noted in Figure 3.8, the value corresponding to sodium bisulphate (0.018) at 58% is
much higher than those reported for sodium nitrate at 48% (0.0018) and 65% (0.0032) RH [9].
These results imply that large quantities of nitrate in aerosols significantly affect the uptake
coefficients. In addition, they also demonstrate that high relative humidity conditions are
necessary to compete with the nitrate effect. This is also verified by the study of Bertram and
Thornton, which also illustrates that water and nitrate in an internally mixed particle compete to
either promote or suppress N2O5 reactivity, respectively [14]. Given that it is suspected that
large amounts of nitric acid were present during this study, the data acquired by Mentel et al. and
Bertram et al. suggest that the values reported here might be lower than most literature values
0
0.01
0.02
0.03
0.04
0.05
0.06
20 25 30 35 40 45 50 55 60 65
γ N2O
5
Relative Humidity (%)
Ammonium BisulphateASW
Malonic Acid
Sodium Nitrate
Malonic Acid
Sodium Bisulphate
Ammonium Bisulphate
ASW
Ammonium SulphateAmmonium Bisulphate
68
due to high concentrations of nitrate in the kinetics flow tube, probably at very high ppb and/or
moderate ppm levels [9, 14]. Further experiments are required to verify this assumption.
Overall, the ammonium bisulphate study permitted further characterization of the new CIMS by
testing the new detection method of ion cluster formation. The values obtained for the
hydrolysis reaction of N2O5 on ammonium bisulphate aerosols at 30% and 50% RH are in
reasonable agreement with literature values, and thus verify higher reaction probabilities under
wet conditions. Discrepancies are attributed to systematic errors, variable aerosol surface area
concentrations, and high levels of nitric acid, leading to a nitrate effect.
3.3 Motivation for Secondary Organic Aerosol Study It has been demonstrated in the literature that submicron particle mass in northern midlatitudes is
often dominated by organic aerosols (OA) [39]. Data from 37 field campaigns obtained from an
aerosol mass spectrometer (AMS) compiled by Zhang et al., clearly reveal that OA comprises an
average of 45% of non-refractory particle mass in these regions. Following is sulphate at 32%,
nitrate at 10%, ammonium at 13%, and chloride at 0.6%. Data were collected at urban, urban
downwind, and rural/remotes sites across the northern hemisphere. The OA fractions quoted in
the Zhang et al. paper refer to the sum of hydrocarbon-like organic aerosols and oxygenated
organic aerosols, HOA and OOA, respectively [39]. As was stated in Section 1.3.6, primary
organic aerosols (POA) are those directly emitted from biogenic and anthropogenic sources,
whereas secondary organic aerosols (SOA) are formed through the chemical transformation and
condensation of volatile and semi volatile species. To some degree, POA and SOA correspond
to the HOA and OOA fraction, the former corresponding to fossil fuels and other potential
primary sources such as cooking, while the latter is derived mostly from secondary atmospheric
processes [39].
Although HOA data is thus named because its fragmentation pattern resembles that of a
hydrocarbon, it actually may or may not be completely deoxygenated, and it can become OOA
over the course of its lifetime. As particles age, the OOA to HOA ratio increases and the total
OA fraction due to SOA becomes dominant. In fact, Zhang et al. reported that most of the
organics detected were a result of SOA and not POA since the organic aerosols consisted mostly
69
of oxygenated organic aerosols, rather than hydrocarbon-like organic aerosols [39]. As such,
given the high prevalence of SOA in the atmosphere, studies concerning N2O5 hydrolysis
reactions on these particles are required to examine the effect of this heterogeneous reaction on
NOx and O3 budgets, especially since the only study to date is the work conducted by Folkers et
al. concerning SOA produced through the ozonolysis of α-pinene [11]. The study reported here
was thus chosen in order to widen the scope of knowledge regarding this heterogeneous reaction
on SOA particles. It is viewed as preliminary work to future SOA experiments that will be
conducted under varying relative humidity conditions.
3.4 Secondary Organic Aerosol Study: Experimental Approach and Results
Although both studies conducted during this research work employed an entrained aerosol flow
tube coupled to a CIMS, this study was completed using a CIMS previously assembled in the
laboratory by T. Thornberry and J. P. D. Abbatt in 2004, since the new mobile CIMS was not
fully operational at this time [42]. Unlike the mobile CIMS described in Chapter 2, this system
contains the quadrupole (19 mm diameter rods, ABB Extrel Mass Core Spectrometers) in a
tertiary chamber rotated 90° to the path of the incoming ions. As such, a DC-quadrupole
deflector (ABB Extrel, part # 814715) located immediately preceding the quadrupole is used to
turn the ions by 90°. The ions are detected by a pushing dynode/channeltron multiplier
connected to a decoupling-capacitor/preamplifier (Advanced Research Instrument, MTS-100).
All system electronics are controlled via two external Extrel units similar to the mobile CIMS,
and monitored via software designed by Extrel (ABB Extrel Merlin System) [42].
This study investigated the reaction of N2O5 with secondary organic aerosols, which were
produced through the reaction of ozone and alpha-pinene (α-pinene) in a continuous flow
chamber prior to mixing with N2O5. A total of three flow tubes were employed for this
experiment. As described earlier (Sections 3.2.1 and 3.2.2), the first functioned as the site of
N2O5 generation (N2O5 flow tube), the second as the site of kinetic measurements (kinetics flow
tube), and the third as site of reagent ion chemistry (CIMS flow tube), respectively. Detailed
descriptions of SOA formation and the CIMS flow tube are outlined below. The flows moved
independently from the N2O5 flow tube and aerosol mixing chamber to the kinetics flow tube,
70
then to the CIMS flow tube and finally on to the mass spectrometer. Upon exiting the kinetics
flow tube, one third of the flow progressed to the SMPS system in order to measure aerosol
surface area. The overall configuration is illustrated in Figure 3.9.
Figure 3.9: Schematic depicting the overall experimental setup for the secondary organic aerosol study.
3.4.1 Aerosol Formation
Alpha-pinene gas was produced by passing a dry nitrogen flow through a fritted glass bubbler
containing α-pinene liquid (Fluka, 99%). A dry nitrogen dilution flow was also added after the
bubbler, such that in combination with the nitrogen flow entering the bubbler, both flows could
be adjusted to vary the aerosol concentrations. Flows before the bubbler ranged from 20-150
sccm, and those after the bubbler from 500-1500 sccm. As ozone is created from the photolysis
of oxygen at a wavelength of 185 nm, a flow of O2 between 30 to 150 sccm was passed over a
22.9 cm long mercury (Hg) penray lamp to generate ozone. All three flows in various
combinations generated SOA particles ranging in concentration from 0.0061 to 0.0461 cm2/cm3.
The ozone and α-pinene precursor gases reacted in a dark mixing flow tube to yield SOA and the
SMPS
N2O5 Flow Tube
CIMS Flow Tube
Kinetics Flow Tube
SOA Mixing Chamber
71
resulting flow continued on through an ozone denuder to eliminate excess, un-reacted ozone.
After emerging from the denuder some of the excess flow was vented to a dump and the
remainder continued towards the kinetics flow tube. All initial gas flows were regulated by
MKS mass flow controllers. Figure 3.10 illustrates the reaction scheme for SOA formation.
Figure 3.10: Schematic depicting the formation of secondary organic aerosols.
3.4.2 CIMS Flow Tube
The CIMS flow tube, as shown in Figure 3.11, is an additional flow tube detached from the
actual CIMS system. It is positioned prior to the chamber encompassing the biased pinhole
leading into the secondary chamber, and was used as the site of kinetics in previous research
work performed in the laboratory [42, 43]. However, for this study, it functioned solely as the
site of ion chemistry. A critical orifice (Teflon disk) was placed in the Ultra Torr (Swagelok)
union immediately preceding the flow tube to decrease the pressure from atmosphere to 1.2 Torr,
the pressure required for mass spectrometer operation. This aperture ensured that the flow in the
kinetics flow tube, for these particular experiments, was constant at 900 sccm, which
corresponded to a velocity of 0.575 cm/s and a total reaction time of 2.6 minutes. The flow
(Covered with aluminum foil)
Dilution Flow
α-pinene (liq) in bubbler
α-pinene(g)
N2 O2
O3
Dump
α-pinene(g) + O3(g) →SOA
O3 denuder
to Kinetics Flow Tube
Hg Lamp (photolyzing O2 to O3 at 185 nm)
72
exiting the kinetics flow tube was divided such that one third proceeded towards the SMPS
system and two thirds proceeded towards the CIMS. The resulting ions then move on to the
mass spectrometer through another aperture held at a constant voltage.
As illustrated in Figure 3.11, methyl iodide (Sigma Aldrich, 99%) was added from a 25 mL bulb
(rather than from a heated permeation tube) through a separate line in a N2 carrier flow, which
varied from 1-1.5 slpm. Both the permeation tube and CH3I bulb offer similar sensitivities to the
nitrate ion scaled by a factor related to the intensity of the reagent ion signal, and as such, the
variability between the two sources is negligible (see Section 3.4.3). This SOA study was
conducted by detecting the N2O5 signal by means of the dissociative charge transfer method
discussed in Section 2.1, since the ion cluster (I-•N2O5) was not observed at these low pressures
(1.2 Torr) [21]. As such, N2O5 is indirectly measured by detecting the NO3- ion that forms
through the dissociation of N2O5 when it reacts with I-. As previously mentioned, this detection
method necessitates that a correction be made to account for the chemical interference of the
NO3- signal resulting from HNO3 and not N2O5 (Section 2.1) [21].
Figure 3.11: Schematic of the CIMS flow tube.
3.4.3 Results and Discussion for SOA Study
Although calibrations were not performed on this system during this study period, previous
calibrations performed by Ullerstam, Thornberry, and Abbatt permit a rough estimate for the
quantification of N2O5 concentrations in the following way [43]:
Biased Aperture
From Kinetics Flow Tube
Aperture
CH3I + Po-210 → I-
I- I- + N2O5 → NO3- + INO2
CH3I + N2
Po-210 Source
To MS
To SMPS
73
Assuming that the second order rate constant (~ 1x10-9 cm3 molecule-1 s-1) for the reaction of
I- + N2O5 is equivalent to that of SF6- + HNO3 (corresponding to the detection method, reagent
ion, and analyte employed in the Ullerstam et al. study), a scaled increase in the signal of the
reagent ion (I- compared to SF6-) indicates a scaled increase in the signal of the analyte ion
(NO3-). The sensitivity of the nitrate ion for the study reported here therefore also scales by an
equivalent amount. That is, Ullerstam et al. report a sensitivity for HNO3 of 7720 ± 365 cps
hPa-1, and a reagent ion signal (SF6-) of 500 kcps, whereas this study reports a reagent ion signal
(I-) of 800 kcps, thus resulting in a sensitivity of 12,352 ± 584 cps hPa-1, or 0.124 cps/pptv for the
nitrate ion [43]. This value, however, corresponds to the sensitivity in the CIMS flow tube and
not the kinetics flow tube. As such, pressure differences also have to be considered. Given that
the experiments for this research work were performed in the kinetics flow tube (760 Torr) and
not the CIMS flow tube (1.2 Torr) as in the Ullerstam et al. study, the sensitivity (for the nitrate
ion for this SOA study) is actually 2x10-4 cps/pptv. Concentrations quoted in the following
sections have therefore been estimated using this approach.
Kinetics were performed following a similar procedure as outlined in Section 3.2.2 for the
ammonium bisulphate study (steps 1-5). Figure 3.12 illustrates the decays of the NO3- signal
obtained as a function of injector distance. A total of ten kinetic runs were performed at varying
aerosol surface area concentrations, with two of these runs pertaining to background wall loss
measurements (orange data points). Similar to the ammonium bisulphate study, corrections were
performed on the aerosol surface area concentrations, but unlike the first study, the surface areas
only increased by 15% for these experiments. As such, the surface areas ranged from 0.0061 to
0.0461 cm2/cm3.
As was previously stated, this study was performed by measuring the nitrate ion signal, rather
than the I-•N2O5 cluster, as the latter was not detected at these low pressures. The nitrate signals
usually ranged from 800 to 5500 cps (4 to 27.5 ppm N2O5), while the I- signal registered at 800-
850 kcps. The chemical interference inherent to this detection method is, in fact, visible in the
decays. As the reaction drives forward, the N2O5 component is lost, and the NO3- signal levels
off around 60 cm. It is this plateau at the end of the kinetic run that corresponds to a NO3- signal
arising either from nitric acid present in the kinetics flow tube, or from nitric acid present in the
CIMS system, but not from N2O5. This chemical background (nitric acid from the flow tube)
varies for each run and is dependent on the actual N2O5 concentration present at this time. The
74
systemic background (nitric acid inherently present in the CIMS) however, should be constant
throughout all experimental trials. A higher than normal background NO3- signal, evident at the
end of the kinetic run, would therefore be an indication of the amount of HNO3 produced from
N2O5. Essentially, a greater N2O5 concentration will lead to a greater NO3- background
composed of both a chemical and systemic background.
Figure 3.12: Original data of the NO3
- signal vs. injector position (cm) for kinetic runs performed under varying aerosol surface area concentrations.
The decays displayed in Figure 3.12 were therefore adjusted in order to account for the chemical
and systemic interference of NO3- arising from HNO3. A constant value was subtracted from
each of the above kinetic runs corresponding to the nitrate signal detected at the end of the
kinetic run. For example, 170 cps (0.9 ppm N2O5) was subtracted from the run performed with
an aerosol concentration of 0.0147 cm2/cm3 (purple data points) since this was the count detected
at the last data point. This subtraction however, accounts for both the systemic background
corresponding to nitric acid present in the CIMS system, as well as the nitric acid present in the
flow tube produced from N2O5. A more sophisticated approach would entail subtracting a signal
equivalent to the sum of a constant systemic nitrate background and the amount of nitric acid
produced at each injector position, which would naturally vary. However, given that the nitric
acid levels cannot be measured systematically at each position, a rough estimate relating to both
the systemic and chemical background is subtracted. The adjusted decay plots are shown in
100
1000
10000
15 25 35 45 55 65 75 85
NO
3‐Signal (cps)
Distance (cm)
0.0461 cm2/cm3
0.0233 cm2/cm3
0.0147 cm2/cm3
0.0112 cm2/cm3
0.0061 cm2/cm3
no aerosol
75
Figure 3.13 below. As can be seen, the kinetic runs become more linear, enabling the
determination of the pseudo first order decay constant from the slope of each run. The last two
points at 75 and 86 cm shown in Figure 3.12 are omitted from Figure 3.13, as they do not
correspond to an N2O5 signal, but rather to the background nitrate signal.
Figure 3.13: Corrected decay plots of the NO3
- signal vs. injector position. The decays were adjusted to account for the HNO3
- chemical and systemic backgrounds.
As expected, the black and green decays in Figure 3.13 clearly demonstrate that higher aerosol
surface area concentrations produced faster decays and thus steeper slopes. Larger aerosol
concentrations permit more reactions with N2O5 leading to a faster decline in the signal as the
injector is pulled out. Similarly, the slower decays correspond to the lower aerosol surface areas
as is evident in the blue decay (0.0061 cm2/cm3). The most significant contrast, in fact, is seen in
the black and orange data sets. The nitrate signal recorded for the decay obtained in the absence
of aerosols decreases by a factor of 2.5, from 5000 to 1900 cps (25 to 9.5 ppm N2O5), whereas
the signal corresponding to an aerosol concentration of 0.0461 cm2/cm3 (black data points), drops
by a factor of 18, from 1100 to 65 cps (5.5 to 0.3 ppm N2O5). These trends are therefore similar
to those observed in the ammonium bisulphate study.
The slopes shown in Figure 3.13 are then converted into pseudo first order rate constants
following the same procedure as outlined in Section 3.2.5. These calculations include
accounting for the bulk flow velocity in the flow tube according to equation (E3.2), and
10
100
1000
10000
15 20 25 30 35 40 45 50 55 60 65
NO
3‐Signal (cps)
Distance (cm)
0.0461 cm2/cm3
0.0233 cm2/cm3
0.0147 cm2/cm3
0.0112 cm2/cm3
0.0061 cm2/cm3
no aerosol
76
correcting the rate constants for non-uniform velocities within the flow tube, according to the
method described by R. L. Brown [40]. The resulting corrected pseudo first order rate constants
are then plotted against the total aerosol surface area concentration to yield Figure 3.14 below.
As stated earlier, the Brown correction is performed to convert the pseudo first order rate
constants into values that only account for losses due to aerosols, hence the y-intercept should
cross through the origin [12, 40]. As noted in Figure 3.14, however, two irregularities arise:
(i) the y-intercept is not 0, and (ii) some values are now negative. These irregularities are
attributed to systematic error in the velocities and surface area concentrations measured, and are
negligible given the small value of the resulting intercept. Application of the Brown correction
changed the pseudo first order rate constants by approximately 5 to 25%, with some values
increasing and others decreasing.
Figure 3.14: Pseudo first order rate constant vs. aerosol surface area for the hydrolysis of
N2O5 on secondary organic aerosols produced through the reaction of ozone and α-pinene.
The uptake coefficient for the hydrolysis reaction of N2O5 on SOA particles produced through
the reaction of ozone and α-pinene under the conditions outlined above is therefore:
4 4 0.64 0.05
3.0 108.5 10 7 10
kI = (0.64 ± 0.05)x(SA) ‐ 0.004
‐0.01
‐0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.00E+00 5.00E‐03 1.00E‐02 1.50E‐02 2.00E‐02 2.50E‐02 3.00E‐02 3.50E‐02 4.00E‐02 4.50E‐02 5.00E‐02
kI(1/s)
Aerosol Surface Area (cm2/cm3)
77
The uncertainty arises from the precision of the least squares fit. Similarly to the ammonium
bisulphate study, the uptake coefficient can also be calculated using the Fuchs and Sutugin
method as illustrated in equations (E3.3) and (E3.4) [3, 12, 41]. Given that was 262 nm,
equation (E3.4) yields a γN2O5 value of 8.53x10-5. Thus, this correction (<1%) is also negligible
for this study, corroborating the assumption that gas-phase diffusion did not occur with these
aerosols. The method and value obtained through Figure 3.14 are therefore reported as valid for
these experiments [3, 12, 41].
Although this value is quite low, it remains within a reasonable range in comparison to the study
conducted by Folkers et al, wherein they investigated the reaction of N2O5 with SOA produced
through the ozonolysis of α-pinene [11]. The researchers reported a value of 4.5x10-4 at 60%
relative humidity for α-pinene concentrations of 1.22 ppm and ppm levels of N2O5 [11]. As
such, the low value obtained during this work is a result of several factors: (i) dry relative
humidity conditions, (ii) the nitrate effect, and (iii) the organic substrate.
This study was performed under dry conditions, whereas Folkers et al. investigated the reaction
under 60% RH [11]. As was demonstrated by the results obtained for the ammonium bisulphate
study and literature values displayed in Table 3.1 and Figure 3.8, the presence of water on/in
aerosols has a large influence on the uptake coefficient. Low water concentrations directly
decrease the likelihood that this reaction will occur. This is further confirmed by the study of
Bertram and Thornton, which illustrates that mass accommodation of N2O5 is water dependent
up to H2O concentrations of 15 M [14]. Consequently, the uptake coefficient reported here is
justifiably smaller than the Folkers et al. value by a factor of 5.3 due to dry experimental
conditions [11].
An alternate issue that must be considered, but cannot yet be verified until further experiments
are performed, is the nitrate effect. As stated in Section 1.3.5, the nitrate effect is valid when
water is scarce as it increases the likelihood that N2O5(aq) reforms on aerosol surfaces more
easily, rather than going on to react [9,10]. It was noted earlier that the N2O5 signals needed to
be adjusted due to (relatively) high background nitrate concentrations. Since the chemical and
systemic background values were quite high at times, up to 2000 cps (10 ppm N2O5) compared to
signals at the beginning of kinetic runs (4 to 27.5 ppm N2O5), it can be assumed that these large
nitrate levels also adversely affected the hydrolysis reactions within the flow tube, inducing
78
slower kinetics. Once again, the work of Bertram and Thornton support this statement, as they
demonstrated that nitrate concentrations greatly suppress the uptake coefficient [14]. More
importantly, their results confirmed that low water concentrations and high nitrate concentrations
together produced lower γN2O5 values than each alone [14].
The method employed to account for background nitrate values may also be of consequence to
the low uptake coefficient reported here. The values subtracted for each kinetic run do not take
into account the nitric acid concentrations produced at each injector position. As such, the γN2O5
value quoted here is a lower estimate because if nitric acid is contributing to the signal, then
γN2O5 is being underestimated. A more robust method to account for nitric acid at each position
might lead to steeper slopes in Figure 3.12, and consequently a higher uptake coefficient.
The composition of the aerosols must also be addressed. Given that these particles contain large
organic fractions, it is expected that this substrate will impede the reaction from proceeding by
preventing the diffusion of N2O5 from the aerosol surface into the bulk volume by limiting either
the N2O5 surface accommodation or N2O5 accessibility to water [4, 11, 30, 31]. Overall, all three
factors (the nitrate effect, aerosol composition, and low relative humidity conditions) must be
considered in unison in order to obtain a better understanding of the hydrolysis reaction of N2O5
with these particular aerosols.
The computer model simulations conducted by Dentener and Crutzen suggest that an uptake
coefficient of 8.5x10-5 for this reaction would not have a large impact on the O3 and NOx budgets
[26]. The O3 and NOx budgets decreased by 9% and 50%, respectively, if γN2O5 = 0.1 was used
in the calculations. Alternatively, a γN2O5 value of 0.01 led to 4% and 40% decreases in O3 and
NOx, respectively. These results thus suggest that the reductions would probably be much
smaller for an uptake coefficient of 0.001, and lower still if γN2O5 were even smaller [26]. This
implies that the uptake coefficient reported here for this SOA study will not greatly affect the O3
and NOx budgets in the model.
79
CHAPTER 4 : CONCLUSIONS
4 Future Studies of the Heterogeneous Chemistry of N2O5
4.1 Summary The hydrolysis reaction of N2O5 on aerosols has been examined extensively both in the
laboratory and in the field. This heterogeneous reaction affects the O3 and NOx budgets in the
troposphere and therefore holds great consequence on the global air quality. Dependencies of
this reaction probability on factors such as aerosol size and composition, as well as relative
humidity and temperature have been studied by several researchers [9-12, 29-33]. As such,
various experimental techniques and detection methods have been employed to investigate the
kinetics of this reaction, including aerosol flow tube measurements and aerosol chamber
experiments, as well as FTIR absorption spectroscopy and chemical ionization mass
spectrometry, respectively. In particular, approaches have been developed to measure this
hydrolysis reaction on ambient particles to determine key influences. In fact, several field and
modeling studies, such as those conducted by Brown et al., Dentener and Crutzen, and Evans et
al. have demonstrated the importance of this reaction in the troposphere and the need to further
quantify relationships both in the laboratory and in the field [1, 26, 28].
The work conducted during this research period investigated the hydrolysis of N2O5 on aerosols
produced in the laboratory through two distinct studies. The first study investigated the
hydrolysis reaction on ammonium bisulphate aerosols under two relative humidity conditions.
The uptake coefficients reported are 0.0067 ± 0.0002 and 0.0120 ±0.0014 for 30% and 50% RH,
respectively, obtained using sub-ppm levels of N2O5. Although these values lie at the lower end
of the spectrum compared to previously cited values for similar studies, they are still within a
reasonable range [3, 8, 9, 11, 12, 32]. Discrepancies are attributed to high nitric acid levels, and
systematic errors associated with the velocities and surface area concentrations measured. More
importantly, this study investigated the aerosol flow tube approach and validated the new ion
chemistry detection method via cluster formation. It proved to be highly superior to the
dissociative charge transfer detection method [21].
80
The second study examined the reaction of N2O5 (ppm levels) on secondary organic aerosols
produced through the ozonolysis of α-pinene. The resulting uptake coefficient for this particular
reaction was low, 8.5x10-5 ± 7x10-6, compared to the Folkers et al. study, which was carried out
under different conditions [11]. This low uptake coefficient indicates small, if not negligible
impacts on the O3 and NOx budgets in the model simulations performed by Dentener and Crutzen
[26]. Given that the only study in literature conducted with secondary organic aerosols is that of
Folkers et al., and that SOA constitutes a large composite fraction of submicron particle mass,
further studies are required involving SOA particles [39].
Additionally, a significant portion of the research period was allocated to building and
optimizing a mobile Chemical Ionization Mass Spectrometer for the purposes of developing
kinetics experimental techniques. Calibrations performed both in low and high pressure regimes
demonstrated higher sensitivity to the ion cluster I-•N2O5 at higher pressures. The sensitivity and
detection limits for both the ion cluster and nitrate ion are reported as 0.024 Hz/pptv, and
0.072 Hz/pptv, respectively. As such, the instrument requires further modifications to contend
with similar systems cited in literature, including increasing the pressure in the multiplier
chamber where the quadrupole is located [21, 34]. Nonetheless, this mobile CIMS offers the
possibility to conduct field experiments in the future.
4.2 Future Research Although a number of possible routes of study are available for the heterogeneous chemistry of
N2O5, of primary importance is the ongoing investigation of the reaction on secondary organic
aerosols. Future work includes examining this reaction under varying relative humidities, such
that results could be compared to those obtained in the ammonium bisulphate study. It is
expected that the uptake coefficient will increase as the relative humidity increases. Moreover, a
study could be performed to investigate the effect of organic coatings on aerosols surfaces on
γN2O5. Employing ammonium bisulphate as the seed aerosols, organic coatings produced through
the ozonolysis of α-pinene could be added to the particles. The formation of these coatings could
be monitored via increases in aerosol surface area, and kinetic experiments could thus be
conducted as outlined in Chapter 3. Yet a further possibility is conducting these organic coated
ammonium bisulphate measurements under varying relative humidities. Although the uptake
coefficient should decrease with the addition of an organic coating, an increase in the relative
81
humidity should counteract this effect. The degree to which each factor influences γN2O5 would
then be thoroughly addressed by each of the above experiments.
Alternatively, experiments could be performed to investigate the nitrate effect. Since this
dependence has been observed by various researchers, further examination in this field could be
further supported by monitoring both ion cluster I-•N2O5 and the nitrate ion. Kinetics could
again be performed at different relative humidities and with aerosols of varying nitrate molalities
as demonstrated by Mentel et al. [9, 10]. Further research could also be conducted to examine
the reaction of N2O5 with chloride-containing aerosols to investigate the competing effects of
chloride, nitrate, and water in internally mixed aerosols [14]. Although Bertram and Thornton
parameterized each of the three species in ammonium bisulphate and malonic acid particles,
research to parameterize organic coatings on a variety of other aerosol substrates is still pending
[14]. As with the N2O5 cluster, rather than monitoring the ion produced through dissociative
charge transfer, experiments involving chloride species could be conducted by measuring the
I-•ClNO2 cluster instead, as shown by Kercher et al [21].
More importantly however, is the need to conduct measurements on ambient particles, as
demonstrated by Bertram et al. [20]. Further developments of techniques and methods, which
can be used in the field are therefore of consequence to such studies. Instruments such as the
CIMS described in Chapter 2 are necessary for field deployment, thus interest in instrumentation
development is also required. Laboratory experiments must be corroborated by field
measurements, as well as supplemented by computational modeling studies. Field work needs to
be performed at various locations for long sampling times, especially at nighttime given that
N2O5 is a nocturnal species. Tandem measurements of NOx and O3 should also accompany N2O5
ion monitoring.
Essentially, the hydrolysis reaction of N2O5 could be studied with a wide variety of aerosols sizes
and compositions, including soot, mineral dust, humic acid, organic acids, soil, or mixtures of
such compounds [12-19, 44]. Priorities are dependent on regional conditions and major
anthropogenic influences to the area. Coastal regions for example, would benefit from
experiments involving sea salt aerosols, whereas dry desert regions would benefit from research
conducted on varying dust/soil types. The list of possible substrates is extensive, and has yet to
be exhausted.
82
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