aerosol generation and measurement
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Aerosol Generation and Measurement
Material from James Smith and Steven MassiePresented by Steven Massie
NCAR / ACDMarch 7, 2011
massie@ucar.edu, jimsmith@ucar.edu
NCAR is sponsored by the National Science Foundation
Outline
• Aerosol generation techniques– electrospray– atomization– vibrating orifice aerosol generator– fluidized bed
• Aerosol physical properties (number, size)– condensation particle counter– differential mobility analyzer– optical particle counter
• Aerosol optical properties • Aerosol chemical composition
– Nanometer-sized particle composition– Aerosol mass spectrometer – Tandem differential mobility analyzer– TDCIMS (thermal desorption chemical ionization mass spectrometer)
Aerosol generation(smallest to largest particle sizes)
Taylor cone
Wikipedia
Expose a small volume of electrically conductive liquid to an electric field ina capillary tube of ~ mm diameter.When a threshold voltage is exceeded, the slightly rounded tip emits a jet of liquid.The droplets disintegrate and spread apart due to electrostatic repulsion.
These devices are used in low power thrusters on spacecraft.
Electrospray particle generator: dp = ~ 5 – 50 nm
neutralizer used to stop fission process
Neutralizer
Natural aerosols frequently are charged
To transport aerosol particles, it is important to neutralize them
Use e.g. a TSI instrument to do this with a Kr-85 or Po-210 source
Radioactive source ionizes surrounding air into positive andnegative ions. These ions interact with the aerosol particles
Particles discharge by interacting with the ions
aerosol atomizer: ~ 20 nm to 0.5 m
• the particle size changes with respect to air velocity, viscosity and surface tension
• need to include a dryer downstream
• at small sizes contamination may be an issue
Hinds
Fluidized bed aerosol generator: 0.5 – 50 m
• powder disperser• Bed of bronze beads
breaks up powder agglomerates
Vibrating orifice aerosol generator (VOAG): ~1 – 200 m
3/16
f
Qd Lp
diameter can be changed by changing flow rate or frequency to piezo. Q = flow rate, f = frequency Hinds
piezoelectric actuator
Aerosol physical properties(mostly size and number)
Aerodynamic Diameter
Consider an aerosol particle.
Its Aerodynamic Diameter is the diameter of a water dropletthat falls at the same speed as the aerosol particle
Hinds
Water1 gm / cm3
Other ways of measuring size distribution or making size-classifications
• inertial-based methods – see tutorial: http://aerosol.ees.ufl.edu/instrumentation/section01.html
cascade impactorscyclone separators
Inertia based instruments
An Impactor separates the particles into two size ranges,larger or smaller than a cutoff size
Cascade impactor: have multiple impaction stages in series (largest cutoff size is 1st stage, etc). Decrease the nozzle size each stage. Can get access to each impaction plate and then weigh the particles.
Virtual impactor: replace the impaction plate with a collection probe. Particles with sufficient inertia go into the collection probe.
Time – of – flight: have a nozzle emit particles, use two lasers at e.g. 100 m apart used to time the particles travel. Particle’sAerodynamic diameter is based upon it’s travel time between thetwo beams.
Optical Particle Counter (OPC): ~ 100 nm to 5 m
Advantages:• Can detect very small particles• Non-intrusive• Instantaneous and continuous
information
Disadvantages:• too sensitive to small changes in
• refractive index• scattering angle• particle size• particle shape
size limits defined by Mie scattering, which are used to interpret integrated scattered intensity.
Condensation Particle Counter
Saturate an aerosol with water or alcohol vapor
Cool by adiabatic expansion or flow through a cold tube
Nuclei will grow to ~ 10 m
Every nuclei grows to a droplet
Measure the number of droplets with an e.g. single particleoptical counter
Condensation Particle Counter (CPC): ~1.5 nm to 0.5 m
Condensation Particle Counters (CPCs) detects particles by exposing them to a region that is supersaturated with vapor (usually butanol), thus allowing particles to grow to a size that can be optically detected.
Counting efficiency curve: TSI model 3010 Response time: TSI model 3010
Hinds
Signal to Particle Diameter
DMA - Differential Mobility Analyzer
Hinds
A charged particle will be pushed in the direction of VTE by theelectric field E between the two plates.
DMA - Differential Mobility Analyzer
Hinds
Stokes Drag on a particleFd = 3 V d / Cf = viscosity of airV = transverse velocity (going from plate to plate)d = diameter of the particleCf = 1 + (mean free path of particle) / d (correction factor)
Electric force on a particle with charge Q in electric field E isQE
Equate the two forces , solve for V = Q E Cf / 3 d
V = Q E B where B is called the Mobility
Differential Mobility Analyzer (shown below, a “Nano DMA”)
Chen et al., 1998TSI, Inc.
inlet
outlet
HV
sheath air
Efficiently size-selects charged particles for collection and analysis.
Unipolar charger
Chen & Pui, 1999;Smith, et al., AS&T, 2004
An efficient ambientnanoparticle charger
~x10 more efficient than bipolar chargers for sub-20nm particles.
Voltages turned off for particles >20nm due to multiple charging.
210Po source
rings, coupled byresistors
DMA + CPC = Scanning Mobility Particle Sizer (SMPS) or Differential Mobility Particle Sizer (DMPS)
DMPS:• A pre-impactor removes all particles larger than the upper diameter of the size range to be
measured• The particles are brought in the the bipolar charge equilibrium in the bipolar diffusion charger.• A computer program sets stepwise the voltage for each selected mobility bin. • After a certain waiting time, the CPC measures the number concentration for each mobility bin.• The result is a mobility distribution.• The number size distribution must be calculated from the mobility distribution by a computer
inversion routine.
Aerosol Optical Properties
Scattering Geometry
Bohren and Huffman
=scattering angle
Note polarization:
|| Parellel to scatteringplane
Perpendicular toscattering plane
P( ’, ’’ ) = Phase function 1 = (1/4 ) P ( ’, ’’ ) d
Given the direction ’ of an incident beam, and direction’’ of the scattered direction, the scattering angle = ’ - ’’
< 90 for forward scattering
> 90 for backward scattering
The phase function tells you the 3 dimensional angular patternof the scattered light
See Thomas and Stamnes, Radiative Transfer in the Atmosphere and Ocean, Cambridge University Press, 1999.
Phase function
Scattering Angleis ’ - ’’
Ice particleshave sharpforwardscatteringpeak
Thomas and Stamnes, Fig 6.3
Phase Functions
Particle Scattering Patterns
Bohren and Huffman
X= D /
D = particle diameter
= wavelength of light
Mie scattering
Measurement of optical properties: Extinction
Beer’s LawI
ILe
0
exp( )
Extinction Efficiency
particle on theincident lly geometricapower radiant
particle aby absorbed and scatteredpower radiant eQ
Extinction Coefficient
4
2ep
epe
QNdQNA
(monodisperse aerosols)
L = path length, N = number of particles per volume
Extinction-based aerosol instruments
transmissometer (used at airports)
pulsed laser cavity-ringdown spectrometer
stack opacity monitor
Nephelometer: Measuring light scattering
The nephelometer is an instrument that measures aerosol light scattering. It detects scattering properties by measuring light scattered by the aerosol and subtracting light scattered by the gas, the walls of the instrument and the background noise in the detector.
Aerosol chemical composition
Tandem Differential Mobility Analyzer (TDMA)
Volatility (~100 °C) Hygroscopicity
Sulfuric acid Volatile Very hygroscopic
Sulfates
(Totally or partially neutralized by ammonia)
Non-volatile Very hygroscopic
Organic carbons Volatile
Not or only slightly hygroscopic
DMA1 DMA2
Heater
Humidifier
ParticleCounter
Dp Dp Dp
if volatile ...
Dp Dp Dp
if hygroscopic ...
Mass Spectrometer
Have a charged molecule of charge Q
Impose E and B fields. The molecule will spiral in the fields. F = Q (E + (v x B )) (Lorentz force)
The curvature of the path of the molecule is given by F = m Awith A the acceleration (e.g A = v2 / R with radius of curvature R)
(m/Q) A = E + ( v x B )
Express Q = z e
Mass spec data has m / z on the x axis of a graph
Mass Spectrometer
Wikipedia
Old style mass spec
Have B directionperpendicularto the page
Use “Right handRule” to see thedirection ofv x B
Radius R of pathof particle of larger mass differsfrom that of the lighter particles
Quadrupole Mass Analyzer
Wikipedia
Radio frequency voltages are applied between one pair of rods and the other
Only ions of a certain mass-to-charge ratio will pass through the quadrupole (otherswill collide with the rods)
Aerosol Mass Spectrometer (AMS)
• For an excellent review of this and other instruments that measure aerosol composition using mass spectrometry see:– http://cires.colorado.edu/~jjose/Papers/2010-
09_IAC_Aerosol_MS_Tutorial.pdf
Canagaratna et al.,Mass Spec Rev, v26, P185-222,2007.
Field observations
Lab
Lab
AMS mass spectrum from ambient aerosol
• Since the AMS uses electron impact ionization and high temperature, species are modified as they are desorbed and ionized.
• Luckily, marker species and co-varying peaks can be found that uniquely identify compound classes.
• A high-resolution Time-Of-Flight Mass Spectrometer (TOFMS) has been developed for use with the AMS, thus allowing for elemental analyses such as C:O. In the TOFMS, an E field accelerates ions of different mass to the same kinetic energy ½ m v2. Larger mass ions travel at slower v than lighter ions. For each ion, measure the travel time between two laser beams, get v, and then m.
Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS)
Use electrostatic precipitator to collect particles
Use evaporation-ionization chamber to ionize particles
A collision-induced dissociation (CID) chamber is used to strip clusters down to their ion cores
Use triple quadrupole mass spectrometer to sort the particles
Electrostatic precipitator Place a wire in a tubeHave E field between wire and tubeIn the TDCIMS, charged particles go to the wire
Chemical IonizationH3O+ + NH3 -> NH4
+ + H2O
CID - Collision-induced dissociation chamber Accelerate ions and let them collide with neutral e.g. Argon gas. The ions will break apart.
Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS)
an instrument for characterizing the chemical composition of ambient particles from 8 to 50 nm in diameter
Voisin et al., AS&T, 2003; Smith, et al., AS&T, 2004
TDCIMS electrostatic precipitator
no voltage applied to filamentFlows of clean N2 keep ambient air away from ion source and filament.
Concentration of particles exiting precipitator noted for estimating collected fraction.
ion source
collection filament
size-selected nanoparticles from Nano-DMA
de-clustering cell
massspec.
TDCIMS electrostatic precipitator
4000 V applied to filamentCharged particles are attracted to the filament by the electric field.
Collection is done at RT and atm, for ~5 – 15 min in order to collect ~10-100 pg sample.
Concentration of particles exiting precipitator noted for estimating collected fraction.
TDCIMS electrostatic precipitator
collection completefilament moved into ion sourceCharged particles are attracted
to the filament by the electric field.
Collection is done at RT and atm, for ~5 – 15 min in order to collect ~10-100 pg sample.
Concentration of particles exiting precipitator noted for estimating collected fraction.
TDCIMS ion source
Close-up of ion source duringsample desorption
• Pt wire ramped from room temperature to ~550 °C to desorb sample
• Neutral compounds are ionized using chemical ionization, e.g.: (H2O )nH3O+ + NH3 (H2O )mNH4
+ + (H2O)n-m
• Reagent ions are created by particles emitted from the source, generating mostly H3O+, O2
- and NO-, …
• Ionized analyte injected into a triple quadrupole mass spectrometer for analysis
pinhole to vacuum chamber
241Amfoil
de-clusteringcell
Pt filament
Succinic acid C4
Glutaric acid C5
Current Parent M-46 fragment
No
rmal
zed
Co
un
ts
Time (sec)0 300
Dicarboxylic Acid Results C = 4-8
0
Suberic acid C8
Pimelic acid C7
Adipic acid C6
Smith and Rathbone, Int. J. Mass Spectrom., 2008
Filament current550 °C
~100 Hz per pg collected aerosol
Temperature programmed TDCIMS: Soft ionization of dicarboxylic acids
Dicarboxylic acids liketo fragment, typically intoformic acid (HCOOH), which has amass of 46 amu units.
Values are integrated areas ofcurves on the right
References
William Hinds, Aerosol Technology – Properties, Behavior, and Measurement of Airborne Particles, John Wiley, 1999
Air Sampling Instruments for Evaluation of AtmosphericContaminants (9th ed), by American Conference of Governmental Industrial Hygenists Staff, 2001
Baron and Willeke, Aerosol Measurement, 2005
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