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