Aerosol generation (smallest to largest particle sizes)
Taylor cone Wikipedia Expose a small volume of electrically conductive liquid to an electric field in a 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: d p = ~ 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 and negative 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 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 droplet that falls at the same speed as the aerosol particle Hinds Water 1 gm / cm 3
Other ways of measuring size distribution or making size- classifications inertial-based methods – see tutorial: cascade impactors cyclone 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 1 st 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’s Aerodynamic diameter is based upon it’s travel time between the two 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 particle optical 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 3010Response time: TSI model 3010
Hinds Signal to Particle Diameter
DMA - Differential Mobility Analyzer Hinds A charged particle will be pushed in the direction of V TE by the electric field E between the two plates.
DMA - Differential Mobility Analyzer Hinds Stokes Drag on a particle F d = 3 V d / Cf = viscosity of air V = transverse velocity (going from plate to plate) d = diameter of the particle Cf = 1 + (mean free path of particle) / d (correction factor) Electric force on a particle with charge Q in electric field E is QE 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., 1998 TSI, 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 ambient nanoparticle charger ~x10 more efficient than bipolar chargers for sub-20nm particles. Voltages turned off for particles >20nm due to multiple charging. 210 Po source rings, coupled by resistors
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 scattering plane Perpendicular to scattering 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 pattern of the scattered light See Thomas and Stamnes, Radiative Transfer in the Atmosphere and Ocean, Cambridge University Press, Phase function
Scattering Angle is ’ - ’’ Ice particles have sharp forward scattering peak Thomas and Stamnes, Fig 6.3 Phase Functions
Particle Scattering Patterns Bohren and Huffman X= D / D = particle diameter = wavelength of light
Measurement of optical properties: Extinction Beer’s Law Extinction Efficiency Extinction Coefficient (monodisperse aerosols) L = path length, N = number of particles per volume
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 acidVolatileVery hygroscopic Sulfates (Totally or partially neutralized by ammonia) Non-volatileVery hygroscopic Organic carbonsVolatile Not or only slightly 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 A with A the acceleration (e.g A = v 2 / 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 direction perpendicular to the page Use “Right hand Rule” to see the direction of v x B Radius R of path of particle of larger mass differs from 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 (others will 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: – 09_IAC_Aerosol_MS_Tutorial.pdf
Canagaratna et al., Mass Spec Rev, v26, P ,2007. Field observations 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 v 2. 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 tube Have E field between wire and tube In the TDCIMS, charged particles go to the wire Chemical Ionization H 3 O + + NH 3 -> NH H 2 O 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 filament Flows of clean N 2 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 mass spec.
TDCIMS electrostatic precipitator 4000 V applied to filament Charged 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 ~ pg sample. Concentration of particles exiting precipitator noted for estimating collected fraction.
TDCIMS electrostatic precipitator collection complete filament moved into ion source Charged 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 ~ pg sample. Concentration of particles exiting precipitator noted for estimating collected fraction.
TDCIMS ion source Close-up of ion source during sample desorption Pt wire ramped from room temperature to ~550 °C to desorb sample Neutral compounds are ionized using chemical ionization, e.g.: (H 2 O ) n H 3 O + + NH 3 (H 2 O ) m NH (H 2 O) n-m Reagent ions are created by particles emitted from the source, generating mostly H 3 O +, O 2 - and NO -, … Ionized analyte injected into a triple quadrupole mass spectrometer for analysis pinhole to vacuum chamber 241 Am foil de-clustering cell Pt filament
Smith and Rathbone, Int. J. Mass Spectrom., 2008 Filament current 550 °C ~100 Hz per pg collected aerosol Temperature programmed TDCIMS: Soft ionization of dicarboxylic acids Dicarboxylic acids like to fragment, typically into formic acid (HCOOH), which has a mass of 46 amu units. Values are integrated areas of curves on the right
References William Hinds, Aerosol Technology – Properties, Behavior, and Measurement of Airborne Particles, John Wiley, 1999 Air Sampling Instruments for Evaluation of Atmospheric Contaminants (9 th ed), by American Conference of Governmental Industrial Hygenists Staff, 2001 Baron and Willeke, Aerosol Measurement, 2005