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Aerosol Generation and Measurement

Aerosol Generation and Measurement. Material from James Smith and Steven Massie Presented by Steven Massie NCAR / ACD March 7, 2011 massie@ucar.edu, jimsmith@ucar.edu. NCAR is sponsored by the National Science Foundation. Outline. Aerosol generation techniques electrospray atomization

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Aerosol Generation and Measurement

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  1. Aerosol Generation and Measurement Material from James Smith and Steven Massie Presented by Steven Massie NCAR / ACD March 7, 2011 massie@ucar.edu, jimsmith@ucar.edu NCAR is sponsored by the National Science Foundation

  2. 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)

  3. Aerosol generation(smallest to largest particle sizes)

  4. 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.

  5. Electrospray particle generator: dp = ~ 5 – 50 nm neutralizer used to stop fission process

  6. 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

  7. aerosol atomizer: ~ 20 nm to 0.5 mm • 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

  8. Fluidized bed aerosol generator: 0.5 – 50 mm • powder disperser • Bed of bronze beads breaks up powder agglomerates

  9. Vibrating orifice aerosol generator (VOAG): ~1 – 200 mm piezoelectric actuator diameter can be changed by changing flow rate or frequency to piezo. Q = flow rate, f = frequency Hinds

  10. Aerosol physical properties(mostly size and number)

  11. 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 Water 1 gm / cm3 Hinds

  12. Other ways of measuring size distribution or making size-classifications • inertial-based methods – see tutorial: http://aerosol.ees.ufl.edu/instrumentation/section01.html cascade impactors cyclone separators

  13. 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’s Aerodynamic diameter is based upon it’s travel time between the two beams.

  14. Optical Particle Counter (OPC): ~ 100 nm to 5 mm size limits defined by Mie scattering, which are used to interpret integrated scattered intensity.

  15. 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

  16. Condensation Particle Counter (CPC): ~1.5 nm to 0.5 mm 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

  17. Signal to Particle Diameter Hinds

  18. DMA - Differential Mobility Analyzer A charged particle will be pushed in the direction of VTE by the electric field E between the two plates. Hinds

  19. DMA - Differential Mobility Analyzer Stokes Drag on a particle Fd = 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 Hinds

  20. Differential Mobility Analyzer (shown below, a “Nano DMA”) inlet sheath air HV outlet Efficiently size-selects charged particles for collection and analysis. TSI, Inc. Chen et al., 1998

  21. Unipolar charger 210Po source 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. rings, coupled byresistors Chen & Pui, 1999; Smith, et al., AS&T, 2004

  22. 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.

  23. Aerosol Optical Properties

  24. Scattering Geometry • =scattering angle • Note polarization: • || Parellel to scattering • plane • Perpendicular to • scattering plane Bohren and Huffman

  25. Phase function 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, 1999.

  26. Phase Functions Scattering Angle is ’ - ’’ Ice particles have sharp forward scattering peak Thomas and Stamnes, Fig 6.3

  27. Particle Scattering Patterns X=  D /  D = particle diameter  = wavelength of light Bohren and Huffman

  28. Mie scattering

  29. Measurement of optical properties: Extinction Beer’s Law Extinction Coefficient (monodisperse aerosols) Extinction Efficiency L = path length, N = number of particles per volume

  30. Extinction-based aerosol instruments transmissometer (used at airports) pulsed laser cavity-ringdown spectrometer stack opacity monitor

  31. 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.

  32. Aerosol chemical composition

  33. Tandem Differential Mobility Analyzer (TDMA)

  34. 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 = 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

  35. Mass Spectrometer 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 Wikipedia

  36. 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)

  37. 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

  38. Field observations Lab Lab Canagaratna et al., Mass Spec Rev, v26, P185-222,2007.

  39. 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.

  40. 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

  41. 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 H3O+ + 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.

  42. 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

  43. TDCIMS electrostatic precipitator no voltage applied to filament Flows of clean N2 keep ambient air away from ion source and filament. Concentration of particles exiting precipitator noted for estimating collected fraction. de-clustering cell ion source mass spec. collection filament size-selected nanoparticles from Nano-DMA

  44. 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 ~10-100 pg sample. Concentration of particles exiting precipitator noted for estimating collected fraction.

  45. TDCIMS electrostatic precipitator collection completefilament 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 ~10-100 pg sample. Concentration of particles exiting precipitator noted for estimating collected fraction.

  46. TDCIMS ion source pinhole to vacuum chamber 241Amfoil de-clustering cell Pt filament 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 byaparticles emitted from the source, generating mostly H3O+, O2- and NO-, … • Ionized analyte injected into a triple quadrupole mass spectrometer for analysis

  47. 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. ~100 Hz per pg collected aerosol 550 °C Filament current Values are integrated areas of curves on the right Smith and Rathbone, Int. J. Mass Spectrom., 2008

  48. References William Hinds, Aerosol Technology – Properties, Behavior, and Measurement of Airborne Particles, John Wiley, 1999 Air Sampling Instruments for Evaluation of Atmospheric Contaminants (9th ed), by American Conference of Governmental Industrial Hygenists Staff, 2001 Baron and Willeke, Aerosol Measurement, 2005

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