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Aerosol Fundamentals

Aerosol Fundamentals. Material from James Smith and Steven Massie Presented by Steven Massie NCAR / ACD February 22, 2011 massie@ucar.edu, jimsmith@ucar.edu. NCAR is sponsored by the National Science Foundation.

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Aerosol Fundamentals

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  1. Aerosol Fundamentals Material from James Smith and Steven Massie Presented by Steven Massie NCAR / ACD February 22, 2011 massie@ucar.edu, jimsmith@ucar.edu NCAR is sponsored by the National Science Foundation

  2. “We have in this fine dust [aerosols] a most beautiful illustration of how the little things in the world work great effects by virtue of their numbers.” -John Aitken, 1880 John Aitken (1839-1919)

  3. Why should we care about aerosol? Human health Air quality Global Cimate Radiation, Chemistry, Rainfall Aerosols are important from the molecular to the global scale

  4. Aerosols : solid and liquid particles suspended in the air Size: nm to 100 microns (range of 105) Lifetime: Troposphere (days to weeks) Stratosphere (year) Primary aerosol: emitted directly into the air Secondary aerosol: gas to particle conversion Composition: sulfate, ammonium, nitrate, sodium, trace metals, carbonaceous, crustal, water Carbonaceous elemental: emitted directly into the air (e.g. diesel soot) organic: a) directly by sources (e.g combustion, plant leaf) b) condensation of low volatile organic gases

  5. “fine” diameters D < 2.5 microns sulfate, ammonium, organic carbon, elemental carbon Nuclei mode 0.005 to 0.01 microns condensation of vapors Accumulation mode 0.1 to 2.5 microns coagulation “coarse” diameters D > 2.5 microns natural dust (e.g. desert) mechanical processes crustal materials biogenic (pollen, plant fragmets)

  6. Aerosols come from a variety of sources, and reside in the atmosphere for weeks aerosols = particles suspended in a gas Seinfeld and Pandis

  7. Aerosols and human health 1952: the “London smog disaster” Air pollution O3, CO, NO2, SO2 aerosol

  8. Submicron aerosols are primarily responsible for visibility reduction. Aerosols are the principle component of what we perceive as “smog” Pasadena, CA, on a clear day (hills are 7 km away) Environmental Protection Agency (EPA) PM2.5 15 g /m3 (annual average) PM10 150 g /m3 (24 hour) regulations. See national map of compliance at: http://en.wikipedia.org/wiki/File:Pm25-24a-super.gif Pasadena, CA, on a bad smog day

  9. Developments in Asia 1000 cars / day are added to the Beijing road system China GDP and NO2 trends ~ 10 % / year Massie

  10. Submicron aerosols can penetrate to the deepest parts of the lung whereupon they can affect the pulmonary part of the respiratory system. For this reason many, including the EPA, consider them dangerous air pollutants. Aerosols and human health

  11. nanoaerosols and human health • Nanoparticles have the ability to translocate from the lung to different parts of the body such as the heart, liver, bone marrow and brain. (Oberdörster, Env. Health Perspect., 2005)

  12. Mt. Pinatubo June 1991

  13. Impact of Pinatubo aerosols on average global surface air temperature: ~0.5 °C! June 12, 1991 eruption

  14. Polar Stratospheric Cloud (PSC) g, gas s, particle ClONO2 (g) + HCl (s) (PSC)  Cl2(g) + HNO3(s) Cl2 + hv  2Cl Leads to the “Ozone Hole” every Spring

  15. Aerosols and climate: indirect effect “Ship Tracks” off the coast of Washington • aerosols are the “seeds” upon which water vapor condenses to form a cloud (these are called “cloud condensation nuclei, or CCN). • If people make more aerosols, we make more cloud droplets, but because there is a fixed amount of water vapor in the air these droplets will be smaller. • smaller droplets scatter light more efficiently! • smaller cloud droplets may also impact rain from these clouds. • very difficult effect to observe and model!

  16. Aerosol “indirect effect” on climate • clean cloud (few particles): • large cloud droplets • low albedo • efficient precipitation • polluted cloud (many particles): • small cloud droplets • high albedo • suppressed precipitation • (very controversial)

  17. Aerosols and climate • Drives Global Warming Direct effect – Light is scattered and absorbed IPCC, AR4

  18. “Warm clouds” clouds with liquid droplets “Mixed phase clouds” clouds with liquid and ice

  19. Optical Depth I = I0 exp [ -(scattering + absorption) (n L) ] Beer’s Law I0 original intensity of light that goes into the cell I observed intensity of light after it travels through the cell L, path length of cell (cm) n, number density of particles (cm-3) scattering scattering cross section (cm2) absorption absorption cross section (cm2) = (scattering + absorption) (n L) Optical depth (unitless) a = (scattering ) / (scattering + absorption) Single scattering albedo K = (scattering + absorption) n Extinction coefficient (1 / km)

  20. “remote continental air” Size Distribution <100 nm: ultrafine Number Density n Surface Area (n π r2) Volume Density (n 4/3π r3) 100 nm<dp<1 mm: accumulation sub-2.5 mm: fine coarse Seinfeld and Pandis

  21. Size Distribution Log-normal particle size distribution dN/dr =  (Ni / (2 ) 1/2 r ln) exp {- (ln r/ln r0)2/2ln2) Units: number per cm-3 per microns i=1,2 2 modes Ni, total number of particles for mode i (cm-3) r, radii (microns) , mode width (dimensionless) r0, modal radii (microns)

  22. Optical Properties • ext() = 103  Qext (r,) r2 dN/dr dr extinction (units km-1) • r particle radius (m) • dn / dr particle size distribution (# cm-3m-1) • Qext (r ,  ) extinction efficiency (from Mie theory) • Qext = Qsca + Qabs (sca=scattering, abs = absorption) • Q is a function of the complex index of refraction • i.e. composition • Size parameter x = 2  r /  • Optical depth contribution in path length ds is  () ds

  23. Mie Scattering Mie scattering (spherical particles) Sensitivity to m2 m, complex index of refraction Increase absorption, m2 m = 1.33 + i m2 The fine details become smoother Note as particle size becomes large, Qext -> 2 Each curve is offset by 1 m2=0.001 0.01 0.03 0.10 Qext Bohren and Huffman

  24. General Results Size parameter x = 2  r /  for  = wavelength For very small particles x << 1 (Rayleigh scattering) scattering  1 / 4 The sky is Blue For medium size particles (aerosols), x ~1 (Mie scattering) scattering ~ 1/  The sky is grey For big size particles (cloud droplets, cirrus) x >> 1 Qext -> 2 independent of  Clouds are white

  25. aerosol optics particles scatter most efficiently in the part ofthe size spectrum withthe longest atmosphericlifetimes!

  26. Global Sulfur Cycle OCS Image courtesy of Hugh Powell, Univ. of Durham, UK

  27. Global Nitrogen Cycle PSCs Image courtesy of Hugh Powell, Univ. of Durham, UK

  28. 450 / 3100 = 15% Seinfeld and Pandis

  29. Annual mean PM2.5 concentration (2002) derived from MODIS satellite instrument data

  30. The chemical properties of atmospheric aerosols: North Americahttp://eosweb.larc.nasa.gov/PRODOCS/narsto/table_narsto.html Annual mean PM2.5 concentrations (NARSTO, 2004)

  31. Black carbon primary emissions (estimates based on satellite obs) DIESEL DOMESTIC COAL BURNING BIOMASS BURNING

  32. Mean sea salt aerosol concentrations Lower marine boundary layer (0-100 m) Roaring 40’s Max 50 S wind Alexander et al. [2005]

  33. Global dust emissions (modeled) 1 US Penny = 2.5 g Fairlie et al. [2007]

  34. Secondary aerosols: Those derived from the condensation of atmospheric trace gases ammonium sulfate aerosol pKa= -3 IMPROVE network (http://vista.cira.colostate.edu/improve/) Seinfeld and Pandis

  35. The formation of ammonium nitrate aerosol mixed NH4+/SO42-/NO3- system pKa= -1.5 IMPROVE network (http://vista.cira.colostate.edu/improve/) Seinfeld and Pandis

  36. Impact of aerosol acidity: Acid deposition (1985) sulfate ammonium nitrate National Acid Deposition Program (http://nadp.sws.uiuc.edu/)

  37. Impact of aerosol acidity: Acid deposition (2005) sulfate ammonium • From 1985 to 2005 … • Significant increases in precipitation ammonium concentrations at 64% of sites. • Statistically significant decreases in sulfate at 89% of sites. • Ammonium now exceeds sulfate over more than half of the continental U.S. nitrate National Acid Deposition Program (http://nadp.sws.uiuc.edu/)

  38. Carbonaceous aerosol: Secondary and primary ORGANIC CARBON (OC) ELEMENTAL CARBON (EC) GLOBAL 22 Tg yr-1 130 Tg yr-1 UNITED STATES 0.66 Tg yr-1 2.7 Tg yr-1

  39. Secondary Organic Aerosol (SOA) SOA accounts for large fraction of submicron particulate mass Mixture of hundreds of compounds Aging of SOA is important (there’s a complicated time history) Take e.g. toluene, isoprene, .. + ozone participate in very many gas phase reactions, produce products with high and low vapor pressures (e.g oxalic, adipic acid). The low vapor pressure products will go into particles. Jimenez, Science, 326, Dec 2009 Odum, Env Sci Tech, 30, 2580, 1996 Ervens, JGR, 109, 2004 See Alex Gunther’s and Mary Barth’s lectures.

  40. Mexico City : organic species and nanoparticle growth Size (nm) TDCIMS Organics 82% Sulfate 10 % Nitrate 8% Smith, Geophy Res Lett, 35, 2008

  41. The one-slide story behind SOA formation VOC -> a1P1 + a2P2 In a model use: Caer = Y ΔVOC for a specific VOC Find a1 and a2 by least squares fit to lab data Aerosol mass yield(Y) as a function of: caer: the total organic aerosol mass concentration ci°:the saturation mass concentration of product i in the aerosol ai: molar yield of product i Mi: molecular weight of product i pi°: equilibrium vapor pressure of project i T: temperature “two-product model” works pretty well Y Y Caer Caer Odum et al., EST, 1996

  42. Modeling Organic Aerosol : What are the challenges? Secondary Organic Aerosol Partitioning Semi-Volatile Organic Vapors Nucleation Cloud Processing Deposition Oxidation by OH, O3, NO3 Surface / multiphase reactions Evaporation upon dilution VOCs Isoprene Monoterpenes Sesquiterpenes Aromatics Alkanes POA POA For MILARGO application see: Hodzic, ACP,2009 SOA is underestimated Direct Emission Forest Traffic Industries Biological Debris Biomass Burning Alma Hodzic

  43. Nanoparticle Formation • Nanoparticles form in the atmosphere by condensation to stable clusters formed by nucleation. They can also be emitted directly, e.g., by diesel engines. • So how are stable clusters formed in the atmosphere? • The formation of stable clusters from low vapor pressure atmospheric species is known as homogeneous nucleation. McMurry, Smith et al. in Aerosol Measurement Techniques, 2009.

  44. One important, and poorly understood, source of Cloud Condensation Nuclei is new particle formation BEACHON Manitou Forest Observatory New Particle Formation event on Dec 10, 2008 • Model estimates suggest that new particle formation can contribute up to 40% of the CCN at the boundary layer, and 90% in the remote troposphere (Pierce and Adams, ACP, 2007). • New particle formation is estimated to add as much as a 8 times more particles to the remote southern ocean atmosphere than anthropogenic primary particles (Spracklen et al., ACP, 2006). diameter that can activate into a cloud droplet at 0.2% supersaturation Smith, unpublished

  45. Which compounds lead to particles? What are the barriers to particle formation? Saturation vapor pressure = P i, sat A high saturation vapor pressure of a substance means it is volatile – molecules easily escape from its liquid surface, and do not stick back easily on the surface. Saturation ratio Sr = Pi / Pi, sat Pi, partial pressure When Sr > 1 (supersaturation), particles will form. What controls Sr?

  46. Saturated Vapor Pressure H2SO4 1.3 x 10-8 atm Water 0.02 atm (20 C) Ethanol (liquor) 0.05 atm Channel #5 expensive

  47. The energetics of particle formation Nature follows the lowest energy path “Why pay more?” If you form a particle, you add mass. What is the cost of doing this? Gibbs Free Energy G dG= -S dT + Vdp +  dni S entropy, V volume T temperature Chemical Potential  Increase of energy of a mass when a single particle is added to the mass. Units: energy/particle G = G (droplet) – G ( pure vapor) Seinfeld and Pandis

  48. Critical cluster energy barrier subsaturated supersaturated R particle radius T temperature Sr saturation ratio ( Pi / Pi, sat ) V volume of liquid molecule  surface tension Seinfeld and Pandis

  49. Getting to the Critical Nucleus Particlesreadily grow if they are past the G* nucleation barrier But what is the chemical composition of the critical nucleus? Recent work points to how organic acids + sulfuric acid leads to particle creation faster than by just sulfuric acid alone Organic acid : e.g. contains carboxyl group –COOH, benzoic acid Organic acid concentrations > sulfuric acid concentrations Zhang, Science, vol 328, June 2010 Zhang, Science, vol 304, June 2004

  50. ok Cloud droplet formation is heterogeneous Observations: supersaturation in natural clouds rarely exceeds a few percent Consequence: cloud droplets in natural clouds do not form by homogeneous nucleation of pure water Instead they form on atmospheric aerosol particles (cloud condensation nuclei or CCN): heterogeneous nucleation The CCN must be large and wettable

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