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Aerosols By Elizabeth Dahl (2005) Edited by Ted Dibble (2008)

Aerosols By Elizabeth Dahl (2005) Edited by Ted Dibble (2008). What’s driving aerosol research?. transport of bomb radionuclides geochemical cycles metals, nutrients, organics acidification (sulfur, nitrogen) climate change direct/indirect effects

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Aerosols By Elizabeth Dahl (2005) Edited by Ted Dibble (2008)

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  1. AerosolsBy Elizabeth Dahl (2005)Edited by Ted Dibble (2008)

  2. What’s driving aerosol research? • transport of bomb radionuclides • geochemical cycles • metals, nutrients, organics • acidification (sulfur, nitrogen) • climate change • direct/indirect effects • aerosol optical properties, aerosol/cloud interactions • human health • air quality, airborne pathogen transport

  3. Terminology • Aerosol – a dispersion of solid and liquid particles suspended in gas (air). note: “aerosol” is defined as the dispersion of both particles and gas, but in common practice it is used to refer to the particles only! • Primary aerosol – atmospheric particles that are emitted or injected directly into the atmosphere. • Secondary aerosol – atmospheric particles that are created by in situ aggregation or nucleation from gas phase molecules (gas to particle conversion). Either type may be natural or anthropogenic or both How much aerosol is there? typically ~10’s of ug/m3 (air density ~1kg/m3)

  4. Global Particle Production(Table 2.19 from Seinfeld and Pandis)

  5. Log-normal distributions Aitken mode Number distribution  nn(log Dp)=dN/d log Dp Surface area distribution  ns(log Dp)= dS/d log Dp Volume distribution  nv(log Dp)=dV/d log Dp Accumulation mode Coarse mode

  6. The Aerosol Modes • Aitken mode – 0.01-0.1 m • Accumulation mode – 0.1-1 m • Coarse mode - >1 m and sometimes, the elusive • nucleation mode <0.01 um

  7. Removal mechanisms... gravitational settling coarse particles • 10 m particle  1000 cm hr-1 • 1 m particle  10 cm hr-1

  8. Diffusion/Coagulation You can estimate the distance a particle will diffuse in a given time from the equation: fine particles where D is the diffusion coefficient

  9. impaction, settling diffusion, coagulation Why is there an “accumulation” mode?

  10. So lifetimes are …. • Aitken nuclei – hours to days (diffusion/coagulation) • Accumulation mode – weeks • Coarse mode – hours to days (deposition) • Ultrafine – minutes to hours

  11. Secondary organic aerosol (SOA)formation • SOA is produced in gas phase from oxidation of parent organic gases. • Partitioning to aerosol phase depends on vapor pressure • High equilibrium vapor pressure  tendency to stay in gas phase • Low equilibrium vapor pressure  partitions to aerosol phase – non-volatiles • Production of the organic aerosol compound depends on the gas phase chemistry of it’s parent (on aerosol phase chemistry too!) • Large organics (C> 6) tend form aerosols while organics C<6 do not.

  12. Aerosol chemistry…

  13. Aqueous Aerosol • Partitioning between gas and aerosol phases (AgAaq) depends on • liquid water content (L=g of H2O/m3 of air) • L=0.1-0.3 in clouds • L=0.02-0.5 in fogs • Henry’s law constant (H) • HA=[A] (M)/PA (atm)

  14. A few Henry’s law constants… • HO2=1.3x10-3 M/atm • HO3=1.1x10-2 M/atm • HNH3=62 M/atm • HH2O2=7x104 M/atm • HH2CO=2.5 M/atm • Calculate the concentration of ozone in 1 ml of pure water for 10 ppbv ozone, assume ideal gas.

  15. Formaldehyde in aqueous aerosol In water, H2CO mostly adds H2O to form H2C(OH)2

  16. The chemical perspective ... a chemical size distribution 1. chemical size distributions resemble mass, not number 2. sulfate and organics dominate the accumulation mode, but there’s a surprising amount of seasalt 3. there are a lot of unidentified organics 4. the coarse mode has the expected mechanically generated aerosols, but also nitrate and sometimes sulfate Mass (C. Leck)

  17. Mineral Dust • Dust (mineral aerosols) • diameter size: 2-300 µm • main material: sand, silt, clay • includes essential trace metals such as Fe • consists of insoluble and soluble fractions

  18. Organic aerosols - burning soot – “elemental carbon” formed in flames little spectral dependence carbon-only “brown carbon”: sugars alcohols aromatics di/tri acids ketoacids hydroxyacids

  19. Seasalt aerosols... wind bubbles spray whitecap coverage W α U3+ seasalt production via bubble bursting... • film drops (many, small, organics) • jet drops (fewer, larger)

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