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VIII. Aerosols Size distribution Formation and Processing Composition Aerosol phase chemistry

VIII. Aerosols Size distribution Formation and Processing Composition Aerosol phase chemistry. Importance of aerosols. human health air quality, airborne pathogen transport climate change direct/indirect effects aerosol optical properties, aerosol/cloud interactions geochemical cycles

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VIII. Aerosols Size distribution Formation and Processing Composition Aerosol phase chemistry

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  1. VIII. Aerosols Size distributionFormation and ProcessingCompositionAerosol phase chemistry

  2. Importance of aerosols • human health air quality, airborne pathogen transport • climate change direct/indirect effects aerosol optical properties, aerosol/cloud interactions • geochemical cycles metals, nutrients, organics • acidification (sulfur, nitrogen)

  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. Aerosol Size Distributions Number distribution  nn(Dp)=dN/dDp Surface area distribution  ns(Dp)= dS/dDp S=Dp2 Volume distribution  nv(Dp)=dV/dDp V=(/6)*Dp3

  6. 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

  7. 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

  8. A process oriented view of aerosol size distribution

  9. deliquescence efflorescence Humidity and aerosol size... • hygroscopic aerosols grow/shrink with RH (with hysteresis!) • aerosol size strongly affects light scattering cross-section

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

  11. 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

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

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

  14. Secondary organic aerosol formation • VOC oxidized to less-volatile OC • Partitioning to aerosol phase depends on vapor pressure • High equilibrium vapor pressure  high tendency to stay in gas phase • Low equilibrium vapor pressure  partitions to aerosol phase – non-volatiles • Large organics (C> 6) tend form aerosols while organics C<6 do not. • Oligomerization on/in acid aerosol

  15. Aqueous Aerosol • Thermodynamic partitioning (AgAaq) • 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)/A (atm)

  16. 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 Exercise: Calculate the concentration of ozone in pure water in equilibrium with 10 ppbv ozone, assume ideal gas.

  17. Formaldehyde…

  18. Acids…

  19. Because Keq2/H+>>1 nearly all nitric acid will exist as nitrate.

  20. 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)

  21. 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

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

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

  24. The sulfur story (in brief) ... • emissions: fossil fuel SO2, volcanic SO2, oceanic DMS • DMS oxidation ... gas phase ... complex! (mod. from Yin et al., 1990)

  25. SO2 oxidation in the gas phase is simple... but most SO2 oxidation occurs in the aqueous phase...

  26. heterogeneous oxidation of SO2 • in-cloud oxidation • weakly buffered, pH ~4 • oxidation by H2O2 • seasalt aerosols • strongly buffered by carbonate system • rapid oxidation by O3 • slower oxidation by H2O2 (also OH, halogen radicals...) • growth of existing particles, inhibits nucleation of new particles (Chameides and Stelson, 1992)

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