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 metals, nutrients, organics acidification (sulfur, nitrogen)

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/m 3 (air density ~1kg/m 3 )

Global Particle Production (Table 2.19 from Seinfeld and Pandis)

Aerosol Size Distributions Number distribution  n n (D p )=dN/dD p Surface area distribution  n s (D p )= dS/dD p S=  D p 2 Volume distribution  n v (D p )=dV/dD p V=(  /6)*D p 3

Log-normal distributions Aitken mode Accumulation mode Coarse mode Number distribution  n n (log D p )=dN/d log D p Surface area distribution  n s (log D p )= dS/d log D p Volume distribution  n v (log D p )=dV/d log D p

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

A process oriented view of aerosol size distribution

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

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

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

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

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

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

Aqueous Aerosol Thermodynamic partitioning (A g  A aq ) –liquid water content (L=g of H 2 O/m 3 of air) L= in clouds L= in fogs –Henry’s law constant (H) H A =[A] (M)/  A (atm)

H O2 =1.3x10 -3 M/atm H O3 =1.1x10 -2 M/atm H NH3 =62 M/atm H H2O2 =7x10 4 M/atm H H2CO =2.5 M/atm Exercise: Calculate the concentration of ozone in pure water in equilibrium with 10 ppbv ozone, assume ideal gas. A few Henry’s law constants…

Formaldehyde…

Acids…

Because K eq2 /H + >>1 nearly all nitric acid will exist as nitrate.

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)

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

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

Seasalt aerosols... seasalt production via bubble bursting... film drops (many, small, organics) jet drops (fewer, larger) wind  bubbles  spray whitecap coverage W α U 3+

The sulfur story (in brief)... emissions: fossil fuel SO 2, volcanic SO 2, oceanic DMS DMS oxidation... gas phase... complex! (mod. from Yin et al., 1990)

SO 2 oxidation in the gas phase is simple... but most SO 2 oxidation occurs in the aqueous phase...

heterogeneous oxidation of SO 2 in-cloud oxidation –weakly buffered, pH ~4 –oxidation by H 2 O 2 seasalt aerosols –strongly buffered by carbonate system –rapid oxidation by O 3 –slower oxidation by H 2 O 2 (also OH, halogen radicals...) –growth of existing particles, inhibits nucleation of new particles (Chameides and Stelson, 1992)