ATMOSPHERIC AEROSOLS: ensembles of condensed-phase particles suspended in air

1. ATMOSPHERIC AEROSOLS:ensembles of condensed-phase particles suspended in air number Typical aerosol size distribution area volume Aerosols are the visible part of the atmosphere: California fire plumes Dust off West Africa Pollution off U.S. east coast

2. ORIGIN OF THE ATMOSPHERIC AEROSOL Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop) Soil dust Sea salt

3. WHY CARE ABOUT ATMOSPHERIC AEROSOLS? Public health Chemistry Visibility Cloud formation Ocean fertilization Climate forcing

4. AIR POLLUTION IN THE US :Ozone and fine particulate matter (PM2.5) are the two main pollutants 75 ppb (8-h average) 15 mg m-3 (1-y av.) http://epa.gov/airtrends/2010/ PM2.5 Ozone

5. By scattering solar radiation, aerosols decrease visibility and increase the Earth’s albedo SCATTERING OF RADIATION BY AEROSOLS • Scattering efficiency is maximum when particle radius = l • particles in 0.1-1 mm size range are efficient scatterers of solar radiation 2 (diffraction limit) green light (λ = 0.5 µm)

6. EARTH HOW TO OBSERVE AEROSOLS FROM SPACE? Solar occultation (SAGE, POAM…) Active system (CALIOP…) Solar back-scatter (MODIS, MISR…) laser pulse Surface Surface Pros: high S/N, vertical profiling Cons: sparse sampling, cloud interference, low horizontal resolution Pro: vertical profiling Con: sparse sampling, low S/N Pro: horiz. resolution Con: daytime only, no vertical resolution

7. Aerosol observation from space by solar backscatter Relatively easy to do qualitatively for thick plumes over dark ocean… California fire plumes Pollution off U.S. east coast Dust off West Africa …but difficult quantitatively! Fundamental quantity is aerosol optical depth (AOD) Il () Measured top-of-atmosphere reflectance = f (AOD, aerosol properties, surface reflectance, air scattering, gas absorption, Sun-satellite geometry) aerosol scattering, absorption Il (0)=Il()exp[-AOD]

8. MODIS AEROSOL RETRIEVAL OVER LAND 1. Use top-of-atmosphere (TOA) reflectance at 2.13 mm (transparent atmosphere) to derive surface reflectance 2. Assume fixed 0.47/2.13 and 0.65/2.13 surface reflectance ratios to derive atmospheric reflectances at 0.47 and 0.65 mm by subtraction 3. Assume generic aerosol optical properties to convert atmospheric reflectance to AOD TOA reflectance 0.47 mm 0.65 mm 2.13 mm SURFACE

9. Aerosol optical depths (AODs) measured from space Jan 2001 – Oct 2002 operational data MODIS (c004) return time 2x/day; nadir view known positive bias over land 550 nm AODs MISR 9-day return time; multi-angle view better but much sparser van Donkelaar et al. [2006]

10. ANNUAL MEAN PM2.5 CONCENTRATIONS (2002)derived from MODIS satellite instrument data

11. Statistics for 20% worst visibility days Background; includes transboundary pollution 2001 observations Natural Deciviews 300 150 80 40 20 Visual range (km) VISIBILITY IN U.S. WILDERNESS AREAS EPA Regional Haze Rule requires that natural visibility be achieved in all US wilderness areas by 2064 Park et al. [2006]

12. RAOULT’S LAW solute molecules in green water saturation vapor pressure over pure liquid water surface water saturation vapor pressure over aqueous solution of water mixing ratio xH2O An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio

13. HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY SOLUBILITY EQUILIBRIA Consider an aqueous sea salt (NaCl) particle: it must satisfy This requires: At lower RH, the particle is dry.

14. UPTAKE OF WATER BY AEROSOLS: HAZE NaCl/H2O Deliquescence RH; depends on particle composition