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
Pollution off U.S. east coast
Dust off West Africa

2. ORIGIN OF THE ATMOSPHERIC AEROSOL
Size range: 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
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.)
PM2.5
Ozone

5. SCATTERING OF RADIATION BY AEROSOLS
By scattering solar radiation, aerosols decrease visibility and increase the Earth’s albedo
Scattering efficiency is maximum when particle radius = l
particles in mm
size range are efficient scatterers of solar radiation
2 (diffraction limit)
green light
(λ = 0.5 µm)
Highest when particle r = wavelength (pi*d)—surface wave, diffraction. Rayleigh=inefficient

6. HOW TO OBSERVE AEROSOLS FROM SPACE?
Solar occultation
(SAGE, POAM…)
Active system
(CALIOP…)
Solar back-scatter
(MODIS, MISR…)
laser
pulse
EARTH
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. VISIBILITY IN U.S. WILDERNESS AREAS
Statistics for 20% worst visibility days
Deciviews
2001 observations
Natural
Background; includes
transboundary pollution
Visual
range (km)
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:
insert clapeyron eq
At lower RH, the particle is dry.

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