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

2. Evolution of aerosol size distribution
Brasseur and Jacob, chapter 5

3. AIR POLLUTION TODAY: Ozone and fine particulate matter (PM2
AIR POLLUTION TODAY: Ozone and fine particulate matter (PM2.5) are the major pollutants
US population exposed to air pollutants
in excess of national ambient air quality standards (NAAQS), 2015
PM2.5
PM10
SO2
Lead
Ozone CO
NO2
108M
31M 1M 5M
11M
70 ppb (8-h average)
12 µg m-3 (annual), 35 µg m-3 (24-h)
Million environmental deaths per year
worldwide
2010 data
US EPA [2017], OECD [2012]

4. PM2.5 and PM10
by volume
PM10
PM2.5
This actually dN/d(log10(Diam)) .
PM2.5 = concentration of particulate matter less than 2.5 µm diameter
PM10 = concentration of particulate matter less than 10 µm diameter

5. Annual mean PM2.5 observed from satellite
US air quality standard

6. Why PM2.5? Airflow interception obstacle diffusion impaction
Wiley Interdisciplinary Reviews

7. Life Expectancy vs annual PM2.5 1978-82
EPA NAAQS
Boston
Topeka
Steubenville, OH
Pope, Ezzati, Dockery. NEJM 2009; 360:376

8. Life Expectancy vs annual PM2.5 1997-2001
EPA NAAQS
Boston
Steubenville
Topeka
Pope, Ezzati, Dockery. NEJM 2009; 360:376

9. Life Expectancy vs annual PM2.5 1980-2000
Boston
Topeka
Steubenville
Pope, Ezzati, Dockery. NEJM 2009; 360:376

10. FINE AEROSOL COMPOSITION IN NORTH AMERICA
Annual mean PM2.5 concentrations (2013)
Current air quality standard is 12 mg m-3
Eloise Marais, Harvard

11. PM2.5 composition
Wang et al. [2014]
Two components dominate PM2.5 mass under almost all conditions:
Sulfate-nitrate-ammonium (SNA)
Organic carbon (OC)

12. Historical radiative forcing (1750-2011) based on emissions
IPCC [2014]

13. Aerosols are major contributors to climate forcing and its uncertainty
IPCC (2014)

14. 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

15. 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

16. 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.

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

18. 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]

19. Measurement of aerosol optical depths (AODs) 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

20. Global aerosol optical depth from MODIS