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

Evolution of aerosol size distribution Brasseur and Jacob, chapter 5

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]

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

Annual mean PM2.5 observed from satellite US air quality standard http://www.nasa.gov/topics/earth

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

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

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

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

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

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

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

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

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 0.1-1 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

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

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.

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

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

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

Global aerosol optical depth from MODIS http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_AER_OD