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

Typical aerosol size distribution

number

area

volume

Aerosols are the visible part of the atmosphere:Pollution off U.S. east coast Dust off West AfricaCalifornia fire plumes

ORIGIN OF THE ATMOSPHERIC AEROSOL

Soil dustSea salt

Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)

WHY CARE ABOUT ATMOSPHERIC AEROSOLS?

Public health

Visibility

Ocean fertilization

Chemistry

Climate forcing

Cloud formation

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/

OzonePM2.5

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 0.1-1 mmsize range are efficient scatterers of solar radiation

2 (diffraction limit)

green light(λ = 0.5 µm)

HOW TO OBSERVE AEROSOLS FROM SPACE?

EARTH

Solar occultation (SAGE, POAM…)

Pros: high S/N, vertical profilingCons: sparse sampling, cloud interference, low horizontal resolution

Active system(CALIOP…)

Pro: vertical profilingCon: sparse sampling,low S/N

Pro: horiz. resolutionCon: daytime only, no vertical resolution

Solar back-scatter(MODIS, MISR…)

laserpulse

Surface Surface

Aerosol observation from space by solar backscatter

Pollution off U.S. east coast Dust off West AfricaCalifornia fire plumesRelatively easy to do qualitatively for thick plumes over dark ocean…

Il ()

Il (0)=Il ()exp[-AOD]

…but difficult quantitatively! Fundamental quantity is aerosol optical depth (AOD)

aerosol scattering, absorption

Measured top-of-atmosphere reflectance = f (AOD, aerosol properties, surface reflectance, air scattering, gas absorption, Sun-satellite geometry)

MODIS AEROSOL RETRIEVAL OVER LAND

SURFACE

0.47 mm0.65 mm2.13 mm

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

TOAreflectance

Aerosol optical depths (AODs) measured from space

Jan 2001 – Oct 2002 operational data

MODIS (c004)return time 2x/day;nadir viewknown positive biasover land

MISR9-day return time;multi-angle viewbetter but much sparser

van Donkelaar et al. [2006]

550 nmAODs

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

VISIBILITY IN U.S. WILDERNESS AREASStatistics for 20% worst visibility days

Deciviews

2001 observations Natural Background; includestransboundary pollution

300 150 80 40 20 Visual range (km)

Park et al. [2006]

EPA Regional Haze Rule requires that natural visibility be achieved in all US wilderness areas by 2064

RAOULT’S LAW

water saturation vapor pressureover pure liquid water surface

water saturation vapor pressureover aqueous solution of watermixing ratio xH2O

2 ,oH O SATP 2 , 2 2 ,

oH O SAT H O H O SATP x P=

An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio 2 ,

22 , 100

H O SATH O o

H O SAT

P RHxP

= =

solutemoleculesin green

HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY SOLUBILITY EQUILIBRIA

Consider an aqueous sea salt (NaCl) particle: it must satisfy

2

(solubility equilibrium) (electroneutrality)

1 (closure)

Na Cl s

Na Cl

Na Cl H O

x x Kx xx x x

= =

This requires:12100(1 2 ) "deliquescence RH"sRH K

At lower RH, the particle is dry.

UPTAKE OF WATER BY AEROSOLS: HAZE

Deliquescence RH;depends on particlecomposition

NaCl/H2O