AT737

Aerosols

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Importance of Aerosols

Aerosols directly change the radiation budgetAerosols modify cloud properties (and clouds modify aerosol properties)Aerosols affect atmospheric chemistryAerosols affect biological systems, including human health

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Characteristics of Aerosols

• Sizes range from 0.01 to 10 or more micrometers• Chemical composition:

NOTE: VERY CHEMICALLY HETEROGENEOUS”PHYSICS OF DIRT”

o      Sulfate (SO4) o      Nitrateo      Soil and mineral dust (silicates, clays)o      Carbonaceous compounds (elemental and organic carbon)o      Sea-salt (NaCl)

SOURCE (of this and next four slides): Dr. Bill Collins, NCAR

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Sources

Sulfate: Oxidation of sulfur dioxide from fossil fuel burning Oxidation of DMS released from ocean micro-organisms Volcanic emissions Manmade now 4 times natural (IPCC time series)

Nitrate: Combustion

Soil and mineral dust: Dry lake beds from prehistoric lakes Desertification

Carbonaceous compounds Organic compounds released from forests Natural biomass burning Anthropogenic biomass burning Fossil fuel combustion

Sea-salt: oceans, of course

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Sinks

Lifetime in the atmosphere: about 5-7 days. Stratosphere: x100

Removal mechanisms: Sedimentation (settling) – about 10-20% by mass

V = 50 cm/s for large silicates (50 microns) V =0.03 cm/s for small particles (1 micron)

“Scavenging” by precipitation – about 80-90% by mass

Consequence: aerosol is very uniformly mixed

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Properties of Atmospheric Aerosols

Type Altitude (km)

Horiz scale (km)

Freq. Compo-sition

Mass loading (mg m−3)

Optical depth

Mean particle

Size

Trop. Aerosols 0-10 1000-10000

1 Sulfate, nitrate,minerals

0.01 to 1 ~0.1 0.1-1

Dust storms 0-3 10-1000 Sporadic Silicates, clays

<1 to 100 1-10 1-10

Volcanic 5-35 1000-10000

Sporadic Mineral ash, sulfates

<1 to 1000 0.1 to 10 0.1 to 10

Smoke 0-10 1-100 Sporadic Soot, ash,tars

0.1 t 1 0.1 to 10 0.1 to 1

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Types of Aerosol

Continental: sulfate, nitrate, carbonaceous, mineralMarine: sea saltStratospheric: e.g., sulfate formed from volcanic eruptions

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Aerosol Variablility

Size spectrumCompositionNumber density

Makes remote sensing difficult, perhaps more difficult than sensing clouds

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Stratospheric Aerosols

Stratospheric aerosols are few in numberThe long path length of limb sounding is useful

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Solar Occultation

Measures transmittance of solar radiation as a function of tangent heightSelf calibrating

dxhh e ln

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Abel’s Equation

dhrzrhdh

dz ee

ze

21

22ln1,

Very peaky weighing functions

0

10

20

30

40

50

60

70

0 0.01 0.02 0.03

Weight (1/km)

Alti

tud

e (

km)

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Corrections

Need to correct for Rayleigh scattering and atmospheric gas absorption

gasesaerosolRayleighe

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Results

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Tropospheric Aerosols

If you can see them you can measure them, right?

MODIS truecolor 12/17/04 05:20 UTC

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Another Example

MODIS truecolor 3/10/07

Note land/ocean differences!

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Processes

Aerosols scatter solar radiation toward satellite

sat

aerosolsunsunaerosol

aerosolRayleighmeasured

pEL

L

LLLL

4

~surface)ocean (dark 0 Assume

0

0

00

Depends on aerosol properties as well as optical depth

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NOAA POES AOD

SOURCE: http://www.osdpd.noaa.gov/PSB/EPS/Aerosol/Aerosol.html

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NOAA GOES AOD

SOURCE: http://www.orbit.nesdis.noaa.gov/smcd/emb/GASP/RealTime.html

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Good Paper

King et al., 1999: Remote sensing of tropospheric aerosols from space: Past, present, and future. BAMS, 80, 2229-2259.

“Reflectance Function”

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Aerosols Over Land

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Color

Color Ratio

Angstrom exponent

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Forecasts of Aerosols

http://www.nrlmry.navy.mil/aerosol_web/loop_html/globaer_world_loop.html

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National Park Service

http://www.cira.colostate.edu/nps.html

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CALIPSO

Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)

532 nm Backscatter

Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation

http://www-calipso.larc.nasa.gov/