Aerosols are important in Earth’s radiation budget in two ways. First, they react with incoming solar radiation. They scatter solar radiation, reflecting some back into space. A few aerosols, like black soot and some types of dust, absorb some incoming solar radiation, reradiating it in all directions. Despite this, the net effect of aerosols is to reduce the amount of solar radiation reaching Earth’s surface. A second way aerosols affect climate is their interaction with clouds.

Clouds with low aerosol concentration (left) and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface. Clouds with high aersol concentration (right) reflect up to 90% of light back to space.
courtesy of NASA Earth Observatory

Aerosols can affect the reflectivity and longevity of clouds. Although aerosols are required for cloud formation as they act as cloud condensation nuclei (CCN), too many aerosols can result in the available liquid water in a cloud to be spread over too many droplets. Since a cloud’s reflectivity is dependent upon not only its liquid water content, but also the number of water droplets in the cloud, the reflectivity of the cloud would be increased. An example can be seen not only in the cloud formation, but the cloud persistence that occurs as a result of aerosols in ship exhaust. (See http://geo.arc.nasa.gov/sge/jskiles/fliers/all_flier_prose/aerosols_ackerman/aerosols_ackerman.html.)

Also, in some situations, small CCN resulting from biomass burning can result in a delay of precipitation onset because cloud droplets are prevented from growing to a large enough size.

Anyone who has ever traveled to Hawaii or the Caribbean has probably noticed that often a very shallow cloud would move overhead and subsequently large drops of rain would fall. In mid-latitudes, especially over the continents, this would be very rare. (In mid-latitudes clouds usually must reach well above the freezing level, so that the ice crystal process can initiate growth of precipitation sized particles.) Over the tropics, large sea salt nuclei that range in size from 1µm to 10µm, result in clouds having fewer droplets of much larger size than found over the mid-latitude continents. These clouds also contain a broader range of drop size distribution. Since droplets fall at different speeds, this allows the larger droplets to fall, collide with smaller droplets, and grow bigger. This process is called collision and coalescence. When many small CCN are present, it is hard for this process to work as an initiator for rainfall. It is feared that large-scale biomass burning in tropical areas may delay the onset of precipitation and increase the reflectivity and longevity of clouds. (Detailed discussion is here:
http://dionysos.mpch-mainz.mpg.de/smocc/fSMOCC-PartB.pdf)

URLs: Aerosols and Climate

From the Earth Observatory, a page dealing with aerosols and their impact on climate. Great links.
http://earthobservatory.nasa.gov/Library/Aerosols/

Presentation by Dr. Stephen Schwartz, Brookhaven National Laboratory before the Committee on Energy and Natural Resources.
http://www.ecd.bnl.gov/steve/senate.html

NOAA site. Contains good graphics on large eddy simulations of aerosol indirect effect on climate and aerosol indirect effect on clouds: climate implications.
http://www.etl.noaa.gov/et2/aca/aie/

From the University of new South Wales: Atmospheric aerosols and global climate.
http://www.phys.unsw.edu.au/RESEARCH/ATMOSPHERIC/atmospheric_research.html

An article by Barry Huebert, University of Hawaii offering us a perspective about why becoming involved in research on aerosols and climate is so important to us
http://www.igac.noaa.gov/newsletter/17/intro.php

Research suggests aerosol pollutants in atmosphere likely masking green house warming.
http://www.colorado.edu/news/releases/2000/104.html

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