Aerosols have both direct and indirect effects on the atmosphere. The cooling or warming of the atmosphere due to the reflective or absorbent properties of the particles are considered direct effects. Reflective particles cool the atmosphere by scattering energy from the sun back into space. Absorbent particles have the opposite effect. They add to atmospheric heating by absorbing the sun’s energy. An indirect effect of aerosols is their roles as cloud condensation nuclei (CCN). Clouds with more CCN are larger and more reflective than those with fewer CCN, so they also add to cooling in the atmosphere.

Aerosol size and travel distance from origin are closely related properties that act as controls on the net effect of atmospheric cooling and heating. For example, volcanic eruptions release a wide size range of particles. The larger, silica rich particles absorb the sun’s energy, but generally remain in the atmosphere for a short period of time. Due to their short residence time the particles travel a limited distance. Smaller sulfate aerosols are also released in volcanic eruptions and these particles may remain in the atmosphere for years. Sulfate aerosols are very reflective and thought to be a significant contributor to cooling in the atmosphere. After the eruption of Mt. Pinatubo in 1991, a large amount of sulfate aerosols were released into the stratosphere (upper atmosphere). The stratospheric winds distributed the particles across a large portion of the globe. The aerosol’s presence is believed to have caused a 0.5 degree Celsius mean reduction in global temperature.

Just as the production of natural aerosols has cooling effects, there are also some warming effects. The particles pushed into the atmosphere from a dust storm are made of minerals with both reflective and absorbent properties. The ability of the particles to absorb sunlight is thought to have a net warming effect on area of the atmosphere they occupy. The force of the dust storm can bring the particles into the mid-troposphere (lower atmosphere) where they can be carried hundreds to thousands of miles from their source. Soil particles released into the atmosphere from farm fields will have a similar effect to that of a dust storm. Soil originates from rocks and minerals and will also have both reflective and absorbent properties.

A large contributor to human produced aerosols is the burning of fossil fuels (oil and coal). This combustion is another source of sulfate aerosols, but, because they are not emitted with the force of a volcanic eruption, they generally remain in the troposphere. While suspended in the atmosphere, many of these particles act as CCN and increase cloud cover. The aerosols usually remain in the atmosphere less than a week before they are washed out by rain. The cooling effect of these aerosols is located near the point of origin or a short distance down wind. Because so many industrial facilities in North America use fossil fuels there is a high concentration of sulfate aerosols in this area.

URLs: Effects of Aerosols

http://www.aip.org/history/climate/20ctrend.htm#L_0338

http://www.aip.org/history/climate/pdf/aerosol.pdf

http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/asian_dust.html

http://www.che.caltech.edu/faculty/jhs/research.html

http://www.unfccc.int/resource/iuckit/fact03.html

Impact of Aerosols
http://climate.gsfc.nasa.gov/research/aerosols.php

Impact of Aerosols on Clouds and Climate
http://earth.rice.edu/mtpe/atmo/atmosphere/topics/aerosols/aero_clouds.html

Clouds Formed from Aerosols and Reflecting Sunlight
http://www-sage3.larc.nasa.gov/solar/text/text-learning-aerosol.html

Atmospheric Aerosols Found to Brighten Clouds
http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr022802.htm

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