The Atmosphere

Water Vapor

Water vapor is an extremely important gas found in the atmosphere. It can vary from 4% in the steamy tropics to nearly nonexistent in the cold dry regions of the Antarctic. Water vapor is a good absorber of Earth's outgoing radiation and thus is considered a greenhouse gas. When water vapor is converted to a liquid during condensation, clouds are formed. Clouds are good absorbers of radiation given off by the Earth's surface. The absorption of this energy raises the temperature of the air. But clouds are generally light-colored and hence reflect incoming solar radiation off their tops. The reflected light is sent back to space, never reaching the ground to warm the Earth. Thus clouds can have either a warming or a cooling effect on air temperature. It has been thought that these effects balance one another out but National Public Radio's All Things Considered report suggests that this might not be true, forcing climatologists to rethink the issue. 

Particulates and Aerosols

Atmospheric particulates and aerosols are very small particles of solid or liquid suspended in the air. Particulates and aerosols play several important roles in atmospheric processes. Particulate matter includes dust, dirt, soot, smoke, and tiny particles of pollutants. Major natural sources of particulates are volcanoes, fires, wind-blown soil and sand, sea salt, and pollen. Human sources such as factories, power plants, trash incinerators, motor vehicles, and construction activity also contribute particulates to the atmosphere.

Figure 3.6 Sources of particulates and aerosols. Courtesy NASA Earth Observatory (Source)

Particulates are very effective at altering the energy and moisture balances of the Earth system. Particulates diffuse sunlight reducing the amount and intensity of solar radiation reaching the Earth's surface. The most spectacular sunrises and sunsets are a result of light being refracted from particulates in the atmosphere. Particulates will also reflect sunlight back out to space, never letting it reach the surface. Decreasing significant amounts of incoming solar radiation can cause global temperatures to decrease. The eruption of Mt. Pinatubo in 1991 caused a .5^o C decrease in global temperatures. However, particulates can absorb longwave radiation emitted by the Earth, causing the atmosphere to warm as well. Particulates serve as condensation nuclei for water. In order for water to change from a gas to a liquid, a nucleus upon which water vapor can attach itself is nearly always required. Without particulates, little water would condense to form clouds and precipitation. 

Figure 3.7 Sunset over the Atlantic 
(Source: NOAA)

NASA scientists using satellite data and computer models found black soot from incomplete combustion may be contributing to changes in sea ice, snow and atmospheric temperatures near the North Pole. They found the timing and location of rising temperatures and loss of sea ice during the end of the 20th century is consistent with a significant rise in human produced aerosols. Their models suggest that one third of the soot comes from South Asia, one third from biomass burning, and the rest from Russia, Europe, and North America. Soot deposited on snow and sea ice decreases the surface reflectivity causing more sunshine to be absorbed. Airborne soot warms the Arctic atmosphere and affects weather patterns and clouds.

Video: Climate Change and Aerosols

Dr. Jim Haywood, Aerosol Research scientist at the Met Office talks about climate change and aerosols.

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