From: Andy Soos, ENN
Published November 15, 2011 01:51 PM

Floods, Droughts, and Air Pollution

Certain types of air pollution can serve as nuclei which aid in the formation of rainfall. Therefore increases in air pollution and other particulate matter in the atmosphere can strongly affect cloud development in ways that reduce precipitation in dry regions or seasons, while increasing rain, snowfall and the intensity of severe storms in wet regions or seasons, says a new study in Nature Geoscience. The research provides the first clear evidence of how aerosols — soot, dust and other small particles in the atmosphere — can affect weather and climate; and the findings have important implications for the availability, management and use of water resources in regions across the United States and around the world.


"Using a 10-year dataset of extensive atmosphere measurements from the U.S. Southern Great Plains research facility in Oklahoma, we have uncovered, for the first time, the long-term, net impact of aerosols on cloud height and thickness, and the resultant changes in precipitation frequency and intensity," says Zhanqing Li, a professor of atmospheric and oceanic science at Maryland and lead author of the study.

The scientists obtained additional support for these findings with matching results obtained using a cloud-resolving computer model.

"These new findings of long-term impacts, which we made using regional ground measurements, also are consistent with the findings we obtained from an analysis of NASA's global satellite products in a separate study. Together, they attest to the needs of tackling both climate and environmental changes that matter so much to our daily life," says Maryland's Li, who is also affiliated with Beijing Normal University.

"We have known for a long time that aerosols impact both the heating and phase changes [condensing, freezing] of clouds and can either inhibit or intensify clouds and precipitation," says Russell Dickerson, a professor of atmospheric and oceanic science at Maryland. "What we have not been able to determine, until now, is the net effect. This study by Li and his colleagues shows that fine particulate matter, mostly from air pollution, impedes gentle rains while exacerbating severe storms. It adds urgency to the need to control sulfur, nitrogen, and hydrocarbon emissions."

An earlier study on Indian monsoons indicated that a reduction in rainfall over India in the past 50 years. This was published online by the journal Science. It is based on a study done by Massimo Bollasina of Princeton university in the U.S., along with Yi Ming and V.Ramaswamy of the Geological Fluid dynamics Laboratory at Princeton,used a state-of-the-art global climate model to simulate what was happening in the Indian ocean with and without human-generated aerosols.

Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. They include soot, dust and sulfate particles, and are what we commonly think of when we talk about air pollution. Aerosols come, for example, from the combustion of fossil fuels, industrial and agricultural processes, and the accidental or deliberate burning of fields and forests.

Aerosol particles also affect the Earth's surface temperature by either reflecting light back into space, thus reducing solar radiation at Earth's surface, or absorbing solar radiation, thus heating the atmosphere. This variable cooling and heating is, in part, how aerosols modify atmospheric stability that dictates atmospheric vertical motion and cloud formation. Aerosols also affect cloud microphysics because they serve as nuclei around which water droplets or ice particles form.

Both processes can affect cloud properties and rainfall. Different processes may work in harmony or offset each other, leading to a complex yet inconclusive interpretation of their long-term net effect.

Greenhouse gases and aerosol particles are two major agents dictating climate change. The mechanisms of climate warming impacts of increased greenhouse gases are clear (they prevent solar energy that has been absorbed by the earth's surface from being radiated as heat back into space), but the climate effects of increased aerosols are much less certain due to many competing effects outlined above.

Until now, studies of the long-term effects of aerosols on climate change have been largely lacking and inconclusive because their mechanisms are much more sophisticated, variable, and tangled with meteorology.

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