Saturday, November 26, 2011

Dirty air fosters precipitation extremes

Even clouds can suffer from inhaling air pollution, a new study finds, resulting in extreme rainfall patterns that appear to be altering climate across the globe.

Farmers, municipal water authorities and others who depend on rainfall prefer moderate, dependable precipitation. But as soot and other minute airborne particles — a class of pollutants known as aerosols — get sucked into clouds, the pollution can dramatically alter when clouds deposit rain. The discovery emerged from analyzing every one of thousands of clouds passing over federal monitoring instruments at a site in the western United States over a 10-year period, explains Zhanqing Li of the University of Maryland in College Park.

“Haze, storms, drought and flood: We found very strong evidence that they are well connected,” he said in Washington, D.C., on November 10 at the Symposium on Stratospheric Ozone and Climate Change. He and colleagues published the findings online November 13 in Nature Geoscience.

“This is the first study to clearly establish the link between aerosols, precipitation and climate,” says Renyi Zhang of Texas A&M University in College Station.

The effects Li and his colleagues saw depended not only on pollution concentrations but also on moisture levels and cloud types. In relatively low-lying clouds where the moisture is liquid in the form of tiny droplets, increasing aerosol concentration tended to suppress precipitation, especially when relative humidity was low.

In clouds, water molecules latch onto aerosols and continue to grow until they collect enough moisture and form rain. But Li and his colleagues found that if there were too many aerosols in a cloud, not much water attached to any individual aerosol. That left water droplets that were too small to fall as rain. more