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Air pollution chokes rain, may temper greenhouse warming
Urban and industrial air pollution stifles precipitation and may nullify greenhouse warming, according to a report by an atmospheric scientist in today's issue of the journal Science.
"The impact of pollution on these clouds is sufficiently strong to completely shut off precipitation from them," said Daniel Rosenfeld, a scientist at Hebrew University of Jerusalem in Israel.
In making his case, Rosenfeld presents satellite images and measurements of "pollution tracks" that have cut off precipitation downstream from major air pollution sources such as power plants, lead smelters and oil refineries.
The tracks have shut off precipitation because they contain tiny water droplets that are unable to coalesce into raindrops large enough to fall to the ground before they evaporate.
To form rain, each cloud droplet must form on a pre-existing particle, and approximately 1 million small droplets must coalesce to form an ample raindrop.
Pollutants such as sulfates and sulfuric acid increase the number of water droplets in clouds. These small droplets float in the air with a low probability of bumping into each other and merging into raindrops, Rosenfeld reports.
Aerosol pollutants also reflect sunlight back to space, rendering a cooling effect on Earth that may cancel out the warming effect of greenhouse gases, said Owen Toon, an atmospheric scientist at the University of Colorado at Boulder, in an accompanying article.
The satellite imagery is the first direct evidence of the effect of urban and industrial pollution on rainfall levels, a source of scientific debate for several decades.
"Now I did the very first step, taking the clearest possible case, to tell us all: 'Hey, it is happening without any doubt. Now let's go in earnest to investigate that,' " said Rosenfeld.
Previous studies suggest pollution increases rainfall. This is true of very large aerosol pollutants such as those emitted by paper mills, which induce the formation of very large water droplets that collect smaller droplets as they fall.
"Unfortunately, my satellite measurements reveal that almost all pollution sources produce many small aerosols that work to reduce the size of the cloud droplets, thus the precipitation," Rosenfeld said.
The scientist based his study on data collected by Advanced Very High Resolution Radiometers onboard NASA's Tropical Rainfall Measuring Mission satellite. The spacecraft was launched in 1997 to study rainfall between 35 degrees north latitude and 35 degrees south latitude.
Images were recorded over regions of Turkey, Canada and Australia that contain known sources of industrial or urban air pollution. The tracks stream away from these pollution sources in long, narrow plumes.
Rosenfeld notes that the most prominent pollution tracks are seen in Australia, the least polluted of continents that are inhabited. This is probably because the pollution is seen against a backdrop of pristine clouds, he said.
Other areas of the world such as the northeastern United States are at least as tainted with aerosols, but the pollution tracks are nearly invisible because of the perpetual pollution that hangs in the atmosphere.
Rosenfeld's study suggests that human activity may be affecting rainfall patterns on a global scale.
"I would not go to the extent of global desertification, because preventing rainfall in a particular region would leave the vapor in the air and that would eventually rain more elsewhere," said Rosenfeld. "So it is more likely that the increased pollution will cause more extreme weather, leading to more droughts in some places and more floods in others."
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Daniel Rosenfeld's study in the March 10 issue of the journalScience.
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