New observations with space-borne instruments by Daniel Rosenfeld of the Hebrew University of Jerusalem showing that aerosols over land can modify clouds, suppress precipitation and reflect light is a "huge leap forward" in understanding the interplay among pollutants, clouds and greenhouse warming, said University of Colorado Professor Owen B. Toon.
A professor in CU-Boulder聮s Laboratory for Atmospheric and Space Physics, Toon authored a perspective article in the March 10 issue of Science magazine tied to a research paper by Rosenfeld in the same Science issue on his new findings. Rosenfeld聮s research shows that aerosol particulates from urban and industrial areas modify clouds over large land areas, suppress rain and snow and are responsible for reflecting significant amounts of sunlight back to space.
Toon, who has studied similar modification of clouds over the ocean and their effects, said aerosol particulates in the atmosphere are reflecting sunlight back into space, directly cooling Earth. The new findings by Rosenfeld are more evidence that aerosol-cloud processes may be diluting and perhaps even canceling out greenhouse warming, Toon said.
Unlike greenhouse gases -- which stay in the atmosphere for long periods and are fairly evenly distributed -- aerosols are more concentrated near their sources and variable in space and time, making it difficult to quantify their impacts. "But their cooling effect may be as large as the warming effect created by humans pumping greenhouse gases into the atmosphere during the last century," said Toon, also a professor in CU-Boulder聮s Program for Atmospheric and Ocean Sciences.
"Computer models could not explain the lack of warming that had been predicted because they were overlooking the opposing forces due to aerosol pollution," Toon said.
In the 1980s, long, bright lines in ocean clouds seen from the air were determined to be the tracks of ships, which were sending exhaust into the atmosphere, including low-lying cloud decks. Because the particulates triggered new cloud formation and redistributed water in existing clouds, they became brighter and more reflective, Toon said.
In 1998, Rosenfeld detected similar "pollution tracks" over land areas of Earth for the first time. "We had known that aerosol particulates emitted by ships can trigger changes in clouds and even their formation over the ocean," said Toon. "What Rosenfeld has done in his latest paper is to use satellite data to demonstrate a clear, widespread influence of aerosol pollution on continental precipitation. "
Rosenfeld used data taken by Advanced Very High Resolution Radiometers onboard several U.S. weather satellites and NASA聮s new TRMM satellite for his Science study. The images showed pollution tracks caused by urban and industrial activity in areas of Turkey, Australia and Canada. They also indicated that clouds making up the pollution tracks were prohibiting rain and snow from falling downwind from the sites.
Since each cloud droplet must form on a pre-existing particle, additional aerosols in clouds like sulfates or sulfuric acid increase the number of water droplets in clouds, said Toon. Because temperatures and atmospheric motions driving cloud formation control the mass of water condensing in the clouds, the droplets formed on aerosol particulates tend to be smaller in size.
Clouds harboring smaller droplets have larger surface areas, making them more reflective and sending more sunlight back to space, said Toon. Because of their diminutive size, the chances of the droplets coagulating into raindrops large enough to fall as precipitation are greatly diminished.
A typical cloud droplet, nearly microscopic, can travel little more than an inch through dry air before evaporating. "About 1 million cloud droplets must collide and coalesce in order to form a precipitation-sized drop," said Toon, who noted that a typical raindrop -- which is about the size of a rice grain -- can fall a mile before evaporating.
The rate at which droplets collide and coalesce depends on their size and the number of other similar droplets in their path, he said. Normal-sized droplets in an unperturbed cloud would sweep up about 64 times the volume of air containing other droplets than would droplets half that size inside a polluted cloud. This would make the aerosol-filled cloud much less likely to rain, said Toon.
One of Rosenfeld聮s satellite images shows a pollution track emanating from a mining and smelting company in Flin-Flon, Manitoba. Another shows pollution tracks from several sources near Istanbul, Turkey. A third image shows a track originating in the vicinity of a brown coal power plant, the world聮s largest smelter and refinery, a huge cement plant and a major oil refinery near Adelaide, South Australia.
Other areas of the world are at least as tainted with aerosols. But pollution tracks from huge, nearly adjacent cities such as those in the Northeast United States, for example, are virtually invisible because of the perpetual pollution that hangs in the atmosphere, he said.
"Rosenfeld聮s work points to locales where in situ observations should be made to pinpoint the mechanisms by which pollution affects clouds," Toon concluded in the Science article. "Such knowledge may allow us to estimate how widespread the aerosol interaction with cloud precipitation may be in our globally polluted world."