Particles of soot produced in southern Asia are significantly reducing sunlight reaching Earth's surface, and the effect may have important consequences for the region's climate. That is an observation drawn from the international Indian Ocean Experiment (INDOEX) and published in the May 4 edition of the journal Nature.

V. Ramanathan and S.K. Satheesh of the Center for Clouds, Chemistry and Climate (C4) at Scripps Institution of Oceanography at the University of California, San Diego, used data from satellite measurements and five surface instruments to pinpoint a three-fold decrease in solar radiation reaching the earth's surface as compared with the top of the atmosphere. The difference, the authors say, is largely due to man-made soot aerosol particles that absorb sunlight in the atmosphere.

"The atmospheric heating over the northern Indian Ocean is surprisingly large compared to other oceanic regions and is comparable in magnitude with that observed over the coastal regions of the Atlantic Ocean," said Ramanathan.

The authors propose that the disruption caused by the soot aerosols may have several consequences for the region's climate, including slowing down the natural hydrological cycle and breaking up cloud cover. Although the researchers documented aerosol particles such as sulfate, nitrate, organics, and ash, the sunlight absorption was largely due to combustion-derived soot.

INDOEX is a cooperative program involving scientists from the United States, Europe, India, and the Maldives. The American component of INDOEX was funded by the National Science Foundation, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration, and NASA.

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