By Michael Kahn

LONDON, Jan 22 (Reuters) - Wood and dung burned for home heating and cooking makes up most of a huge brown cloud of pollution that hangs over South Asia and the Indian Ocean during the winter months, researchers said on Thursday.

The study in the journal Science solves the mystery of what makes up the soot in the brown haze linked to hundreds of thousands of deaths -- mainly from lung and heart disease -- each year in the region, they said.

"Doing something about this brown cloud has been difficult because the sources are poorly understood," said Orjan Gustafsson, a biogeochemist at Stockholm University.

Gustafsson led a Swedish and Indian team that used a newly developed radiocarbon technique to measure atmospheric soot particles collected from a mountaintop in western India and on the Maldives.

They found that two-thirds of the particles in the cloud was made up of so-called biomass, or organic matter like wood or dung, and the rest from fossil fuels.

The effects of the cloud, which towers up to 5 km above the ground, on regional climate warming were significant, Gustaffson said.

He said green technology, such as solar power, could quickly make a difference because the particles only linger in the air for a few weeks.

"You can get clear skies within a few weeks. That would be a huge benefit to the humans and the climate in the region," Gustafsson said."

The brown haze blankets much of the region in the winter months when temperatures drop and the air is dry. In wetter months the rain washes away the soot.

The phenomena is most persistent in Southeast Asia, but has also been identified in large metropolitan areas in Egypt, China, South Korea and other tropical locations, Gustaffson said. (Editing by Katie Nguyen)

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