Study Gets Inside the World's "Brown Cloud"

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A thick photochemical smog comprised of respirable suspended particulate matter and nitrogen dioxide hangs thick over the Causeway Bay district of Hong Kong, 22 January 2009

Pilot John Horwood says the worse part about flying into Hong Kong is the suffocating, two-mile-thick blanket of pollution that hovers between 15 and 18,000 feet. "The whole cockpit fills with an acrid smell," says Horwood, who started noticing the cloud in 1997. "Each year it just gets worse and worse." What comprises this nuisance — a sprawling high-altitude mass of air pollution that stretches from the Arabian peninsula to the western Pacific Ocean — has long captured the curiosity of scientists. A report released in the Jan. 23 issue of Science breathes fresh air into that ongoing study, confirming that the mass, nicknamed the 'Brown Cloud' but comprised of several small, local clouds, is soot from human burning of wood, dung and crop residue, as well as industrial processes and traffic pollution.

Örjan Gustafsson, the study's lead author, and his colleagues at Stockholm University conducted research with Indian scientists from January to April 2006 to determine that two-thirds of the cloud's soot particles come from biomass combustion like household cooking and slash-and-burn agriculture. The researchers confirmed that the layer of haze — which many have blamed for the world's increasingly extreme weather patterns — makes rain both more rare during the dry season and more intense during monsoons. And in South Asia, the cloud's net effect on climate change, says the study, rivals that of carbon dioxide.

Scientists have been gathering different kinds of information about the mass for years. A 2008 study by the United Nations Environment Program, for instance, warned that 340,000 people in China and India alone die annually from cardiovascular and respiratory diseases that can be traced to human-induced emissions of combustible particles in these atmospheric brown clouds. It concluded that regional pollution's impact can go global since winds blow soot across the world within weeks, and it, too, noted that brown clouds exacerbate deadly flooding and droughts.

But that report, like others before it, was inconclusive about what causes the clouds in the first place. "The new research is incredibly important because it refines our understanding of the clouds' components," says Achim Setiner, executive director of the U.N. Environment Program. "In each part of the world there are different atmospheric mixes that add to brown clouds' cocktail, so this research helps show where governments should direct their money and efforts."

The new findings imply that controlling biomass combustion, particularly the small-scale burning of wood and dung for home heating and cooking common throughout Asia and Africa, will be an important step towards improving the world's air quality. Gustafsson, a professor of biogeochemistry at Stockholm University, urges environmentalists not to limit their efforts to curbing car traffic and coal-fired power plants. He says fighting poverty and spreading green technologies that limit emissions from small-scale biomass burning are equally important. "More households in South Asia need to be given the possibility to cook food and get heating without using open fires of wood and dung," says Gustafsson. "The international community should step in and contribute to technology transfers towards solar heating and biogas, especially since the developed world is responsible for the climate situation that exists today."

The silver lining of this massive brown cloud is that a boost in those technologies could decrease soot emissions — fast. "We can't fix every environmental problem, but we can make a huge and immediate change by reducing black carbon," says Veerabhadran Ramanathan, Director of the Center for Clouds, Chemistry & Climate at the University of California at San Diego. (Black carbon manifests as soot particles that comprise brown clouds.) While carbon dioxide levels in the atmosphere respond on a sluggish 100-year timescale to reductions in emissions, soot particles, whose effects are equivalent to roughly half the warming damage that carbon dioxide does, have a shelf life of only a few weeks. Environmentalists have joined scientists' call to urge governments to cut pollution by introducing more efficient heating stoves in developing countries and turning to solar power and other clean sources of energy. "We need sustainable growth in every city in this world," says Edwin Lau Che Feng, director of the environmental group Friends of the Earth in Hong Kong. "Richer countries have a responsibility to transfer cleaner technology to developing nations and help them reduce emissions because developing nations are the world's factory today."