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American Association for the Advancement of Science

The makings of a deadly brown cloud

View from Sinhagad sampling site near Pune, India showing local combustion sources.

For years, a huge "Brown Cloud" of pollution has hung over South Asia and the Indian Ocean during the winter months. Its cancer-causing soot has affected people beneath it for years as well, and it can even be traced to the deaths of many people in China and India from heart and lung diseases.

Sunset at Sinhagad aerosol sampling site, near Pune, India with brown haze obscuring the sun.

Researchers have wondered exactly what this brown cloud is made of. Is it soot from burning fossil fuels like that from car exhaust? Or is it soot from burning organic materials like wood?

Finally, we have an answer. A group of researchers say that it is now clear that the burning of biomass, or organic matter like wood and dung, is mostly responsible for the huge, harmful cloud of soot. Örjan Gustafsson and colleagues caught soot particles from the brown cloud on a mountaintop in western India and also on the island of Maldives. They used radiocarbon measurements to find the source of the soot, and discovered that the burning of biomass was responsible for a full two thirds of the Brown Cloud's soot.

This finding suggests that we should address the issue of small-scale wood and dung burning for home heating and cooking in order to reduce the size of the harmful brown cloud over South Asia. The scientific results make it clear that efforts should not be limited to car traffic and coal-fired power plants. Instead, these authors recommend fighting poverty and spreading Asia-appropriate green technology to limit soot emissions from small biomass fires.

Because the brown cloud's soot particles only stay in the atmosphere for days or weeks at a time, there is hope for a quick change the climate system once biomass burning is reduced. Possibly, if we are able to limit the burning of biomass and organic matter in China and India, we could see the brown cloud disappear before our eyes.


This research appears in the 23 January issue of Science.