Study Pinpoints Main Source of Asia’s Brown Cloud

South Asia has a cloud over its head — an unpleasant, unhealthy and climate-affecting soup of sooty haze that envelops the region, particularly in winter.

Scientists have studied what’s called the “brown cloud” for years, yet there has always been uncertainty about it. How much of the soot and other carbon-containing aerosols that make up the haze comes from the burning of fossil fuels in cars, power plants and the like, and how much comes from burning wood and other biomass for cooking and agriculture?

Orjan Gustafsson of Stockholm University in Sweden and colleagues have now removed the cloud of uncertainty hanging over the brown cloud. Burning of biomass, they report in Science, is the greater culprit.

The researchers used carbon-14 dating of atmospheric soot sampled in early 2006 at Sinhagad in western India and Hanimaadhoo island in the Maldives. They made use of the fact that fossil fuels are millions of years old, and thus the carbon-14, a radioactive isotope with a half-life of 5,700 years, has decayed away. On the other hand, vegetation that is burned when fields are cleared, and wood and dung that are used for cooking, contain “young” carbon, with plenty of C-14.

They found that biomass combustion produced about two-thirds of the pollution, a much larger proportion than found in earlier studies that used different methodologies.

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