Every winter, a thick cloud of brown smog settles over South Asia, stretching from southern China, across India and Pakistan, to the northern reaches of the Indian Ocean. For everyone who lives with the so-called “Asian brown cloud,” this air pollution is just a fact of life. 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” [Time]. But scientists have long puzzled over the cloud’s source: Is it produced by burning biomass, or by the combustion of fossil fuels?
Now researchers have analyzed the cloud’s composition, and found that two-thirds of the haze is produced by burning biomass, primarily the wood and dung burned to heat houses and cook food throughout the region. This research is the first step to doing something about the brown haze, which is 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 [Reuters], the study’s lead author.
For the study, published in Science [subscription required], researchers looked at the proportion of an isotope called carbon-14, or radiocarbon, in soot particles collected from a mountaintop site at Sinhagad, India, and from the Maldives in the Indian Ocean. Radiocarbon decays with a half-life of around 5700 years. In fossil fuels there is very little of the isotope because it has had millions of years to decay, whereas the relatively youthful nature of biomass (plants and animals) means the radiocarbon signature is high [New Scientist]. (Carbon-14 is produced high in the atmosphere by incoming cosmic rays, and living plants and animals take the isotope in through the air.) The analysis found that two-thirds of the airborne soot had high radiocarbon content, indicating that its source was burning biomass.
The cloud not only has a serious impact on public health, it also changes the climate of Asia in dangerous ways. 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 [Time]. However, the news isn’t all bad: Researchers say the annual cloud could be quickly diminished if people switch to solar- or gas-powered cooking stoves, because the particles remain in the air for just a few weeks.
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Image: AAAS / Science