Biomass-burning ‘behind Asian brown clouds’

Biomass-burning ‘behind Asian brown clouds’

26 January 2009

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India — Burning biomass is the main cause of the dense ‘brown clouds’ that plague South Asia each winter, and both biomass and fossil fuel burning should be targeted to combat climate change and improve air quality.

These are the conclusions of a study published today (23 January) in Science. The study, conducted at two sites in South Asia, attempted to find the main source of the carbon soot particles that comprise much of the clouds.

While the brown cloud acts as a ‘global dimmer’ by absorbing heat trapped by greenhouse gases, it also affects the regional climate by melting glaciers, affecting crop growth and impacting the Asian monsoon.

Researchers from India, the Maldives and Sweden analysed the amount of radiocarbon — an unstable form of carbon that decays — in air from a mountain top in India and an island in the Maldives to determine whether the soot came from fresh biomass or the older carbon in fossil fuels.

They found biomass-burning produces two-thirds of the carbon soot and a half to two-thirds of a type of soot called black carbon, whose role in climate change is under debate (see Black carbon climate danger ‘underestimated’).

Burning biomass such as dried twigs, leaves and dung, and agricultural slash-and-burn practices, are common across poor, rural Asian areas.

The brown clouds affect the health of people inhaling the pollutants, causing bronchitis and heart disease. Women and children are particularly vulnerable, as they breathe in the soot at home during cooking and heating.

Previous data on the source of the soot was confusing, with atmospheric data suggesting 50—90 per cent of the South Asian brown cloud comes from fossil fuels; compared with 10—30 per cent estimated from emission inventories.

“This is the first time this difficult technique has been used for black carbon studies in India,” P. S. P. Rao, a scientist at the Indian Institute of Tropical Meteorology in Pune, and one of the researchers, told SciDev.Net.

“We should not only look to traffic and coal combustion but particularly at small-scale burning practices,” lead researcher Örjan Gustafsson, at the department of applied environmental science at Stockholm University, Sweden, told SciDev.Net.

Rao says another study will commence this year to confirm the findings, but preliminary indications are that Indian policymakers should discourage biomass burning and support a switchover to cleaner sources.

An upcoming pilot study to test whether low-tech solar and biogas cookers could reduce the adverse impacts on climate and human health could offer some insights, adds Gustafsson.

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