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Biomass, biomass, burning black

indianfire.JPGA study published in Science today (subscription) uses carbon-14 measurements to figure out where the black carbon drifting in the haze above South Asia is coming from. That’s a prerequisite to cleaning it up – which, as we’ve reported here, could be a major boon to a very vulnerable region. The light-absorbing compound not only causes cancer (among other ill health effects), but reportedly warms some places as much as greenhouse gases do. Because its lifespan in the atmosphere is far shorter than carbon dioxide’s, these impacts could potentially be reduced quickly – if we knew where to clamp down.

Writing in Science, Örjan Gustaffson of Stockholm University and colleagues call black carbon the “dark horse” in the current climate debate, saying “substantial uncertainties exist about its atmospheric longevity, aerosol mixing state, measurement, and sources.” Black carbon is unusually concentrated in the ‘brown cloud’ over South Asia, they note. But estimates vary wildly on how much of this is from fossil fuel burning and how much from smoky fires of wood, dung and other biomass: the ratio ranges from about 1:10 to 10:1, depending on the technique and study area.

Radiocarbon measurements give a more reliable read on this question than other methods, according to Sönke Szidat of the University of Bern, who discusses the new paper in an accompanying Perspective. The principle is simple: fossil fuels are ancient enough that all their carbon-14 has decayed away, whereas freshly gathered biofuels have plenty of the isotope. And while other chemical clues about the brown cloud can change as it wafts around, carbon-14 stays stable.

The carbon-14 levels tell Gustaffson et al. that around half of the black carbon in the cloud, or more, is from biomass. They took samples at ground stations in the Maldives and atop a West Indian mountain, downwind of the rest of South Asia. The 50% figure, Gustaffson says, comes from a method of isolating black carbon that picks up airborne coal dust as well as combusted carbon in soot; in soot-only samples, about two-thirds was from biomass.

Says Szidat:

The study shows that the importance of biomass burning for local and global BC [black carbon] budgets has been underestimated. This was previously pointed out for urban, rural, and remote areas in Europe, but never were the consequences as severe as for the Asian haze.

Regular readers may notice I’m on aerosol patrol this week – see my post on links between pollution, fog, and temperature in Europe. On a related note, one of the reports in a suite just released by the US Climate Change Science Program stresses the need for better research on aerosols, particularly in the modelling world. “The influence of aerosols on climate is not yet adequately taken into account in our computer prediction,” says lead author Mian Chin of NASA.

Anna Barnett

Photo: Agricultural fires in northern India / NASA


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