Reining In Brown Clouds—Cuts in regional, black-carbon-laden air pollution could…help curb…warming

August 24, 2009 - Volume 87, Number 34 - pp. 26 - 27

Reining In Brown Clouds

Cuts in regional, black-carbon-laden air pollution could quickly help curb global warming

Cheryl Hogue
Across parts of Earth are massive swaths of brownish air covering thousands of square miles. Thick with lung-choking pollutants, these so-called brown clouds dim the surface below them and contain particles that are potent agents of global warming. In addition, some scientists say brown clouds are altering regional precipitation patterns in Asia.

Calls are growing to clean up emissions that form brown clouds. This, proponents says, will help curb global warming in the short term, significantly improve people’s health, and help protect the flow of major rivers that millions of people depend on for water.

A brown cloud is a soup of pollutants. It contains nanometer- to micrometer-sized soot particles, sulfates, nitrogen oxides, ground-level ozone, and fly ash, which is a residue from coal burning. Imparting the clouds’ brownish color are nitrogen dioxide gas and aerosols including fly ash, soil particles that are suspended in air, and black carbon, a product of incomplete combustion. These particulates are linked to respiratory problems. And black carbon, which absorbs sunlight, is a major contributor to human-induced climate change, ranked as second after carbon dioxide (Nat. Geosci. 2008, 1, 221).

The first brown cloud was identified about a decade ago over Southern Asia. But such clouds are not limited to Asia. Brown clouds are all over the world, a recent report by the http://www.unep.org/">United Nations Environment Program (UNEP) points out.

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