August 6, 2007

Volume 85, Number 32

p. 9

Climate Change

Aerosols Warm Up Atmosphere

Study of 'brown clouds' over Asia contradicts cooling hypothesis

Rachel Petkewich

AEROSOLS CAUSE as much warming in the lower atmosphere as anthropogenic greenhouse gases, and both appear to contribute equally to regional climate change, according to a direct aerial study of so-called brown clouds near Asia (Nature 2007, 448, 575). On the basis of satellite and model data, researchers had surmised that aerosols have a net cooling effect on the troposphere, whereas greenhouse gases are known to increase heating.

Brown clouds, which hang in the troposphere over the Indian Ocean, are not well understood. These hazy mixtures of light-absorbing and light-scattering particles likely come from burning biomass and fossil fuels.

To learn more about brown clouds, Veerabhadran Ramanathan at Scripps Institution of Oceanography and colleagues at the University of California, San Diego, and NASA simultaneously flew three lightweight, unmanned aerial vehicles in a stacked formation below, into, and above the clouds between 0.5 and 3 km above the Indian Ocean. In a total of 18 flights, instruments on board the vehicles collected the first vertically resolved data from brown clouds, including temperature, aerosol concentrations, and solar radiation fluxes.

Ramanathan and colleagues also plugged their new data into well-established models, along with information collected by other researchers. The results indicated that brown clouds enhance lower atmospheric solar heating by about 50%.

"The major uncertainties in our understanding of the climate system and human influences on it have to do with how atmospheric aerosol particles interact with solar radiation," says James Donaldson, an atmospheric chemist at the University of Toronto. Because different regions of the atmosphere can experience varying heating rates, directly collecting data at various altitudes is critical to unraveling this complex effect, he adds.

Pinpointing the radiative effects of aerosols depends on determining their chemical composition, says Adrian Tuck, a meteorological chemist at the National Oceanic & Atmospheric Administration.

Chemical & Engineering News

ISSN 0009-2347

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