Sulfate aerosols' role in climate change studied
March 10, 1999
Web posted at: 10:00 AM EST
A team of researchers equipped with a host instruments and a research aircraft is in the Indian Ocean trying to determine how much sulfate aerosols -- a form of pollution -- cool the climate.
Sulfur dioxide levels from industrial and auto emissions, biomass burning and soil dust are expected to increase in Asia as its population rises. Sulfur dioxide is converted into sulfate aerosols when emitted into the air.
The ability of sulfate aerosols to reflect the sun's radiation may be one reason that increasing greenhouse gases have not warmed the Earth as much as some climate models have predicted. Sulfates also contribute to local pollution and acid rain.
"In the future, pollution in the tropics will increase, so we'd better understand it now. The chemistry in the tropics is severely under sampled," said National Center for Atmospheric Research's Jeffrey Kiehl, a principal investigator for the Indian Ocean Experiment.
Kiehl and his colleagues want to understand how sulfate aerosols influence global climate so they can improve climate change models. Some physical and chemical processes in the Earth system are so complicated that modelers cannot simulate each detailed step.
"With INDOEX data, we can actually test the way we treat aerosols in computer models against observations," said Kiehl.
The $25 million Indian Ocean Experiment is based in the Republic of the Maldives, an archipelago southwest of India's southern tip.
The observation region is downwind of the Indian subcontinent during the spring and extends into the pristine Southern Hemisphere. With a forecast of pleasant weather -- calm, with little rain -- the investigators should be able to sample both polluted and clean air in clouds and clear sky.
"It's a natural laboratory for studying direct and indirect effects of aerosols," said Andrew Heymsfield, another INDOEX researcher. The direct effect of aerosols is the scattering that occurs when solar radiation bounces off particles in clear air. The indirect effects have to do with sulfates' interactions with clouds.
Globally, aerosols are an important source of nuclei around which cloud droplets can condense, and in the tropics they are the chief source. The more cloud condensation nuclei, the brighter the cloud, that is, the more solar radiation reflected back into space before it reaches the Earth's surface.
This radiative effect is what makes clouds, and the indirect effects of aerosols, so important in climate change research. However, indirect effects are so little understood that estimates in global climate models vary from almost no effect to more than enough cooling to offset global warming resulting from greenhouse gas increases.
"A host of changes in cloud physical and microphysical properties are lumped under the term 'indirect effects,'" said National Center for Atmospheric Research scientist William Collins.
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