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A Matter Of Size

Aerosol particle size, not chemistry, is most important in cloud droplet formation

by Elizabeth K. Wilson
June 5, 2006 | A version of this story appeared in Volume 84, Issue 23

Aerosol Factory
Particles that seed clouds come from burning fossil fuels and natural sources such as volcanic eruptions.
Particles that seed clouds come from burning fossil fuels and natural sources such as volcanic eruptions.

The ability of atmospheric aerosol particles to seed the formation of the droplets that collect into climate-influencing clouds depends primarily on the particles' size, not their chemical compositions. This finding suddenly makes studying aerosol particles' effects on climate much easier.

Meinrat O. Andreae, director of the biogeochemistry department at Max Planck Institute for Chemistry in Mainz, Germany, and colleagues there and at other institutions sampled the atmosphere from a German observatory, categorizing the sizes of particles and their ability to grow into cloud droplets. This nucleating ability increases with particle size, and only 4-16% of this ability can be attributed to a particle's chemical composition, they report (Science 2006, 312, 1375).

Aerosols, which are produced by natural phenomena such as volcanic eruptions and by human-generated processes such as the burning of fossil fuels, are noxious pollutants and play an important role in shaping Earth's climate. The clouds they seed can mask the effects of global warming by reflecting sunlight back into space.

Despite the importance of aerosols, scientists' understanding of how they influence climate remains murky. Indeed, aerosols "represent the greatest uncertainty in our understanding of the climate system," notes Daniel Rosenfeld, a professor at Hebrew University's Earth Sciences Institute, in Jerusalem, in a perspective accompanying the Andreae group's paper.

Now, atmospheric models that had been greatly complicated by attempts to include aerosol chemistry can be created more simply. And relatively easy measurements of particle size will allow researchers to estimate cloud droplet concentrations in the atmosphere; these measurements are vital for understanding the complex relationships between the droplets and clouds, Andreae says.

"Their findings are very important," Ulrike Lohmann, professor at the Institute for Atmospheric & Climate Science at the Swiss Federal Institute of Technology, Zurich, tells C&EN. "They suggest we do not need to track every organic species in the climate models."



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