Taming polar clouds in the lab.

Scientists know that chlorofluorocarbons and other manufactured compounds cause the Antarctic ozone hole each year, but these pollutants couldn't do their damage without help from nature. Tiny cloud particles in the frozen reaches of the atmosphere play a critical role in the ozone drama by liberating destructive forms of chlorine from otherwise benign molecules. A team of Canadian chemists has now succeeded in producing these cloud particles in the lab for the first time, opening the door to more detailed studies of how chemicals destroy ozone.

Ever since atmospheric researchers discovered the importance of these so-called polar stratospheric clouds (PSCs), they have attempted to model them in the lab. But studies to date have made imperfect versions of PSCs because they have produced extremely thin films instead of actual microscopic cloud particles. "Thin films can't mimic the way a particle behaves in the real atmosphere," says James J. Sloan of the University of Waterloo in Ontario.

Sloan and his colleagues used a number of difficult techniques to make PSC particles, To produce tiny droplets of ice, they mixed a stream of water vapor with air cooled to -87 degrees C. To fashion a different type of PSC, they forced nitric acid vapor into supercooled air. By studying the way these particles absorb and scatter light, the researchers confirmed that the simulated clouds matched the composition and size of PSCs, they report in the March 1 GEOPHYSICAL RESEARCH LETTERS.