HR: 08:00h
AN: A31D-01 INVITED     [Abstracts]
TI: Polar Stratospheric Cloud Studies: History and Future
AU: * Toon, O B
EM: toon@lasp.colorado.edu
AF: University of Colorado, Campus Box 392 University of Colorado, Boulder, CO 80309-0392 United States
AB: Polar stratospheric clouds (PSCs) have been reported in the atmosphere for more than a century. I will provide an overview of their properties and discuss issues that remain to be resolved. It was only with the discovery of Antarctic ozone loss that we realized some PSCs are made of nitric acid, making them the first type of cloud discovered in Earth's atmosphere not composed of liquid water or ice. During the past few decades it has become clear that polar stratospheric clouds drive ozone loss. In the Arctic stratosphere the seasonal ozone loss is directly proportional to the volume filled with these clouds. We have also learned that they dehumidify and denitrify the polar stratosphere, particularly in the Southern Hemisphere. We now know quite a lot about these clouds. They can be large, for example Arctic clouds can cover an area similar to that of the U.S. and have a vertical extent comparable to that from sea level to the top of the Rockies. Unfortunately they can also be small, for example of the scale of mountains, or mountain ranges, making them difficult to treat in global chemical models. The clouds can be composed of ice, and various solutions or hydrates of nitric acid. Particle sizes range from tenths of a micrometer to tens of micrometers. The physical mechanisms controlling many of these properties are understood, and new observations from satellites should greatly expand our ability to test our models. However some cloud properties are not well understood. Most importantly we don't understand the physics that leads to the formation of large solid particles of nitric acid, from small liquid particles. There are nitric acid clouds in the tropics that are similar to PSCs. We know little about their distribution, and effects on the atmosphere. There are also puzzling interactions between nitric acid and ice, which may lead to nitric acid redistribution in the upper troposphere, and may also impact important properties of the clouds, such as ice supersaturation. Nitric acid adsorbed on ice may also serve as a useful clock for cloud lifetimes.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0340 Middle atmosphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005