PHYSICAL-CHEMISTRY OF THE H2SO4 HNO3/H2O SYSTEM - IMPLICATIONS FOR POLAR STRATOSPHERIC CLOUDS/

Polar stratospheric clouds (PSCs) play a key role in stratospheric ozo ne depletion. Surface-catalyzed reactions on PSC particles generate ch lorine compounds that photolyze readily to yield chlorine radicals, wh ich in turn destroy ozone very efficiently. The most prevalent PSCs fo rm at temperatures several degrees above the ice frost point and are b elieved to consist of HNO3 hydrates; however, their formation mechanis m is unclear. Results of laboratory experiments are presented which in dicate that the background stratospheric H2SO4/H2O aerosols provide an essential link in this mechanism: These liquid aerosols absorb signif icant amounts of HNO3 vapor, leading most likely to the crystallizatio n of nitric acid trihydrate (NAT). The frozen particles then grow to f orm PSCs by condensation of additional amounts of HNO3 and H2O vapor. Furthermore, reaction probability measurements reveal that the chlorin e radical precursors are formed readily at polar stratospheric tempera tures not just on NAT and ice crystals, but also on liquid H2SO4 solut ions and on solid H2SO4 hydrates. These results imply that the chlorin e activation efficiency of the aerosol particles increases rapidly as the temperature approaches the ice frost point regardless of the phase or composition of the particles.