UARS MICROWAVE LIMB SOUNDER HNO3 OBSERVATIONS - IMPLICATIONS FOR ANTARCTIC POLAR STRATOSPHERIC CLOUDS

We present Microwave Limb Sounder (MLS) measurements of gas-phase HNO3 obtained at the beginning of five southern hemisphere winters: 1992-1 996. To investigate the composition of the polar stratospheric clouds (PSCs) that formed in early winter each year, the observed evolution o f HNO3 at 465 K is compared against that predicted using nitric acid t rihydrate (NAT), nitric acid dihydrate (NAD), and liquid ternary solut ion models of PSC formation and correlated with temperature histories from three-dimensional back trajectory calculations. The MLS HNO3 obse rvations suggest that the initial composition of PSCs depends on the p hysical state of the background sulfate aerosols. If the preexisting a erosols are liquid, then the formation of ternary solutions is initiat ed as the temperature drops below about 192 K, followed by a gradual c onversion to NAD after exposure to low temperatures for several days. HNO3 uptake into ternary solutions occurs at higher temperatures, and the conversion to NAD is delayed, under conditions of enhanced aerosol loading from the Mount Pinatubo eruption. If a majority of the backgr ound aerosols are frozen, the growth of ternary solutions is inhibited , but formation of a metastable, water-rich, KNO3-containing solid pha se characterized by a relatively high HNO3 vapor pressure (type Ic PSC ) may occur. In general, MLS HNO3 measurements obtained during early s outhern winter indicate a strong correspondence between the area of ga s-phase HNO3 loss and the area of temperatures below 192 K but only a weak correspondence between the area of gas-phase HNO3 loss and the ar ea of temperatures below 195 K, the value commonly assumed as the thre shold for PSC formation. Although temperatures were low enough to main tain NAT PSCs, the MLS data show that they were not forming, at least not over spatial scales comparable to or larger than the similar to 40 0 x 100 x 5 km MLS field of view.