EVOLUTION AND STOICHIOMETRY OF HETEROGENEOUS PROCESSING IN THE ANTARCTIC STRATOSPHERE

Simultaneous in situ measurements of HCl and ClO have been made for th e first time in the southern hemisphere, allowing a systematic study o f the processes governing chlorine activation between 15 and 20 km in the 1994 Antarctic winter. Data for several other gases (O-3, NO, NOy, OH, HO2, N2O, CH4, CO, H2O, CFCs), particulates, and meteorological p arameters were collected from the ER-2 aircraft out of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measure ments of Atmospheric Effects of Stratospheric Ail cl att (ASHOE/MAESA) campaign. Observations from the ER-2 in the fall (April-May), prior t o polar night, show that chlorine activation begins with 60-75% of ino rganic chlorine as HCl. By midwinter (July-August), near-total removal of HCl is observed. The wintertime loss of HCl in air recently expose d to extreme temperatures is found to be correlated with high levels o f reactive chlorine (ClO and its dimer, Cl2O2) in the linear fashion e xpected from the stoichiometry of the heterogeneous reaction of hydroc hloric acid with chlorine nitrate on polar stratospheric ic clouds (PS Cs): HCl + ClONO2 --> Cl-2 + HNO3. To constrain the role of different heterogeneous reactions and PSC types, we have used a photochemical tr ajectory model which includes heterogeneous sulfate and PSC chemistry. Model calculations of the evolution of reactive gases ale compared wi th the in situ observations. In addition, simultaneous measurements of OH and HO2 are used as a diagnostic for the occurrence of the heterog eneous reaction HOCl + HCl --> Cl-2 + H2O, which contributes to suppre ssed levels of HOx inside the vortex. It is shown that the amount of c hlorine activation is not strongly dependent on the composition of PSC s. However, HOx levels exhibit different signatures depending on the t ype of heterogeneous surfaces that affected chlorine activation. Furth ermore, this analysis implies that in the edge legion of the Antarctic vortex, the observed near-total removal of HCl can result from latitu dinal excursions of air parcels in and out of sunlight during the wint er, which photochemically resupply HOCl and ClONO2 as oxidation partne rs for HCl.