Evolution and stoichiometry of heterogeneous processing in the Antarctic stratosphere

Creators: Jaeglé, L.; Webster, C. R.; May, R. D.; Scott, D. C.; Stimpfle, R. M.; Kohn, D. W.; Wennberg, P. O.; Hanisco, T. F.; Cohen, R. C.; Proffitt, M. H.; Kelly, K. K.; Elkins, J.; Baumgardner, D.; Dye, J. E.; Wilson, J. C.; Pueschel, R. F.; Chan, K. R.; Salawitch, R. J.; Tuck, A. F.; Hovde, S. J.; Yung, Y. L.

Abstract

Simultaneous in situ measurements of HCl and ClO have been made for the first time in the southern hemisphere, allowing a systematic study of the processes governing chlorine activation between 15 and 20 km in the 1994 Antarctic winter. Data for several other gases (O_3, NO, NO_y, OH, HO_2, N_(2)O, CH_4, CO, H_(2)O, CFCs), particulates, and meteorological parameters were collected from the ER-2 aircraft out of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements of Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign. Observations from the ER-2 in the fall (April–May), prior to polar night, show that chlorine activation begins with 60–75% of inorganic chlorine as HCl. By midwinter (July–August), near-total removal of HCl is observed. The wintertime loss of HCl in air recently exposed to extreme temperatures is found to be correlated with high levels of reactive chlorine (ClO and its dimer, Cl_(2)O_2) in the linear fashion expected from the stoichiometry of the heterogeneous reaction of hydrochloric acid with chlorine nitrate on polar stratospheric clouds (PSCs): HCl + ClONO_2 → Cl_2 + HNO_3. To constrain the role of different heterogeneous reactions and PSC types, we have used a photochemical trajectory model which includes heterogeneous sulfate and PSC chemistry. Model calculations of the evolution of reactive gases are compared with the in situ observations. In addition, simultaneous measurements of OH and HO_2 are used as a diagnostic for the occurrence of the heterogeneous reaction HOCl + HCl → Cl_2 + H_(2)O, which contributes to suppressed levels of HO_x inside the vortex. It is shown that the amount of chlorine activation is not strongly dependent on the composition of PSCs. However, HO_x levels exhibit different signatures depending on the type of heterogeneous surfaces that affected chlorine activation. Furthermore, this analysis implies that in the edge region of the Antarctic vortex, the observed near-total removal of HCl can result from latitudinal excursions of air parcels in and out of sunlight during the winter, which photochemically resupply HOCl and ClONO_2 as oxidation partners for HCl.

Additional Information

© 1997 American Geophysical Union. Manuscript Accepted: 27 March 1997; Manuscript Received: 20 March 1996. The authors would like to thank H. Nair for providing the original modifications to the photochemical Lagrangian code. K. Carslaw made available his code for ternary solution growth. We thank D. Fahey for the ASHOE/MAESA NO and NO_y data, E. Keim and L. Del Negro for valuable discussions, and L. Mickley for helpful comments. The authors also thank two reviewers for their comments. Additional field operations support of ALIAS was provided by M. Tuchscherer and G. Flesch. Part of the research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). This research is also supported in part by NASA grant NAGW-413 to the California Institute of Technology.

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