Mid-Winter Chemistry

It is still believed that Type I (NAT) and Type II (water ice) PSCs predominate in the polar regions. Balloon measurements by Hofmann and Deshler [1991] and Deshler et al. [1994a], measurements from the Stratospheric Aerosol Measurement II (SAM II) sensor on the Nimbus 7 satellite from 1978 to 1989 [ Poole and Pitts, 1994], and observations from the ground [ Collins et al., 1993] continue to provide an altitudinal and temporal PSC climatology for the polar regions, and the basic view summarized by Brune [1991] has not changed. PSCs occur more readily over the Antarctic, where temperatures are consistently lower than over the Arctic, and the PSC season is a few months longer over Antarctica, beginning in mid-May and ending in November. Finally, Poole and Pitts [1994] show that the occurence temperature for PSCs remains relatively constant over the Arctic whereas it drops significantly over Antarctica as the winter season progresses. This supports the view that large-scale dehydration and denitrification occur over Antarctica, but not over the Arctic.

Observations of ClO over Antarctica and the Arctic from UARS [ Waters et al., 1993] confirm that a large fraction of the available inorganic chlorine within both vortices was in reactive forms by mid-winter. Abundances of ClO from UARS have exceeded 2 ppbv, in excess of values observed at slightly lower altitudes by the ER-2 aircraft. These large values are near the limit of what can be explained by current photochemistry. Averaged UARS ClO and ER-2 ClO during the 1991-92 arctic winter agree well where the two data sets overlap in altitude and solar zenith angle. The two measurements also agree very well on the geographic location of enhanced ClO.

As shown by Newman et al. [1993], a limited number of well-situated PSC events can process a significant fraction of the vortex in less than a month; therefore, extensive PSC activity is not necessary to process nearly all of the air in the vortex. However, if the perturbations are to extend into springtime, when ozone loss would be more severe, then NO must be irreversibly removed from the air; otherwise, photochemically produced NO will tie up reactive chlorine into ClONO.

Over the Arctic, data from UARS [ Roche et al., 1994 and Santee et al., 1995] show that nitric acid is high throughout the vortex in mid-winter, so there is no large-scale denitrification. NO and NO correlations from the ER-2 [ Loewenstein et al., 1993] confirm this view. However, Kawa et al. [1992b] and Murcray et al. [1994] show evidence for redistribution of NO in some arctic airmasses, presumably due to sedimentation and evaporation of larger particles. In these regions ozone losses could be significant.