Observed Hemispheric differences in the role of PSCs in denitrification and dehydration

A technique has been developed by Bevilacqua et al. [2002] to infer stratospheric denitrification from Northern Hemisphere (NH) Polar Ozone and Aerosol Measurement (POAM) III polar stratospheric cloud (PSC) data. The technique is based upon the assumption that in a denitrified stratosphere the nitric-acid-trihydrate NAT saturation temperature is lower than with normal ambient nitric-acid abundances. This in turn lowers the PSC occurrence temperature for non-ice PSCs, which allows for a calculation of the relative change in HNO3 required to account for the decrease in PSC occurrence temperature. This technique has been used for the 1999/2000 [Bevilacqua et al., 2002] and the 2002/2003 [Alfred et al., 2006] NH winters. In this paper we extend this analysis to the POAM Southern Hemisphere (SH) PSC data. Hemispheric differences in the evolution of the PSC occurrence temperature are observed, suggesting significant micro-physical differences governing PSC occurrence. Notably, we infer that significant levels of irreversible denitrification can occur in the NH in the absence of ice PSCs, while in the SH the levels of denitrification in the absence of ice PSCs are not as large. It is not until the onset of ice PSCs that significant levels of denitrification are inferred, and denitrification in the SH is largely coupled with dehydration. We discuss on possible causes of this difference.