Interhemispheric differences in springtime production of HCl and ClONO2 in the polar vortices

UARS observations of O₃ and ClO (Microwave Limb Sounder), ClONO₂ and HNO₃ (Cryogenic Array Etalon Spectrometer), NO, NO₂, and HCl (Halogen Occultation Experiment), and model calculations are used to produce an exposition of the different processes through which the reservoir gases ClONO₂ and HCl are reformed at the end of the polar winter. Comparison of the observations within the polar vortices shows that HCl increases more rapidly in the Antarctic vortex in spring than in the Arctic vortex. Model analysis shows that this occurs because the O₃ concentrations in the southern vortex fall well below those in the northern vortex. The Cl/ClO fraction calculated for the southern hemisphere is therefore up to 30 times higher, leading to rapid HCl formation by Cl + CH₄. The concentrations of NO observed by HALOE are substantially lower for the northern hemisphere than for the southern hemisphere, even for similar values of the concentration of HNO₃ and the production of NO_X from HNO₃ through photolysis and reaction with OH. This is consistent with the dependence of the NO/NO_X ratio on the O₃ concentration, i.e., the daytime production rate of NO₂ via NO + O₃ is reduced, leading to higher NO in the southern hemisphere. This higher concentration of NO also contributes to the rapid HCl increase as Cl production from ClO + NO is enhanced.