Six years of UARS Microwave Limb Sounder HNO3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere

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Abstract
We present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 in the lower stratosphere based on measurements of gas‐phase HNO3 made by the UARS Microwave Limb Sounder (MLS) through six complete annual cycles in both hemispheres. Outside of the winter polar regions, zonal‐mean HNO3 mixing ratios on the 465‐K potential temperature surface are comparable in the two hemispheres in all latitude bands and in all years examined. Except at high latitudes, interannual variability is minimal, and there is no significant hemispheric asymmetry in the overall HNO3 distribution or its seasonal cycle. Although the Antarctic experiences widespread severe denitrification, the MLS data indicate that the denitrification is not complete; that is, not all polar stratospheric cloud (PSC) particles sediment out of the lower stratosphere. Replenishment of HNO3 at 465 K during the mid‐ to late‐winter period (when temperatures, though still low, are generally rising) is most likely achieved through a combination of PSC evaporation and continuing weak diabatic descent. Despite large interhemispheric and interannual differences in the extent and duration of PSC activity and denitrification, HNO3 recovers to similar values at the end of every winter in both the Arctic and the Antarctic. Zonal‐mean HNO3 values for the two hemispheres are virtually indistinguishable for the latitudes equatorward of 65°, even during the winter months. Thus the effects of severe denitrification are confined in both space and time to the regions poleward of 65°S during the winter and early spring.