Millimeter wave spectroscopic measurements over the South Pole. 5. Morphology and evolution of HNO3 vertical distribution, 1993 versus 1995

We compare differences and similarities in the annual stratospheric HNO 3 cycle derived from ground-based measurements at the South Pole during 1993 and 1995, after correcting an error in earlier published profile retrievals for 1993 which led to under estimation of mixing ratios. The data series presented here provide profiling over the range ∼16-48 km, and cover the fall-winter-spring cycle in the behavior of HNO 3 in the extreme Antarctic with a large degree of temporal overlap. With the exception of one gap of 20 days, the combined data sets cover a full annual cycle. The record shows an increase in HNO 3 above 30 km occurring about 20 days before sunset, which appears to be the result of higher altitude heterogeneous conversion of NO x as photolysis diminishes. Both years show a strong increase in HNO 3 beginning about polar sunset, in a layer peaking at about 25 km, as additional NO x is heterogeneously converted to nitric acid. When temperatures drop to the polar stratospheric cloud (PSC) formation range near the end of May, gas phase HNO 3 is rapidly reduced in the lower stratosphere, although at least 2-3 weeks of temperatures ≤192 K appear to be required to complete most of the gas-phase removal at the upper end of the depletion range (22-25 km). Despite a significant difference in residual sulfate loading from the explosion of Mount Pinatubo, there appears to be little gross difference in the timing and effects of PSC formation in removing gas phase HNO 3 in these 2 years, though removal may be more rapid in 1995. Incorporation of gas phase HNO 3 into PSCs appears to be nearly complete up to ∼25 km by midwinter. We also see a repeat of the formation of gas phase HNO 3 in the middle stratosphere in early midwinter of 1995 with about the same timing as in 1993, suggesting that this phenomenon is driven by a repetition of dynamical transport and appropriate temperatures and pressures in the polar night, and not (as has been suggested) by ion-based heterogeneous chemistry that requires triggering by large relativistic electron fluxes. High-altitude HNO 3 production peaks during a period of ∼20 days, but appears to persist for up to ∼40 days in the 40-45 km range, ceasing well before sunrise. This HNO 3 descends rapidly throughout the production period, at a rate in good agreement with theoretically determined midwinter subsidence rates. As noted in earlier studies, later warming of this region above PSC evaporation temperatures does not cause reappearance of large amounts of HNO 3 , indicating that most PSCs gravitationally sink out of the stratosphere before early spring. We present evidence that smaller PSCs do evaporate to ∼1 to 3.5 ppbv of HNO 3 in the lower stratosphere, however, working downward from ∼25 km as temperatures rise during the late winter. There is a delay of ∼15 days after sunrise before photolysis causes significant depletion in the altitude range below ∼30 km, where subsidence has carried virtually all higher-altitude HNO 3 by polar sunrise. Some continued subsidence and photolysis combine to keep mixing ratios less than ∼5 ppbv below 30 km until the final breakdown of the vortex in November brings larger amounts of HNO 3 with air from lower latitudes.