South polar firn cores spanning the last millennium have been analyzed to determine the nitrate background level of high-latitude precipitation and its temporal variations. The resulting data reveal no evidence of a positive correlation between solar activity (11-year solar cycle, low solar activity time periods, and solar proton events) and the NO3 content of south polar snow. These data therefore suggest that NOx production in the upper stratosphere, mesosphere, and thermosphere does not contribute significantly to the antarctic NO3 budget. This study of the NO3 content of high latitude precipitation suggests a major contribution by lightning (from a third to a half of the total) and by NOx produced in the lower stratosphere (approximately a third from N2O oxidation and to a lesser extent galactic cosmic rays) to the NO3 budget of this background atmosphere, the remaining portion being related to the present NOx surface sources of the southern hemisphere. For the first time, our data point to a decrease of NO3 content when very large amounts of sulfuric acid are present in South Pole snow layers. This observation suggests a possible interaction (via the hydroxyl radical) between S- and N-derived species when large quantities of SO2 are injected by volcanic eruptions. Although the deposition mechanism of HNO3 in South Pole snow layers is far from well understood, our data suggest that only a minor amount of this compound is deposited directly on surface snow layers. Finally, the role played by particles (volcanic ash, terrestrial impurities, or ice particles) to enhance the uptake of odd nitrogen from the atmosphere by heteregeneous processes is discussed. In particular, it is suggested that in late winter, under certain meteorological conditions leading to ice particle formation, a significant uptake of HNO3 from the lower stratosphere can occur.
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