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Denitrification and nitrification in the Arctic stratosphere during the winter of 1996–1997

The concentrations of HNO3, N2O, ozone, and aerosol in the lower stratosphere inside the Arctic vortex were observed by the Improved Limb Atmospheric Spectrometer (ILAS) in the winter of 1996–1997. These data demonstrate that irreversible loss of reactive nitrogen by sedimentation of HNO3 containing particles (denitrification) at 18–23 km occurred in mid‐late February soon after the Arctic vortex cooled below ice saturation temperature (TICE). Denitrification exceeding 40% was observed only in air masses which experienced temperature below TICE. It occurred within 2 days in some of these air masses. Increases in HNO3 by evaporation of the particles (nitrification) at 13–15 km occurred 0–3 days after denitrification was observed, indicating particle radii of 5–10 µm or larger. It is likely that these particles were composed of nitric acid trihydrate (NAT) or NAT‐coated ice particles, given that the temperatures below 16 km were higher than TICE. Continued exposure of air masses below NAT saturation temperature for 1–4 days did not lead to any significant denitrification as long as the temperature did not fall below TICE, indicating that possible nucleation of NAT at these temperatures within 4 days did not play a significant role in causing denitrification. There was little change in the average HNO3 column from February 11 to 28 since HNO3 decreases at 18–23 km were almost completely offset by increases at 12–17 km.