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

The concentrations of HNO3, N2O, ozone, and aerosol in the lower stratosphe re 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 (T-ICE) De nitrification exceeding 40% was observed only in air masses which experienc ed temperature below T-ICE It occurred within 2 days in some of these air m asses. Increases in HNO3 by evaporation of the pal-rides (nitrification) at 13-15 km occurred 0-3 days after denitrification was observed, indicating particle radii of 5-10 mu m or larger. It is likely that these particles we re composed of nitric acid trihydrate (NAT) or NAT-coated ice particles, gi ven that the temperatures below 16 km were higher than T-ICE Continued expo sure of air masses below NAT saturation temperature for 1-4 days did not le ad to any significant denitrification as long as the temperature did nor fa ll below T-ICE, indicating that possible nucleation of NAT at these tempera tures within 4 days did not play a significant role in causing denitrificat ion. 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 incre ases at 12-17 km.