Redistribution of nitric acid in the Arctic lower stratosphere during the winter of 1996-1997

Vertical profiles of HNO3, N2O, O-3, and the aerosol extinction coefficient at 780 nm were observed by the Improved Limb Atmospheric Spectrometer (ILA S) on board the Advanced Earth Observing Satellite (ADEOS) during the Arcti c winter of 1996-1997. Irreversible redistribution of HNO3 is evaluated usi ng HNO3-N2O and HNO3-O-3 correlations. Denitrification and nitrification st arted to be observed just after the Arctic vortex cooled to below the ice f rost point (T-ICE) on February 10. Trajectory analyses show that denitrific ation occurred only in air masses, which were once cooled to near T-ICE and were kept at temperatures below the nitric acid trihydrate saturation thre shold continuously for more than 4 days. Such a temperature history provide s the necessary conditions for nucleation and growth of particles causing d enitrification. The average extent of denitrification at 19 km reached 43% at the center of the vortex, suggesting that stratospheric ozone could be a ffected by denitrification. deep inside the vortex. Denitrification (>2 ppb v) and nitrification (>1 ppbv) covered 40 +/- 10% and 35 +/- 10% of the vor tex area, respectively. Redistributed numbers of HNO3 molecules at each alt itude were calculated by integrating the area-weighted changes in the HNO3 concentration. The decreases in total HNO3 concentration at 17-21 kin in la te February and early March agreed with the increases at 12-15 kin to withi n 25%, confirming conservation of HNO3 during sedimentation and evaporation of HNO3-containing polar stratospheric cloud particles.