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

Abstract

Vertical profiles of HNO3, N2O, O3, and the aerosol extinction coefficient at 780 nm were observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) during the Arctic winter of 1996–1997. Irreversible redistribution of HNO3 is evaluated using HNO3‐N2O and HNO3‐O3 correlations. Denitrification and nitrification started to be observed just after the Arctic vortex cooled to below the ice frost point (TICE) on February 10. Trajectory analyses show that denitrification occurred only in air masses, which were once cooled to near TICE and were kept at temperatures below the nitric acid trihydrate saturation threshold continuously for more than 4 days. Such a temperature history provides the necessary conditions for nucleation and growth of particles causing denitrification. The average extent of denitrification at 19 km reached 43% at the center of the vortex, suggesting that stratospheric ozone could be affected by denitrification deep inside the vortex. Denitrification (>2 ppbv) and nitrification (>1 ppbv) covered 40±10% and 35±10% of the vortex area, respectively. Redistributed numbers of HNO3 molecules at each altitude were calculated by integrating the area‐weighted changes in the HNO3 concentration. The decreases in total HNO3 concentration at 17–21 km in late February and early March agreed with the increases at 12–15 km to within 25%, confirming conservation of HNO3 during sedimentation and evaporation of HNO3‐containing polar stratospheric cloud particles.