Aircraft measurements of O3, HNO3 and N2O in the winter arctic lower stratosphere during STREAM I

Simultaneous in situ measurements of O3, HNO3, and N2O were performed in the Arctic (68°–74°N) lower stratosphere during February 1993 on board a Cessna Citation aircraft up to 12.5 km altitude, during the first Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) campaign. Strong variations in the concentrations, distributions, and ratios of these trace gases were found from the maximum altitude down to the tropopause. Close to the tropopause, vortex air was present with relatively low N2O concentrations. The observed N2O-HNO3 relation agrees with earlier measurements of total nitrogen and N2O inside the vortex, suggesting subsidence of vortex air across the bottom of the vortex. This air also contained low O3 concentrations relative to N2O, indicating enhanced O3 loss by chemical reactions involving stratospheric particles. Based on trajectory calculations and assuming a potential temperature cooling rate of 0.6 K d−1, we estimate an O3 loss of 4–7 ppbv d−1 (0.9–1.2% d−1), in the Arctic lower stratosphere for the period January–February 1993. Air parcels originating from middle latitudes, containing relatively low O3 and N2O concentrations, may have originated from the vortex earlier in the winter. In addition, the results also show high HNO3 concentrations relative to O3 and N2O. Air parcels originating from high latitudes may have been enriched in HNO3 by sedimentation and evaporation of nitric acid containing particles, which would explain the relatively high HNO3 concentrations and HNO3/O3 ratios measured. Heterogeneous chemistry on sulfuric acid particles, probably enhanced in concentration by gravitational settling of the Pinatubo aerosol, is the most plausible explanation for the observed high HNO3 concentrations relative to N2O in air parcels originating from midlatitudes.