AIRCRAFT MEASUREMENTS OF O-3, HNO3 AND N2O IN THE WINTER ARCTIC LOWERSTRATOSPHERE DURING THE STRATOSPHERE-TROPOSPHERE EXPERIMENT BY AIRCRAFT MEASUREMENTS (STREAM) .1.

Simultaneous in situ measurements of O-3, HNO3, and N2O were performed in the Arctic (68 degrees-74 degrees N) lower stratosphere during Feb ruary 1993 on board a Cessna Citation aircraft up to 12.5 km altitude, during the first Stratosphere-Troposphere Experiment by Aircraft Meas urements (STREAM) campaign. Strong variations in the concentrations, d istributions, and ratios of these trace gases were found from the maxi mum altitude down to the tropopause. Close to the tropopause, vortex a ir was present with relatively low N2O concentrations. The observed N2 O-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 O-3 concentrations relative to N2O, indicating enhanced O-3 loss by chemical reactions in volving stratospheric particles. Based on trajectory calculations and assuming a potential temperature cooling rate of 0.6 K d(-1), we estim ate an O-3 loss of 4-7 ppbv d-1 (0.9-1.2% d-1), in the Arctic lover st ratosphere for the period January-February 1993. Air parcels originati ng from middle latitudes, containing relatively low O-3 and N2O concen trations, may have originated from the vortex earlier in the winter. I n addition, the results also show high HNO3 concentrations relative to O-3 and N2O. Air parcels originating from high latitudes may have bee n enriched in HNO3 by sedimentation and evaporation of nitric acid con taining particles, which would explain the relatively high HNO3 concen trations and HNO3/O-3 ratios measured. Heterogeneous chemistry on sulf uric acid particles, probably enhanced in concentration by gravitation al settling of the Pinatubo aerosol, is the most plausible explanation for the observed high HNO3 concentrations relative to N2O in air parc els originating from midlatitudes.