In situ measurements of total reactive nitrogen, total water, and aerosol in a polar stratospheric cloud in the Antarctic

Abstract

Measurements of total reactive nitrogen (NOy), total water, and aerosol were made as part of the Airborne Antarctic Ozone Experiment during August and September 1987. The measurements were made using instruments located on board the NASA ER‐2 aircraft, which conducted 12 flights over the Antarctic continent, reaching pressure altitudes of 20 km at 72°S latitude. The data presented here focus on a flight during which a polar stratospheric cloud (PSC) was encountered, containing concentrations of 0.8‐ to 2.6‐μm diameter aerosol particles greater than 1 cm−3. The temperatures in the cloud ranged as low as 184 K near 65‐mbar pressure, but they remained above the frost point of water ice, except for short intervals. From knowledge of the vapor pressures over nitric acid condensates, the appearance of aerosol above the background level is consistent with the formation of the trihydrate phase, HNO 3·3H 2 O. The anisokinetic feature of the NOy sample probe enhances the concentration of large aerosol particles in the inlet by a factor of ∼9. NOy levels above 20 parts per billion by volume (ppbv) observed in the PSC indicate that aerosol NOy species contribute substantially to the NOy signal. The amount of aerosol HNO 3·3H 2 O necessary to enhance the NOy signal to observed levels is calculated from aerosol impaction theory and is found to be in satisfactory agreement with the observed aerosol volume. In addition, using the saturation vapor pressures and an estimate of available HNO 3 in the cloud, the predicted volume of HNO 3·3H 2 O also shows satisfactory agreement with the directly measured aerosol volume. These results indicate that substantial aerosol volume containing HNO 3 and H 2 O is formed at temperatures above the frost point in the Antarctic stratosphere in the winter and spring months. Such aerosol formation is thought to be the prerequisite for the production of active chlorine in heterogeneous reactions and for the large‐scale removal of NOy through aerosol sedimentation.