Vortexwide denitrification of the Arctic polar stratosphere in winter 1999/2000 determined by remote observations

Abstract

Denitrification has been studied using measurements of stratospheric HNO 3 and N 2 O by the Airborne Submillimeter Radiometer (ASUR), operated on board the NASA DC‐8 during SOLVE/THESEO 2000. Lidar measurements taken on board the same aircraft have been used to distinguish between temporary uptake of HNO 3 in polar stratospheric clouds (PSCs) and denitrification events. To derive an NO y budget, ClNO 3 data by balloonborne and ground‐based Fourier transform infrared measurements and a model estimate of NO x + 2N 2 O 5 have been considered. The HNO 3 profiles of sporadic ASUR measurements without PSC coverage in January suggest that denitrification had started in the vortex core region by then. Vortexwide denitrification was found in mid‐March 2000. Corrected for diabatic descent using the N 2 O measurements, a vortex‐averaged NO y deficit between 1.2 ± 0.9 ppb at about 16 km altitude and 5.3 ± 2.7 ppb at about 20.5 km altitude was derived compared to December 1999, based on an observed decrease in HNO 3 between 2.2 and 3.5 ppb during this time period. A shift in the NO y partitioning from HNO 3 toward ClNO 3 of about 0.4 to 0.7 ppb was observed in mid‐March compared to December, indicating that chlorine deactivation was occurring. Comparisons with the SLIMCAT three‐dimensional chemical transport model applying denitrification schemes based on ice and nitric acid trihydrate particles in equilibrium, respectively, reveal agreement within the error bars at higher altitudes (∼19 km) but show discrepancies at lower altitudes (∼16 km). It is suggested that more sophisticated denitrification schemes are needed to generally describe denitrification processes.