Vortexwide Denitrification of the Arctic Polar Stratosphere in Winter 1999/2000 determined by Remote Observations

Armin Kleinböhl, Holger Bremer, Miriam von König, Harry Küllmann, and Klaus F. Künzi
Institute of Environmental Physics, University of Bremen, Bremen, Germany

Albert P. H. Goede
Space Research Organization of the Netherlands, Utrecht, The Netherlands, now at: Royal Netherlands Meteorological Institute, De Bilt, The Netherlands

Edward V. Browell and William B. Grant
NASA Langley Research Center, Hampton, VA, USA

Geoffrey C. Toon
Jet Propulsion Laboratory, Pasadena, CA, USA

Thomas Blumenstock
Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe and University of Karlsruhe, Karlsruhe, Germany

Bo Galle
Swedish Environmental Research Institute, Gothenburg, Sweden

Björn-Martin Sinnhuber and Stewart Davies
School of the Environment, University of Leeds, Leeds, UK

Abstract
Denitrification has been studied using measurements of stratospheric HNO₃ and N₂O by the Airborne SUbmillimeter Radiometer ASUR, operated on board the NASA DC-8 during THESEO 2000 / SOLVE. Lidar measurements taken on board the same aircraft have been used to distinguish between temporary uptake of HNO₃ in polar stratospheric clouds (PSCs) and denitrification events. To derive an NO_y budget, ClNO₃ data by balloon borne and ground-based Fourier transform infrared measurements and a model estimate of NO_x + 2 N₂O₅ have been considered. The HNO₃ 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 N₂O 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 HNO₃ between 2.2 and 3.5 ppb during this time period. A shift in the NO_y partitioning from HNO₃ towards ClNO₃ 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 3D 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.