Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere in winter 2015/2016: In-situ observations of nitrification, denitrification and particulate nitrate

The Arctic winter 2015/2016 was characterized by extremely low temperatures in the stratosphere and by a very strong polar vortex, accompanied by extended fields of Polar Stratospheric Clouds.  During this winter, aircraft-based measurements were carried out with the research aircraft HALO (High Altitude and Long-Range Research Aircraft) from Kiruna/Sweden and Oberpfaffenhofen/Germany.

Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. Tracer-tracer correlations were applied to study the vertical redistribution of gas-phase total reactive nitrogen.  The extended observation period from December to March provided the opportunity to study the changing distribution of reactive nitrogen in the lowermost Arctic stratosphere during the course of the winter. In early winter, during December, the lowermost Arctic stratosphere did not show any indications for disturbed conditions.

The situation changed during the observational period in January and February. Tracer-tracer correlations showed elevated levels of total reactive nitrogen of up to 6 nmol/mol. These observations could be interpreted by evaporation of polar stratospheric particles falling down from the stratosphere above and leading to a nitrification of the lowermost stratosphere. During some periods up to 60 % of the observed total reactive nitrogen can be attributed to evaporating particles. The observation of gas phase nitrification was accompanied by the occurrence of particulate nitrate in extended regions at altitudes between about 10 and 14 km. Usually, the occurrence of particulate nitrate is rare at such altitudes. The diameter of these particles was estimated to range between about 9 and 18 µm.

During the late-winter observation period, no indications for polar stratospheric cloud particles at flight altitude were found. However, extended regions with elevated gas-phase concentrations of total reactive nitrogen were still observed. In late winter, the subsidence of air masses from the polar vortex became increasingly important for the distribution of total reactive nitrogen in the lowermost stratosphere. Air masses with substantial denitrification of up to 5 nmol/mol were observed. In these cases, up to 50 % of the undisturbed total reactive nitrogen was missing. Concurrently lower ozone concentrations were observed, indicating destruction of ozone at higher altitudes.

Nitrification and denitrification of the lowermost stratosphere during the course of the winter are linked by heterogeneous processes in the above-lying stratosphere. Simulations with the CLaMS model confirm and complement the findings of the in-situ observations. They also suggest that the observations have been representative of the vortex-wide redistribution of total reactive nitrogen. The aircraft-based in-situ measurements provided a comprehensive picture of the temporal evolution of the distribution of total reactive nitrogen over the entire winter period 2015/2016.