Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere in late winter 2024/2025: Comparison with the findings during the cold winter 2015/2016

Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding the processes controlling the ozone budget in the polar winter stratosphere. Observations in the lowermost stratosphere reflect heterogeneous processes in the stratosphere above, leading to denitrification and later to nitrification below the vortex.

In the late winter of 2025, aircraft measurements were carried out as part of the ASCCI mission (Arctic Springtime Chemistry Climate Investigations) using the HALO (High Altitude and Long-Range Research Aircraft) research aircraft from Kiruna/Sweden and Oberpfaffenhofen/Germany. Tracer-tracer correlations were used to investigate the vertical redistribution of gas-phase total reactive nitrogen.

The winter of 2024/2025 was characterized by low temperatures in the polar vortex at the beginning of the winter, which enabled the formation of polar stratospheric cloud (PSC) particles. In March, the polar vortex collapsed. The observations of total reactive nitrogen in the lowermost stratosphere showed a mixture of different fingerprints of Arctic nitrogen chemistry. Elevated levels of reactive nitrogen are indicative of the evaporation of sinking PSC particles from the middle stratosphere. On the other hand, the sinking air from the polar vortex in late winter can show considerable denitrification.

During the ASCCI field measurement campaign in March, periods of elevated reactive nitrogen concentrations alternated with periods when concentrations were lower than would be expected for undisturbed chemistry. In some cases, more than 40 % of the observed total reactive nitrogen could be attributed to evaporating PSC particles, while in other flights air masses with a deficit of about 30 % of total reactive nitrogen were measured. At the end of the observation period, air masses with undisturbed background concentrations were probed.

These results from 2025 are compared with those obtained during the POLSTRACC aircraft mission in winter 2025/2016. Both missions show similar behavior regarding the redistribution of total reactive nitrogen in the lowermost stratosphere. However, lower values were observed during the most recent mission. The present observations are also compared with CLaMS model simulations.