Scientific Rationale

The Arctic is the region that experiences the largest and most rapid climate change on earth. In addition to the increase in long-lived greenhouse gases, short lived climate pollutants such as ozone and aerosols play an important role in Arctic warming, but many uncertainties about processes and impacts remain with large differences between models and observations of Arctic tropospheric chemistry and composition. Substantial ozone depletion in the Arctic lowermost stratosphere has been observed in several recent winter/spring seasons as a result of anthropogenic ozone depleting substances, not well captured by current earth system models. The expected Arctic ozone recovery in the coming decades will exert a positive radiative forcing, the magnitude of which is still uncertain.

Knowledge on the vertical profile and the latitudinal distribution of trace gas and aerosol changes is critical for assessing the associated radiative forcing. Additional aircraft campaigns providing vertically resolved features of short-lived climate pollutants should be given a top priority for future studies. Both emission sources outside the Arctic and within the Arctic contribute to Arctic pollution. Climate pollutants from mid-latitude sources are primarily transported into the Arctic mid- and upper troposphere due to uplift along sloping isentropic surfaces, impacting on free tropospheric ozone. Emission within the Arctic are largely confined to the Arctic lower troposphere at low potential temperatures (“Polar Dome”). Ozone in the Arctic boundary layer during springtime is strongly influenced by “bromine explosion” and ozone depletion events. Despite progress in understanding of the chemical processes involved in the bromine explosion events, there is still little knowledge on their larger scale impacts.

The HALO campaign POLSTRACC was successfully conducted in the unusually cold Arctic winter 2015/16 and provided important information on transport and chemistry under these conditions. However, observations during the unusually cold stratospheric Arctic winter 2015/16 are not necessarily representative and call for a follow up study in another winter to capture inter-annual variability and changes. In addition to the different year (different meteorological conditions, 7-9 years after POLSTRACC), modified measurement capabilities and refined flight strategies to meet the focus of the current proposal will expand the observational base of the 2015/16 POLSTRACC campaign with a stronger focus on composition changes from the lower stratosphere to the mid troposphere at high Arctic latitudes.

In addition to probing the Arctic during the late winter, we suggest to extend the campaign into later spring (e.g. early May) and mid-latitudes to investigate the impact of stratospheric ozone and ozone anomalies onto upper tropospheric and lower stratospheric ozone in the mid latitude lower stratosphere and upper troposphere during spring time. It has been shown that interannual variability of stratospheric ozone are the largest cause for interannual variability in southern hemispheric upper tropospheric ozone. We suggest to provide a database for investigation of this linkage in the northern hemisphere, by extending the measurements into spring and also by using ozone sonde stations and satellite data.