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Abstract

Halogen radicals can drastically alter the polar atmospheric chemistry. This is made evident by a recurrent destruction of boundary layer ozone during polar springs called ozone depletion events (ODEs). ODEs are caused by enhanced concentrations of reactive halogens, in particular bromine monoxide (BrO) radicals. Measurements suggest that there are two distinct sets of environmental conditions favoring the emissions of bromine to the atmosphere, namely cold and stable meteorological conditions on one side and less stable conditions associated with low-pressure systems on the other. This thesis investigates the importance of these differing environmental conditions on a pan-Arctic scale by comparing TROPOMI satellite observations of BrO with the results of an meterology model coupled with atmospheric chemistry (WRF-CHEM) for the Arctic spring of 2019. For the retrieval of tropospheric BrO from satellite measurements an algorithm is developed which allows to assess the tropospheric partial column without reliance on external input. Compared to other retrieval algorithms, it enables the full utilization of TROPOMI’s high spatial resolution (7 × 3.5 km²) while also avoiding biases from the use of model data. Satellite observations are used to validate model assumptions. It was demonstrated that a bromine release mechanism from the snow-pack employed in numerous models is unfit to predict ODEs in early February at high solar zenith angles. Case studies demonstrated that the observed spatial patterns and large BrO columns observed during late polar spring in association with polar cyclones can be explained by the intrusion of bromine into the free troposphere. It was shown that the magnitude of bromine emissions from blowing snow in polar cyclones is likely overestimated. A seasonal dependence in the environmental conditions favoring bromine release was established. Calm meteorological conditions favor the occurrence of ODEs during early polar spring. During late March and April, ozone was identified as limiting factor for BrO formation and results indicated high wind speeds as favorable meteorological parameter for bromine release. A statistical analysis of spatial extent and shape of ODEs was conducted, showing a scale of 40 km to 1000 km for bromine enhanced air masses.