| Summary: | The chemical composition of the polar troposphere is influenced by reactive halogens such as Cl, Br and I atoms, and their oxides ClO, BrO and IO, which can also exert an influence on climate. I, Br and Cl are involved in catalytic reactions which lead to the near complete destruction of surface ozone in polar regions. In order to get a better knowledge of the levels and seasonal variability of these halogens, we have conducted two years of observations of two important halogens species (BrO and IO) in the Arctic. We built a Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument which has been adapted to the extreme Arctic environmental conditions and installed at Villum Research Station -Station Nord (Greenland), 81°36 N, 16°40W-, in April 2017. During the course of this thesis, we used the DOAS technique, based on the analysis of the narrowband features that trace gases have in their spectral absorptions, to retrieve concentration profiles of these reactive halogen species. Since the MAX-DOAS instrument collects scattered sunlight, it works continuously during sunlit periods, thereby allowing long-term analysis. This thesis reports the first long-term ground-based measurement series (years 2017 and 2018 are presented) of vertical concentration profiles of combined BrO and IO in the Arctic. BrO monthly daytime averages are measured up to 25 pptv in early spring and autumn, while values of about 5 pptv are reached in summer. However, IO, which is found to be ubiquitous during the sunlit period, shows a less pronounced seasonality with a background mixing ratio of about 1 pptv of IO in the boundary layer. We have estimated the ozone loss rate caused by the measured levels of halogens and found that Arctic iodine increases the O3 loss rate by ~30%, as compared to a simulation taking only bromine into account. This thesis also reports halogen oxide results from the first Antarctic Circumnavigation Expedition (ACE) in which we installed a MAX-DOAS instrument on board the research vessel ...
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