| Summary: | The importance of reactive halogen chemistry in our atmosphere has become evident throughout the last decades. The discovery of the chemical reactions leading to the stratospheric ozone hole over the South Pole in spring was followed by the exploration of the Arctic tropospheric ozone holes developing in early spring. The effects of halogen chemistry are not limited to polar regions. The largest chlorine and bromine source to the atmosphere is the sea through sea salt aerosol, thus reactive halogen chemistry is expected to have an influence on global tropospheric chemistry, too. Bromine can react through two major pathways, both leading to ozone loss: firstly, one cycle of bromine release from the aerosol is an autocatalytic ozone destruction cycle producing two Br radicals while destroying one ozone molecule and losing one Br radical. Secondly, in a gas phase cycle BrO, formed by reaction of Br with O 3 , reacts with HO 2 leading to HOBr, which is photolysed back into OH and Br and restarts the ozone destruction cycle. The latter pathway does not only reduce ozone but also influences the oxidising capacity of the atmosphere by transferring HO 2 back to OH. In addition, bromine chemistry enhances the loss of nitrogen oxides to the aerosol by formation of BrNO 3 . For the first time a global model study of halogen chemistry in the marine boundary layer was accomplished, using a prognostically calculated aerosol distribution as well as calculating explicitly halogen release from the aerosol. The general circulation model ECHAM5 is used, linked to the Modular Earth Submodel SYstem (MESSy). The halogen chemistry mechanism including gas and aerosol phase reactions was developed within this thesis work and implemented as a submodel of MESSy. The MESSy submodel M7 - a seven modal aerosol representation - provides the prognostically calculated aerosol distribution. First results show pronounced maxima of reactive halogen species (RHS) in the mid-latitude marine boundary layer in the northern hemisphere, reaching more ...
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