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Mercury is a toxic substance that is ubiquitous in the environment.In the atmosphere mercury exists in three forms: Gaseous Elemental Mercury(GEM),Gaseous Oxidized Mercury(GOM),and Particle Bound Mercury(PBM).GOM and PBM make up only 1 percent of the total.But deposition,which is the only sink for atmospheric mercury,is dominated by these two species.Therefore,oxidation processes are key to understand the behaviour of mercury in the environment.Yet,only few continuous measurements of GOM and PBM are performed.More importantly,there is almost no information on the vertical distribution of mercury.However,the vertical distribution is key to correctly simulate wet deposition.This model study is part of the European Union FP7 Research Project GMOS(Global Mercury Observation System).GMOS focuses on the improvement and validation of mercury models to assist establishing a global monitoring network and to support political decisions.In the course of this study the CTM Community Multiscale Air Quality model(CMAQ)was used to simulate transport,oxidation,and deposition of mercury using different chemical oxidation mechanisms and emission inventories.The model results are evaluated based on ground based long-term measurements of speciated mercury from the GMOS observation network and vertical profiles of GEM from the ETMEP-2(European Tropospheric Mercury Experiment)and the CARIBIC aircraft campaigns.Moreover,during the ETMEP-2 campaign,which took place in August 2013,measurements of GOM were performed in the PBL and the lower free troposphere.The capability of the CTM to reproduce the vertical structure of mercury concentrations,both GEM and GOM,are investigated and the influence of the meteorology,chemistry,and emissions are evaluated.