AbstractAtmospheric deposition is the most important pathway by which Hg reaches marine ecosystems, where it can be methylated and enter the base of food chain. The deposition, the transport and chemical interactions of atmospheric Hg has been simulated over Europe for the year 2013 in the framework of the Global Mercury Observation System (GMOS) project, performing 14 different model sensitivity tests using two high resolution three-dimensional Chemical Transport Models (CTMs), varying the anthropogenic emissions data sets, atmospheric Br input fields, the Hg oxidation schemes and the modelling domain boundary condition input. Sensitivity simulation results were compared with observations from 28 monitoring sites in Europe, to assess model performance and particularly to analyse the influence of anthropogenic emission speciation and the Hg0(g) atmospheric oxidation mechanism. The contribution of anthropogenic Hg emissions, their speciation and vertical distribution is crucial to the simulated concentration and deposition fields, as is also the choice of Hg0(g) oxidation pathway. The areas most sensitive to changes in Hg emission speciation and the emission vertical distribution are those near major sources, but also the Aegean and the Black Seas, the English Channel, the Skagerrak Strait and the North German coast. Considerable influence was found also evident over the Mediterranean, the North and Baltic Sea, some influence is seen over continental Europe, while this difference is least over the north-western part of the modelling domain, which includes the Norwegian Sea and Iceland. The Br oxidation pathway produces more HgII(g) in the lower model levels, but overall wet deposition is lower in comparison to the simulations which employ an O3/OH oxidation mechanism. The necessity to perform continuous measurements of speciated Hg, to investigate the local impacts of Hg emissions and deposition, as well as interactions dependent on land use and vegetation, forests, peat bogs etc. is highlighted in this study.