Abstract
Mercury is a toxic substance that is ubiquitous in the environment. In the atmosphere mercury exists mainly in the form of gaseous elemental mercury (GEM). Deposition is dominated by oxidized mercury species although they make up for only 1% of the total mercury in the atmosphere. The situation in the aquatic environment is inverse. Here, mercury exists mainly in its oxidized state HgII. Due to photolysis and biological activity mercury in the Ocean is reduced to dissolved elemental mercury (DEM). As mercury is constantly cycling between the ocean and the atmosphere it is important to include both compartments into a chemistry transport model in order to understand it’s environmental fate. For this study, we coupled the atmospheric chemistry transport system CMAQ to the three dimensional Eulerian ocean-ecosystem model ECOSMO. We implemented photolysis, chemical reactions, and biologically induced transformation for elemental, oxidized, and methylated mercury species into the ocean model. Based on wind speed and temperature elemental mercury is exchanged between the ocean and the atmosphere. The model was set up for a regional domain covering the North- and Baltic Sea region and was run for a period of 14 years from 1993 to 2005. The ocean model was evaluated using DEM observations from a series of six cruises (MNB = 0.21 MNE = 0.53). Furthermore, we compared model results with and without ocean coupling to GEM observations at 5 EMEP stations. We found, that the coupled model system is able to reproduce GEM peaks which the uncoupled CTM was missing. However, the effect was limited to stations in a vicinity of 100 km to the coast (e.g. at the EMEP station DE09 in Zingst the model bias was reduced from −0.11 to 0.02 for the year 2000 and from −0.10 to −0.03 for 2005). On average, atmospheric GEM concentrations were increased by 5% in the North and Baltic Sea region.
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