Elemental mercury fluxes over a sub-alpine grassland determined with two micrometeorological methods

The exchange of gaseous elemental mercury (GEM) over a sub-alpine grassland in central Switzerland was measured over a full year. Seasonal and diurnal variability were measured with two micrometeorological techniques: the aerodynamic method and the modified Bowen ratio method. With these two methods mean deposition rates of 4.3 and 1.7ngm-2h-1 were calculated throughout the vegetation period. As no significant GEM exchange occurred under snow covered conditions the mean annual deposition fluxes reached 2.9 and 1.1ngm-2h-1, respectively. A fair weather period in October 2005 was selected to specifically study diurnal patterns of GEM exchange. During this time vertical day-time GEM gradients averaged 0.01ngm-4 and night-time gradients 0.07ngm-4, but no clear diurnal pattern of the GEM fluxes was observed. The measured gradients were very small, which entailed considerable uncertainties in the calculated GEM fluxes. However, the observed exchange pattern is verified by the agreement of the applied methods regarding the direction of the flux and the pronounced seasonal trends. Complementary measurements of total mercury in precipitation during three rain events revealed that dry deposition of GEM would account for 67% of a total annual input of 0.26gha-1 and would therefore constitute a major deposition pathway.