Some contaminants are also deposited in a gaseous phase, particularly mercury. Various EU ELOISE (European Land-Ocean Interaction Studies) projects (http://www. eloisegroup.org) indicate that deposition of gaseous mercury can be an important transport pathway for this element from the atmosphere to terrestrial aquatic surfaces. The MERCYMS (An Integrated Approach to Assess the Mercury Cycle into the Mediterranean Basin) indicates that the deposition flux of gaseous Hg to the Mediterranean Sea is relatively constant (http://www.cs.iia.cnr.it). The Hg deposition velocity is governed by the atmospheric resistances in the case of deposition over the sea.
Deposition of gaseous Hg is more complicated over terrestrial surfaces. The stomata uptake would need to be taken into account in addition to the atmospheric resistances. The process is bidirectional as there are also fluxes of gaseous Hg from canopy, which are dependent on evapotranspiration rates. On the other hand, gaseous Hg is emitted from soil and the net flux needs to be estimated. The soil emissions depend on soil temperature and moisture, solar radiation, and Hg concentrations in the upper layer of soil.
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