Global simulation of atmospheric mercury concentrations and deposition fluxes

Results from a numerical model of the global emissions, transport, chemistr y, and deposition of mercury (Hg) in the atmosphere are presented. Hg tin t he form of Hg(0) and Hg(LT)) is emitted into the atmosphere from natural an d anthropogenic sources (estimated to be 4000 and 2100 Mg yr(-1), respectiv ely). It is distributed between gaseous, aqueous and particulate phases. Re moval of Hg from the atmosphere occurs via dry deposition and wet depositio n, which are calculated by the model to be 3300 and 2800 Mg yr(-1), respect ively. Deposition on land surfaces accounts for 47% of total global deposit ion. The simulated Hg ambient surface concentrations and deposition fluxes to the Earth's surface are consistent with available observations. Observed spatial and seasonal trends are reproduced by the model, although larger s patial variations are observed in Hg(0) surface concentrations than are pre dicted by the model. The calculated atmospheric residence time of Hg is sim ilar to 1.7 years. Chemical transformations between Hg(0) and Hg(II) have a strong influence on Hg deposition patterns because Hg(II) is removed faste r than Hg(0). Oxidation of Hg(0) to Hg(II) occurs primarily in the gas phas e, whereas Hg(II) reduction to Hg(0) occurs solely in the aqueous phase. Ou r model results indicated that in the absence of the aqueous reactions the atmospheric residence time of Hg is reduced to 1.2 from 1.7 years and the H g surface concentration is similar to 25% lower because of the absence of t he Hg(II) reduction pathway. This result suggests that aqueous chemistry is an essential component of the atmospheric cycling of Hg.