Results from a numerical model of the global emissions, transport, chemistry, and deposition of mercury (Hg) in the atmosphere are presented. Hg (in the form of Hg(0) and Hg(II)) is emitted into the atmosphere from natural and anthropogenic sources (estimated to be 4000 and 2100 Mg yr-1, respectively). It is distributed between gaseous, aqueous and particulate phases. Removal of Hg from the atmosphere occurs via dry deposition and wet deposition, which are calculated by the model to be 3300 and 2800 Mg yr-1, respectively. Deposition on land surfaces accounts for 47% of total global deposition. 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 spatial variations are observed in Hg(0) surface concentrations than are predicted by the model. The calculated atmospheric residence time of Hg is ~1.7 years. Chemical transformations between Hg(0) and Hg(II) have a strong influence on Hg deposition patterns because Hg(II) is removed faster than Hg(0). Oxidation of Hg(0) to Hg(II) occurs primarily in the gas phase, whereas Hg(II) reduction to Hg(0) occurs solely in the aqueous phase. Our 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 Hg surface concentration is ~25% lower because of the absence of the Hg(II) reduction pathway. This result suggests that aqueous chemistry is an essential component of the atmospheric cycling of Hg. ¿ 1999 American Geophysical Union