Formulation of bi-directional atmosphere-surface exchanges of elemental mercury

A formulation of estimating bi-directional air-surface exchange of elemental mercury (Hg0) for natural surfaces was developed. This approach calculates emission and dry deposition respectively for various types of land cover from meteorological data and Hg0 concentrations, and can be used to provide lower boundary conditions for regional/global atmospheric transport and deposition models. The emission from bare soil was modeled as a function of soil temperature. For plant canopies, the emission of Hg0 was formulated as a function of the rate of evaportranspiration and the Hg0 concentration in the surface soil water. A resistance model was used for dry deposition to land surfaces. The air–water exchange of Hg0 was parameterized with wind speed, whitecap coverage, and Hg0 concentration in air and water. Using this method, the mass transfer velocities and exchange fluxes of Hg0 for five ground cover types were calculated for a region in the northeast United States during a summer week of 1995. The calculated dry deposition velocities and net exchange rates were similar in magnitude to published data. The simulation results indicated that the emission from natural surfaces is comparable in magnitude with the total anthropogenic emission from the region. The simulation also illustrated some of the general characteristics of the air-surface exchange of Hg0 over different land covers. The average net emission from plant canopies were estimated as 22 ng m−2 h−1, in comparison with 2.6 ng m−2 h−1 for water surfaces. For plant canopies, both emission and deposition followed a diurnal cycle with higher values during daytime. Emissions were predominant during daytime, but not at night. The air-canopy exchange of Hg0 was also highly sensitive to the soil water deficit and surface wetness. For water surfaces, the rates of emission were always higher than that of dry deposition.