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An alternative, independent estimation of the kinetics of aquatic Hg(II) photochemical reduction featuring dissolved gaseous mercury (DGM) emission from water in consideration was obtained by using a mass balance box model. An interactive Excel spreadsheet was constructed to implement the model equations to yield the rate constants and the rates of the Hg(II) photoreduction. The model calculations used field-measured data of DGM paired with its emission flux coupled with the corresponding field sampling times. This data set came from a previous, separate, year-long field study conducted at a southern reservoir lake (Cane Creek Lake, Cookeville, Putnam County, TN). The mean value of the model-calculated rate constants (kDGM) of the Hg(II) photoreduction for the warm season (June–August) (4.5 fM h−1/pg L−1) is higher than that for the cold season (October–January) (2.2 fM h−1/pg L−1). The rate constants were found to be the highest (22.5 fM h−1/pg L−1) in August whereas the lowest (0.03 fM h−1/pg L−1) in January. The model-calculated rate constants are clearly higher in value than but comparable in order of magnitude to the published kinetic data. The model-calculated rates (rDGM) of the Hg(II) photoreduction are significantly higher, by one order of magnitude (102 vs. 101) than the apparent rates calculated using the same field DGM data without consideration of the Hg emission from the water. A sensitivity analysis of the model parameters points to a high sensitivity of Hg emission flux to the rate constant under modeled realistic environmental conditions. The initial Hg(II) concentration is also a sensitive model parameter under certain conditions. The results of our model study support the conclusion that DGM emission from water has a strong impact on the kinetics of aquatic Hg(II) photoreduction and the model calculation can provide an independent, valuable approach for estimating the kinetics of aquatic Hg(II) photoreduction.

A year-long field study of mercury (Hg) air/water exchange was conducted at a southern reservoir lake, Cane Creek Lake (Cookeville, TN, USA). The Hg air/water exchange fluxes and meteorological data including solar radiation (global solar radiation, Rg and ultraviolent radiation, UVA), water and air temperatures, relative humidity, and wind speed were collected to study the daily and seasonal trends of the Hg air/water exchange at the lake in relation to solar radiation and wind speed. The Hg exchange fluxes generally exhibited diurnal patterns with a rise in the morning, a peak around noontime, and a fall in the afternoon through the evening, closely following the change of solar radiation. There were cases that deviated from this general daily trend. The Hg emission fluxes were all below 3 ng m&minus, 2 h&minus, 1 with the daily mean fluxes &lt, 2 ng m&minus, 1. The fluxes in the summer (mean: 1.2 ng m&minus, 1) were higher than in the fall (mean: 0.6 ng m&minus, 1) and winter (mean: 0.7 ng m&minus, 1). The daily and seasonal trends of the Hg air/water exchange fluxes are similar to the trends of the changes of the dissolved gaseous mercury (DGM) concentrations in the lake observed in our previous study. Solar radiation was found to exert a primary control over the Hg air/water exchange, while wind speed appeared to have a secondary effect on the Hg exchange. The two-thin-film model was used to calculate Hg emission fluxes from the Cane Creek Lake water.