Mercury water-air exchange over the Upper St. Lawrence River and Lake Ontario

The paper presents an analysis of a set of data that includes both Hg profiles and modeled fluxes over water surface (using the two-layer model) and develops some insights regarding mercury water-air exchanges in Lake Ontario and the Upper St. Lawrence River (USLR). This paper presents the first in situ estimation of the water-air Hg gas exchange in Lake Ontario. It shows that water-air exchange fluxes over Lake Ontario and the Upper St. Lawrence River contributed to atmospheric mercury built-up over water bodies. TGM concentrations over Lake Ontario were significantly higher on the South shore than on the North shore, whereas the degree of saturation was higher on the North shore. Moreover TGM concentrations over water bodies were significantly higher than the reference station located on the ground surface (St. Anicet) indicated. TGM gradient measurements suggested that the South shore of Lake Ontario was mostly in evasion mode, whereas on the North shore and the Upper St. Lawrence River, bath evasion and deposition were observed. However, Hg gas evasion should be far larger than gas deposition. Hg fluxes were calculated through the two-layer model. Model calculation indicated Hg supersaturation (476-2163%) and Hg fluxes between similar to 0 and 9.28 ng/ m(2)/h (median 2.88 ng/m(2)/h). Hg gradients were weakly related to solar radiation, whereas the calculated Hg fluxes were not. Modeled Hg fluxes are related to Henry's law and wind speed. It is suggested that the two-layer model gives the order of magnitude of the Hg flux but cannot express adequately the fine structure of Hg water-air fluxes. Further research is needed to elucidate the fine structure of the Hg water-air gas exchange flux. This study points out the complexity of Hg water-air gas exchange flux processes over large lakes or rivers acid that Hg gas exchange is dynamic, changing in space and time.