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, both evasion a...