Estimation of mercury loadings to Lake Ontario: Results from the Lake Ontario atmospheric deposition study (LOADS)

Atmospheric mercury (Hg) loadings to Lake Ontario were estimated using data measured at two land-based sites: Sterling, NY and Point Petre, Ont., as part of the Lake Ontario air deposition study (LOADS) between April 2002 and March 2003. These loadings were compared with those estimated using intensive data measured onboard the R/V Lake Guardian in April 2002, September 2002, and July 2003 (each approximately one week). Measured concentrations and modeled mass transfer coefficients of elemental mercury (Hg0), reactive gaseous mercury (RGM) and particulate mercury (Hg(p)) in air and total Hg in precipitation were incorporated into a total deposition model including wet deposition, air–water gas exchange and particle dry deposition. Urban/rural Hg concentration ratios were assumed based on literature values. Assuming that 10% of the lake was influenced by urban areas, the annual net Hg atmospheric loadings of wet deposition, net air–water gas exchange of Hg0 (deposition=300 kg yr−1 and emission=410 kg yr−1) and RGM, and Hg(p) dry deposition to Lake Ontario were estimated to be 170, −110, 68, and 20 kg, respectively, resulting in a net loading of 150 kg yr−1. Net Hg loadings were largest in the fall (46 kg) and smallest in the summer (20 kg). Hg0, wet, RGM and Hg(p) deposition contributed 55%, 30%, 12%, and 3.6% of the total Hg deposition, respectively. The net loading was found to be most sensitive to the assumed urban/rural concentration ratios, wind speed, DGM concentration and Hg0 transfer velocity. An increase in the influence of urban areas from 0% to 30% resulted in a 90% increase in the total loading demonstrating the complexity and non-linearity of the atmospheric deposition of mercury to Lake Ontario and the importance of quantifying the urban footprint.