Atmospheric mercury near Salmon Falls Creek Reservoir in southern Idaho

Gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM) were measured over 2-week seasonal field campaigns near Salmon Falls Creek Reservoir in south-central Idaho from the summer of 2005 through the fall of 2006 and over the entire summer of 2006 using automated Tekran Hg analyzers. GEM, RGM, and particulate Hg (HgP) were also measured at a secondary site 90 km to the west in southwestern Idaho during the summer of 2006. The study was performed to characterize Hg air concentrations in the southern Idaho area for the first time, estimate Hg dry deposition rates, and investigate the source of observed elevated concentrations. High seasonal variability was observed with the highest GEM (1.91 ± 0.9 ng m−3) and RGM (8.1 ± 5.6 pg m−3) concentrations occurring in the summer and lower values in the winter (1.32 ± 0.3 ng m−3, 3.2 ± 2.9 pg m−3 for GEM, RGM, respectively). The summer-average HgP concentrations were generally below detection limit (0.6 ± 1 pg m−3). Seasonally averaged deposition velocities calculated using a resistance model were 0.034 ± 0.032, 0.043 ± 0.040, 0.00084 ± 0.0017 and 0.00036 ± 0.0011 cm s−1 for GEM (spring, summer, fall and winter, respectively) and 0.50 ± 0.39, 0.40 ± 0.31, 0.51 ± 0.43 and 0.76 ± 0.57 cm s−1 for RGM. The total annual RGM + GEM dry deposition estimate was calculated to be 11.9 ± 3.3 μg m−2, or about 2/3 of the total (wet + dry) deposition estimate for the area. Periodic elevated short-term GEM (2.2–12 ng m−3) and RGM (50–150 pg m−3) events were observed primarily during the warm seasons. Back-trajectory modeling and PSCF analysis indicate predominant source directions to the SE (western Utah, northeastern Nevada) and SW (north-central Nevada) with fewer inputs from the NW (southeastern Oregon and southwestern Idaho).