Observations of speciated atmospheric mercury at three sites in Nevada: Evidence for a free tropospheric source of reactive gaseous mercury

Air mercury (Hg) speciation was measured for 11 weeks (June-August 2007) at three sites simultaneously in Nevada, USA. Mean reactive gaseous Hg (RGM) concentrations were elevated at all sites relative to those reported for locations not directly influenced by known point sources. RGM concentrations at all sites displayed a regular diel pattern and were positively correlated with ozone (O-3) and negatively correlated with elemental Hg (Hg-0) and dew point temperature (T-dp). Superimposed on the diel changes were 2- to 7-day periods when RGM concentrations increased across all three sites, producing significant intersite correlations of RGM daily means (r = 0.53-0.76, p < 0.0001). During these periods, enhanced O-3 concentrations and lower Tdp were also observed. Back trajectories were applied to develop gridded frequency distribution (GFD) plots and determine trajectory residence times (TRT) in specific source boxes. The GFD for the upper-quartile RGM daily means at one site showed a contributing airflow regime from the high-altitude subtropics with little precipitation, while that developed for the lower-quartile RGM concentrations indicated predominantly lower-altitude westerly flow and precipitation. Daily mean TRT in a subtropical high-altitude source box (>2 km and <35 degrees N) explained a component of the daily mean RGM at two sites (r(2) = 0.37 and 0.27, p < 0.05). These observations indicate that long-range transport of RGM from the free troposphere is a potentially important component of Hg input to rural areas of the western United States.