Gaseous Oxidized Mercury Dry Deposition Measurements in the Four Corners Area, U.S.A., after Large Power Plant Mercury Emission Reductions.

Gaseous oxidized mercury (GOM) dry deposition measurements using surrogate surface passive samplers were collected at six sites in the Four Corners area, U.S.A., for the two-year period August, 2017-August, 2019, after the implementation of large power plant mercury emission reductions across the U.S.A. Two-year baseline GOM dry deposition measurements at the same six sites in the Four Corners area, taken before the implementation of U.S.A. power plant mercury control regulations, were conducted earlier from August, 2009-August, 2011. The GOM dry deposition rate estimate decreased at the Four Corners area high elevation remote mountain site of Molas Pass, Colorado (3249 m asl) from 0.4 ng/mh for August, 2009-August, 2011 to 0.3 ng/mh for August, 2017-August, 2019. In contrast, GOM dry deposition rate estimates for the remaining five sites increased for August, 2017-August, 2019, ranging from 0.8-1.3 ng/mh, up from the August, 2009-August, 2011 range of 0.6-1.0 ng/mh. Comparisons of median GOM dry deposition values showed a statistically significant decrease of 17 ng/m at the Molas Pass site between August, 2009-August, 2011 and August, 2017-August, 2019, and a statistically significant increase of 66 ng/m and 64 ng/m, respectively, at the Mesa Verde National Park and Farmington Substation sites between August, 2009-August, 2011 and August, 2017-August, 2019. For the four years of GOM dry deposition data collected in the Four Corners area annual GOM dry deposition levels ranged from 2237 ng/myr (at the Molas Pass high elevation remote mountain site) to 11542 ng/myr (at the Mesa Verde National Park site), and the estimates were generally higher in magnitude in the spring and summer compared to the fall and winter. In light of the unexpected increases in GOM dry deposition rates at the non-remote sites, it is suggested that large regional wildfires and local anthropogenic mercury emission sources from cities and oil/gas production areas are possible notable contributors to the GOM dry deposition measurements collected in the Four Corners area.