Diurnal and seasonal variation of mercury species in the Southeast US

Analysis of concentrations of mercury species, from three different SEARCH locations in the Southeast US, shows significant differences in diurnal and seasonal patterns. The coastal rural, inland suburban and inland urban SEARCH sites all show a distinct, average diurnal pattern of divalent reactive gaseous mercury (RGM) peaking at the mid day hours. However, the peak value of RGM is quite different among these sites. Even though the average diurnal patterns and magnitudes of Hg0 are very similar for both the coastal rural and inland rural sites, the mid-day peak value of average RGM at the inland rural site is twice that of the coastal rural site. The differences are even more pronounced at the inland urban site, where the average diurnal RGM amplitude is approximately ten times higher than the coastal site. While episodic signature of halogen chemistry is found in daily observations, the above discussed differences in average patterns of RGM is contradictory to the marine influence hypothesis where halogen species of marine origin are the dominant oxidizer of mercury, with other oxidants having only negligible impact on ambient mercury concentration. Analysis of SEARCH measurements of ambient mercury data suggests that, for the Southeast United States, oxidants other than halogen species dominate the oxidation of mercury at the inland locations as well as at the coastal sites. In addition to the RGM diurnal variation, diurnal variation of Hg0 and particle bound mercury (HgP) both show patterns that deviate from those reported in prior studies, indicating that there are other physical and chemical processes to be considered.

Continuous multi-year observation data of mercury species, along with other pollutants and meteorological variables, are analyzed to examine how the various processes involved in modulating the seasonal variation of the atmospheric mercury cycle in locations of different environments in the Southeast US. The examined processes include emission, dry and wet deposition, boundary layer dispersion, and chemical transformation.