Mercury speciation at a coastal site in the Northern Gulf of Mexico: Results from the grand bay intensive studies in summer 2010 and spring 2011

During two intensive studies in summer 2010 and spring 2011, measurements of mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM), trace chemical species including O₃, SO₂, CO, NO, NO_Y, and black carbon, and meteorological parameters were made at an Atmospheric Mercury Network (AMNet) site at the Grand Bay National Estuarine Research Reserve (NERR) in Moss Point, Mississippi. Surface measurements indicate that the mean mercury concentrations were 1.42 ± 0.12 ng·m-3 for GEM, 5.4 ± 10.2 pg·m^-3 for GOM, and 3.1 ± 1.9 pg·m^-3 for PBM during the summer 2010 intensive and 1.53 ± 0.11 ng·m^-3 for GEM, 5.3 ± 10.2 pg·m-3 for GOM, and 5.7 ± 6.2 pg·m^-3 for PBM during the spring 2011 intensive. Elevated daytime GOM levels (>20 pg·m^-3) were observed on a few days in each study and were usually associated with either elevated O₃ (>50 ppbv), BrO, and solar radiation or elevated SO₂ (>a few ppbv) but lower O₃ (̃20-40 ppbv). This behavior suggests two potential sources of GOM: photochemical oxidation of GEM and direct emissions of GOM from nearby local sources. Lack of correlation between GOM and Beryllium-7 (⁷Be) suggests little influence on surface GOM from downward mixing of GOM from the upper troposphere. These data were analyzed using the HYSPLIT back trajectory model and principal component analysis in order to develop source-receptor relationships for mercury species in this coastal environment. Trajectory frequency analysis shows that high GOM events were generally associated with high frequencies of the trajectories passing through the areas with high mercury emissions, while low GOM levels were largely associated the trajectories passing through relatively clean areas. Principal component analysis also reveals two main factors: direct emission and photochemical processes that were clustered with high GOM and PBM. This study indicates that the receptor site, which is located in a coastal environment of the Gulf of Mexico, experienced impacts from mercury sources that are both local and regional in nature.