Use of criteria pollutants, active and passive mercury sampling, and receptor modeling to understand the chemical forms of gaseous oxidized mercury in Florida

Abstract

Abstract. The highest mercury (Hg) wet deposition in the United States (US) occurs along the Gulf of Mexico, and in the southern and central Mississippi River Valley. Gaseous oxidized Hg (GOM) is thought to be a major contributor due to its high water solubility and reactivity. Therefore, it is critical to understand the concentrations, potential for wet and dry deposition, and GOM compounds present in the air. Concentrations and dry deposition fluxes of GOM were measured at Outlying Landing Field (OLF), Florida, using a Tekran® 2537/1130/1135, and active and passive samplers using cation-exchange and nylon membranes. Relationships with Tekran® derived data must be interpreted with caution, since GOM concentrations can be biased low depending on the chemical compounds in air, and interferences with water vapor and ozone. Only gaseous elemental Hg and GOM are discussed here since the PBM measurement uncertainties are higher. Criteria air pollutants were concurrently measured and Tekran® data were assessed along with these using Principal Component Analysis to identify associations among air pollutants. Based on the diel pattern, high GOM concentrations at this site were associated with fossil fuel combustion and gas phase oxidation during the day, and gas phase oxidation and transport in the free troposphere. The ratio of GEM/CO at OLF (0.008 ng m−3 ppbv−1) was much higher than the numbers reported for the Western United States and central New York for domestic emissions or biomass burning (0.001 ng m−3 ppbv−1), which we suggest is indicative of a marine boundary layer source. Results from nylon membranes with thermal desorption analyses suggest five potential GOM compounds exist in this area, including HgBr2, HgO, Hg(NO3)2, HgSO4, and an unknown compound. This indicates that the site is influenced by different gaseous phase reactions and sources. A~high GOM event related to high CO but average SO2 suggests the air parcels moved from the free troposphere and across Arkansas, Mississippi, and Alabama at low elevation (< 300 m) using back trajectory analysis. We hypothesize this is due to subsidence of Hg containing air from the free troposphere. It is difficult to fully understand GOM dry deposition processes without knowing the actual GOM compounds, and their corresponding physicochemical properties, such as the Henry's Law constant. Overall, measured GOM dry deposition at this site ranged from 4–23% of total Hg wet deposition. The Aerohead sampling system for dry deposition captures primarily GOM since it would only collect fine particulate bound Hg by way of diffusion.