Characterization and cycling of atmospheric mercury along the central US Gulf Coast

Concentrations of atmospheric Hg species, elemental Hg (Hg^∘), reactive gaseous Hg (RGM), and fine particulate Hg (Hg-PM_2.5) were measured at a coastal site near Weeks Bay, Alabama from April to August, 2005 and January to May, 2006. Mean concentrations of the species were 1.6 ± 0.3 ng m⁻³, 4.0 ± 7.5 pg m⁻³ and 2.7 ± 3.4 pg m⁻³, respectively. A strong diel pattern was observed for RGM (midday maximum concentrations were up to 92.7 pg m⁻³), but not for Hg^∘ or Hg-PM_2.5. Elevated RGM concentrations (>25 pg m⁻³) in April and May of 2005 correlated with elevated average daytime O₃ concentrations (>55 ppbv) and high light intensity (>500 W m⁻²). These conditions generally corresponded with mixed continental-Gulf and exclusively continental air mass trajectories. Generally lower, but still elevated, RGM peaks observed in August, 2005 and January–March, 2006 correlated significantly (p < 0.05) with peaks in SO₂ concentration and corresponded to periods of high light intensity and lower average daytime O₃ concentrations. During these times air masses were dominated by trajectories that originated over the continent. Elevated RGM concentrations likely resulted from photochemical oxidation of Hg^∘ by atmospheric oxidants. This process may have been enhanced in and by the near-shore environment relative to inland sites. The marine boundary layer itself was not found to be a significant source of RGM.

Size segregation determination, using a limited dataset from two different methods, suggested that a significant fraction of particulate Hg was bound to coarse particles (>2.5 μm). A potential source of the large fraction of coarse particulate Hg in the study area is sequestration of RGM within sea salt aerosols. The presence of rapidly depositing RGM and coarse particulate Hg may be important sources of Hg input along the Gulf Coast. However, the impact of these species on deposition rates is yet to be determined.