Coupling atmospheric mercury isotope ratios and meteorology to identify sources of mercury impacting a coastal urban-industrial region near Pensacola, Florida, USA

Identifying the anthropogenic and natural sources of mercury (Hg) emissions contributing to atmospheric mercury on local, regional, and global scales continues to be a grand challenge. The relative importance of various direct anthropogenic emissions of mercury, in addition to natural geologic sources and reemission of previously released and deposited mercury, differs regionally and temporally. In this study, we used local-scale, mesoscale, and synoptic-scale meteorological analysis to couple the isotopic composition of ambient atmospheric mercury with potential sources of mercury contributing to a coastal urban-industrial setting near a coal-fired power plant in Pensacola, Florida, USA. We were able to broadly discern four influences on the isotopic composition of ambient atmospheric mercury impacting this coastal urban-industrial region: (1) local to regional urban-industrial anthropogenic emissions (mean delta Hg-202=0.44 +/- 0.05 parts per thousand, 1SD, n=3), (2) marine-influenced sources derived from the Gulf of Mexico (mean delta Hg-202=0.77 +/- 0.15 parts per thousand, 1SD, n=4), (3) continental sources associated with north-northwesterly flows from within the planetary boundary layer (mean delta Hg-202=0.65 +/- 0.04 parts per thousand, 1SD, n=3), and (4) continental sources associated with north-northeasterly flows at higher altitudes (i.e., 2000m above ground level; mean delta Hg-202=1.10 +/- 0.21 parts per thousand, 1SD, n=8). Overall, these data, in conjunction with previous studies, suggest that the background global atmospheric mercury pool is characterized by moderately positive delta Hg-202 values; that urban-industrial emissions drive the isotopic composition of ambient atmospheric mercury toward lower delta Hg-202 values; and that air-surface exchange dynamics across vegetation and soils of terrestrial ecosystems drive the isotopic composition of ambient atmospheric mercury toward higher positive delta Hg-202 values. The data further suggest that mass-independent fractionation (MIF) of both even-mass- and odd-mass-number isotopes, likely generated by photochemical reactions in the atmosphere or during reemission from terrestrial and aquatic ecosystems, can be obscured by mixing with anthropogenic emissions having different MIF signatures.