Wet and Dry Deposition of Atmospheric Hg at Three Sites Across the Southeastern U.S.

Eric S. Edgerton
ARA, Inc., 410 Midenhall Way, Cary, NC, 27513


Atmospheric Hg inventories show that anthropogenic sources are important on local, regional and global scales. As important as the magnitude of emissions is their chemical form. Elemental Hg has a long atmospheric lifetime (months) and is therefore broadly dispersed from the point of emission. Reactive gaseous Hg (RGM) and particulate mercury (HgP), in contrast, are removed from the atmosphere much more quickly (days-weeks) and thus have more local to regional ranges of influence than elemental Hg. Long-term measurements of all three forms of Hg therefore are needed to understand the sources, transformations, impact of mitigation strategies and spatial distribution of atmospheric Hg. This presentation will: 1) describe measurement and data reduction protocols for atmospheric Hg; and 2) analyze 3 years of speciated Hg data from 3 sites across the southeastern U.S.; and) 3 compare measured wet deposition rates with inferred dry deposition rates. Elemental Hg (Hg0), reactive gaseous Hg (RGM) and fine particulate Hg (HgP) measurements are made at the three sites on an hourly time base using a Tekran 2537A, equipped with 1130 and 1135 speciation modules. The three sites are Yorkville, GA, a rural site 65 km NNW of Atlanta; OLF, FL, a suburban site 15 km NW of Pensacola, and BHM, AL an urban-industrial site in Birmingham. Analyzers are calibrated with Hg0 using an internal permeation device and checked for transmission efficiency by method of addition immediately upstream of the RGM denuder. Additional quality assurance includes comparison with manual samples and desorption of RGM-spiked denuders. Wet deposition is also measured according to Mercury Deposition Network (MDN) protocols and dry deposition is calculated for each species using resistance analogies. Ancillary measurements at both sites include, SO2, NOy, CO and surface meteorology.

Results show striking differences across sites. At the rural site, average Hg0, RGM and HgP concentrations are 1590 pg/m3, 2.7 pg/m3 and 1.2 pg/m3, respectively, while at the urban industrial site average Hg0, RGM and HgP concentrations are 2485 pg/m3, 55.7 pg/m3 and 10.3 pg/m3, respectively. Maximum concentrations of Hg0 and RGM at the urban-industrial site exceed 30,000 pg/m3 and 8500 pg/m3, respectively, indicating significant local sources, some of which have characteristic signatures of SO2, CO and NOy. All sites exhibit weak, but statistically significant, diurnal patterns for Hg0, strong diurnal patterns for RGM and no diurnal pattern for HgP. Limited measurements at the suburban site (20 km from the Gulf of Mexico) suggest there is several times more HgP above 2.5 µm aerodynamic diameter than below. Given that the “standard” Tekran configuration uses a 2.5 µm cutpoint inlet, this observation suggests that HgP concentrations and dry deposition rates are significantly underestimated. Measured wet deposition and calculated dry deposition are factors of about 1.5 and 4 higher at the urban-industrial site than the rural site; however, dry deposition estimates are very sensitive to assumptions regarding transfer rates for Hg0 and RGM. Further work is needed to address overall uncertainties in dry deposition calculations.