Regional Trends of Isotopic Composition in Total Gaseous Mercury Across the United States

The dataset presented here represents the concentrations and stable isotopic composition of gaseous mercury (Hg) for 31 sites across the United States. This national-scale monitoring effort was performed by the National Atmospheric Deposition Program (NADP) and the USGS over a two-year period (March 2016 - May 2018). The network includes a highly diverse set of sites ranging from remote (e.g. Denali National Park, AK and Mauna Loa, HI) to highly urbanized (e.g. Bronx, NY and Boston, MA). Air samples were collected using a custom designed bulk air sampler, in which air was pumped at approximately 1 L/min through particulate and soda lime filters prior to passing through two quartz tubes containing gold coated glass beads where gaseous Hg was collected. Each sampling period was two weeks long. The Hg was thermally desorbed from the gold beads and collected in a liquid oxidizing trap. The total Hg concentration and isotopic composition was determined on this solution. The average total gaseous Hg concentration across sites was 1.23 ± 0.28 ng/m3, but higher concentrations were observed at sites closer to known Hg sources, such as Oak Ridge National Lab (6.53 ng/m3). For all sites, the average odd isotope mass independent fractionation (MIF, Δ199Hg), an indicator of photochemistry, was -0.20 ± 0.07‰ and had a range of -0.43 to 0.01 ‰. Mass dependent fractionation (MDF, δ202Hg), the commonly used isotope ratio for source tracking, was generally positive with a mean value of 0.45 ± 0.40‰, although intermittent negative MDF was also observed at some sites. Urban sites were consistently lighter in δ202Hg (0.34‰) compared to remote background sites (0.65‰), potentially indicating differences between emission sources. Though regional differences in odd MIF and MDF were small, we observed a trend showing enrichment of δ202Hg in the northeastern United States, from the Ohio River Valley to remote sites in northern Maine and Nova Scotia, suggesting a shift from localized emission sources to globally transported Hg across this region.