Contrasting Controls on the Diel Isotopic Variation of Hg0 at Two High Elevation Sites in the Western United States

Abstract

The atmosphere is a significant global reservoir for mercury (Hg) and its isotopic characterization is important to understand sources, distribution, and deposition of Hg to the Earth's surface. To better understand Hg isotope variability in the remote background atmosphere, we collected continuous 12-h Hg-0 samples for 1 week from two high elevation sites, Camp Davis, Wyoming (valley), and Mount Bachelor, Oregon (mountaintop). The samples collected at Camp Davis displayed strong diel variation in delta Hg-202 values of Hg-0, but not in Delta Hg-199 or Delta Hg-200 values. We attribute this pattern to nightly atmospheric inversions trapping Hg in the valley and the subsequent nighttime uptake of Hg by vegetation, which depletes Hg from the atmosphere. At Mount Bachelor, the samples displayed diel variation in both delta Hg-202 and Delta Hg-199, but not Delta Hg-200. We attribute this pattern to differences in the vertical distribution of Hg in the atmosphere as Mount Bachelor received free tropospheric air masses on certain nights during the sampling period. Near the end of the sampling period at Mount Bachelor, the observed diel pattern dissipated due to the influence of a nearby forest fire. The processes governing the Hg isotopic fractionation differ across sites depending on mixing, topography, and vegetation cover.