Use of a global model to understand speciated atmospheric mercury observations at five high-elevation sites

Abstract. Atmospheric mercury (Hg) measurements using the Tekran^® analytical system from five high-elevation sites (1400–3200 m elevation), one in Asia and four in the western US, were compiled over multiple seasons and years, and these data were compared with the GEOS-Chem global model. Mercury data consisted of gaseous elemental Hg (GEM) and "reactive Hg" (RM), which is a combination of the gaseous oxidized (GOM) and particulate bound (< 2.5 μm) (PBM) fractions as measured by the Tekran^® system. We used a subset of the observations by defining a "free tropospheric" (FT) data set by screening using measured water vapor mixing ratios. The oxidation scheme used by the GEOS-Chem model was varied between the standard run with Br oxidation and an alternative run with OH–O₃ oxidation. We used this model–measurement comparison to help interpret the spatio-temporal trends in, and relationships among, the Hg species and ancillary parameters, to understand better the sources and fate of atmospheric RM. The most salient feature of the data across sites, seen more in summer relative to spring, was that RM was negatively correlated with GEM and water vapor mixing ratios (WV) and positively correlated with ozone (O₃), both in the standard model and the observations, indicating that RM was formed in dry upper altitude air from the photo-oxidation of GEM. During a free tropospheric transport high RM event observed sequentially at three sites from Oregon to Nevada, the slope of the RM / GEM relationship at the westernmost site was −1020 ± 209 pg ng⁻¹, indicating near-quantitative GEM-to-RM photochemical conversion. An improved correlation between the observations and the model was seen when the model was run with the OH–O₃ oxidation scheme instead of the Br oxidation scheme. This simulation produced higher concentrations of RM and lower concentrations of GEM, especially at the desert sites in northwestern Nevada. This suggests that future work should investigate the effect of Br- and O₃-initiated gas-phase oxidation occurring simultaneously in the atmosphere, as well as aqueous and heterogeneous reactions to understand whether there are multiple global oxidants for GEM and hence multiple forms of RM in the atmosphere. If the chemical forms of RM were known, then the collection efficiency of the analytical method could be evaluated better.