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

Abstract

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.

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

“…The observed TM profiles are often similar to GEM profiles. Inside the PBL, oxidized mercury (OM) (here, OM is defined as the sum of all oxidized forms of mercury including model species GOM, PBM, and any mercury in the aqueous phase) concentrations are very low and mostly between 20 and 100 pg m −3 in Europe and North America, even in source regions with high anthropogenic emissions (e.g., coal-fired power plants) (Xu et al, 2016;2013;Gay et al, 2013;Tørseth et al, 2012;Prestbo and Gay, 2009;Weiss-Penzias et al, 2015). In China, PBM concentrations up to 1000 pg m 3 and GOM concentrations up to 100 pg m 3 have been observed, but no aircraft observations in the PBL and the LFT are available for this region : (Fu et al, 2015Sprovieri et al, 2016).…”

“…For atmosphere-terrestrial exchange, GEOS-Chem treats the evasion and dry deposition of GEM separately . Dry deposition is parameterized with a resistance-inseries scheme (Wesely, 1989). In addition, an effective GOM uptake by sea salt aerosol is also included over the ocean .…”

“…Chemical initial and boundary conditions were taken from the ECHMERIT model (Jung et al, 2009;De Simone et al, 2014) for Hg species, while boundary conditions for other chemical species were taken from MOZART-4 (Emmons et al, 2010). Dry deposition of gas-phase species is treated using the approach developed by Wesely (1989), multiplying the concentrations in the lowest model layer by the spatially and temporally varying deposition velocity, which is proportional to aerodynamic, sublayer, and surface resistances. The wet deposition of Hg species has been implemented by adding the Hg compounds to the scheme in WRF-Chem for gas and particulate convective transport and wet deposition.…”

Sensitivity model study of regional mercury dispersion in the atmosphere

“…Despite the theoretical doubts of significance of GEM oxidation under atmospheric conditions by O 3 and OH radical, atomic Br is of great relevance to the atmospheric oxidation of GEM is certain (Hynes et al, 2009;Subir et al, 2011Subir et al, , 2012Weiss-Penzias et al, 2015). In particular, for the HgBr * intermediate, Dibble et al (2012) have shown the potential importance of reactions with NO 2 , HOO, ClO and BrO.…”

Dependence of Daily Aerosol Wet Deposition on Precipitation at Appalachian Mountains Site in the United States

“…Observations in mountain areas can provide insight into: a) long-range transport of air pollutants; b) impact of acid rain on alpine forests and their response to pollution; c) fog scavenging and aerosol removal from direct cloud impact with terrain slopes; d) role of orographic convection and its effects on precipitation; e) heterogeneous ice nucleation of various aerosol types (Wrzesinsky and Klemm, 2000;Avila and Rodà, 2002;Igawa et al, 2002;Thalmann et al, 2002;Baumgardner et al, 2003;Burkard et al, 2003;Bayraktar and Turalioglu, 2005;Kim et al, 2006;Fischer et al, 2007;Budhavant et al, 2009;Calvo et al, 2012;Pawar et al, 2012;Hallar et al, 2013;Li et al, 2014;Conen et al, 2015;Weiss-Penzias et al, 2015;Sarkar et al, 2015).…”