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

摘要

Atmospheric mercury (Hg) measurements using theTekran? analytical system from fivehigh-elevation sites (1400–3200 m elevation), one in Asia and four in thewestern US, were compiled over multiple seasons and years, and these datawere compared with the GEOS-Chem global model. Mercury data consisted ofgaseous elemental Hg (GEM) and "reactive Hg" (RM), which is a combinationof the gaseous oxidized (GOM) and particulate bound(< 2.5 μm) (PBM) fractions as measured by theTekran? system. We used a subset of theobservations by defining a "free tropospheric" (FT) data set by screeningusing measured water vapor mixing ratios. The oxidation scheme used by theGEOS-Chem model was varied between the standard run with Br oxidation and analternative run with OH–O₃ oxidation. We used this model–measurementcomparison to help interpret the spatio-temporal trends in, and relationshipsamong, the Hg species and ancillary parameters, to understand better thesources and fate of atmospheric RM. The most salient feature of the dataacross sites, seen more in summer relative to spring, was that RM wasnegatively correlated with GEM and water vapor mixing ratios (WV) andpositively correlated with ozone (O₃), both in the standard model andthe observations, indicating that RM was formed in dry upper altitude airfrom the photo-oxidation of GEM. During a free tropospheric transport high RMevent observed sequentially at three sites from Oregon to Nevada, the slopeof the RM / GEM relationship at the westernmost site was?1020 ± 209 pg ng?1, indicating near-quantitative GEM-to-RMphotochemical conversion. An improved correlation between the observationsand the model was seen when the model was run with the OH–O₃ oxidationscheme instead of the Br oxidation scheme. This simulation produced higherconcentrations of RM and lower concentrations of GEM, especially at thedesert sites in northwestern Nevada. This suggests that future work shouldinvestigate the effect of Br- and O₃-initiated gas-phase oxidationoccurring simultaneously in the atmosphere, as well as aqueous andheterogeneous reactions to understand whether there are multiple globaloxidants for GEM and hence multiple forms of RM in the atmosphere. If thechemical forms of RM were known, then the collection efficiency of theanalytical method could be evaluated better.