Is oxidation of atmospheric mercury controlled by different mechanisms in the polluted continental boundary layer vs. remote marine boundary layer?

Abstract

Deposition of atmospheric mercury is of global concern, primarily due to health effects associated with efficient bioaccumulation of mercury in marine food webs. Although oxidation of gaseous elementary mercury (GEM), the major fraction of atmospheric mercury, is a critical stage in regulating atmospheric mercury deposition efficiency, this oxidation is currently not well-characterized, limiting modeling-based assessments of mercury in the environment. Based on a previous study, we hypothesized that the oxidation of GEM is predominantly controlled by multistep bromine-and chlorine-induced oxidation (MBCO) in the remote marine boundary layer (RMBL), and by photochemical smog oxidants, primarily ozone (O 3 ) and hydroxyl radical (OH), in the polluted continental boundary layer (PCBL). To test this hypothesis, we used the following analyses: (i) application of a newly developed criterion to evaluate the gaseous oxidized mercury (GOM)-O 3 association based on previous studies in the RMBL and PCBL; (ii) measurement-based box simulations of GEM oxidation in the RMBL and at a PCBL site; and (iii) measurement-based analysis of photochemical oxidation vs. other processes which potentially influence GOM. Our model simulations indicated that the MBCO mechanism can reproduce GOM levels in the RMBL, but not in the PCBL. Our data analysis suggested the important role of photochemical smog oxidants in GEM oxidation in the PCBL, potentially masked by the effect of relative humidity and entrainment of free tropospheric air.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

“…Br concentrations are highest near the tropical tropopause due to fast photolysis of BrO and low ozone and temperature. , The OH-initiated oxidation pathway contributes most to Hg 0 oxidation in the tropical free troposphere, as dissociation of HOHg I is fast at lower altitudes . It also dominates in the continental boundary layer, consistent with Gabay et al, shows the zonal distribution of…”

“…88,89 The OH-initiated oxidation pathway contributes most to Hg 0 oxidation in the tropical free troposphere, as dissociation of HOHg I is fast at lower altitudes. 6 It also dominates in the continental boundary layer, consistent with Gabay et al, 90 shows the zonal distribution of BrHg II OH and Hg II (OH) 2 are the main Hg II species initially formed from Hg 0 oxidation, but the Hg II speciation evolves as these species are processed by aerosol and cloud droplets to form Hg II P particles and Hg II X gas. We find that Hg II X (modeled as Hg II Cl 2 ) is the most abundant form of Hg II in the troposphere, comprising 49% of Hg II mass, while Hg II P comprises 22%.…”

Together, not separately, OH and O3 oxidize Hg(0) to Hg(II) in the atmosphere

“…Others have used the two-step model of Br-initiated oxidation put forth by Goodsite et al 12 and first included in modeling by Holmes et al 13 Subsequent comparisons of modeling results to field data made it hard to give up OH and/or O3 as an oxidant in continental regions. [59][60][61][62] The present work shows that OH and O3, together, not separately, can account for significant conversion of GEM to GOM.…”

“…In this manner, OH and O3, together, but not separately, could oxidize GEM to GOM. Demonstrating the feasibility of this mechanism for GEM oxidation would help resolve the discrepancy between modeling that requires OH and/or O3 to explain observations of GOM [59][60][61][62]…”

Evidence against Rapid Mercury Oxidation in Photochemical Smog

“…Also, as discussed in the introduction, evidence for rapid Hg 0 oxidation exists for some extreme particle pollution events . The source of high nighttime Hg II in Jerusalem, Israel, is also uncertain and may be due to rapid, local oxidation, though the oxidants responsible are unknown . All of these cases, however, involve either unusually high concentrations of known Hg 0 oxidants or the possibility of new, unexplored oxidation mechanisms.…”