Mercury Oxidation via Chlorine, Bromine, and Iodine under Atmospheric Conditions: Thermochemistry and Kinetics

Abstract

Emissions of gaseous mercury from combustion sources are the major source of Hg in the atmosphere and in environmental waters and soils. Reactions of Hg(o)(g) with halogens are of interest because they relate to mercury and ozone depletion events in the Antarctic and Arctic early spring ozone hole events, and the formation of Hg-halides (HgX2) is a method for removal of mercury from power generation systems. Thermochemistry and kinetics from published theoretical and experimental studies in the literature and from computational chemistry are utilized to compile a mechanism of the reactions considered as contributors to the formation of HgX2 (X = Cl, Br, I) to understand the reaction paths and mechanisms under atmospheric conditions. Elementary reaction mechanisms are assembled and evaluated using thermochemistry for all species and microscopic reversibility for all reactions. Temperature and pressure dependence is determined with quantum Rice Ramsperger Kassel (RRK) analysis for k(E) and master equation analysis for fall-off. We find that reactions of mercury with a small fraction of the reactor surface or initiation by low concentrations of halogen atoms is needed to explain the experimental conversion of Hg to HgX2 in the gas phase. The models do not replicate data from experiments that do not explicitly provide an atom source. The Hg insertion reaction into X2 (Hg + X2 → HgX2) that has been reported is further studied, and we find agreement with studies that report high barriers. The high barriers prevent this insertion path from explaining the experimental data on HgX2 formation and Hg conversion under atmospheric conditions. Mechanism studies with low initial concentrations of halogen radicals show significant conversion of Hg under the experimental conditions.

Speciated atmospheric mercury and sea–air exchange of gaseous mercury in the South China Sea

“…On the one hand, high wind speed may increase the levels of halogen atoms (Br and Cl etc.) and sea salt aerosols in the marine atmosphere, which in turn were favorable to the production of RGM and formation of Hg P 2.5 (Auzmendi-Murua et al, 2014); on the other hand, high wind speed was favorable to the removal of RGM and Hg P 2.5 in the atmosphere, this was probably the reason for lower RGM and Hg P 2.5 concentrations during 2527 September as compared to those observed during 2022 September (see Fig. 2).…”

“…Numerous previous studies have shown that Hg 0 in the marine boundary layer (MBL) can be rapidly oxidized to form RGM in situ (Hedgecock et al, 2003;Laurier et al, 2003;Sprovieri et al, 2003Sprovieri et al, , 2010Laurier and Mason, 2007;Soerensen et al, 2010a;Wang et al, 2015). Ozone and OH could potentially be important oxidants on aerosols (Ariya et al, 2015), while the reactive halogen species (e.g., Br, Cl and BrO, generating from sea salt aerosols) may be the dominant sources for the oxidation of Hg 0 in the MBL (Laurier et al, 2003;Sander et al, 2003;Holmes et al, 2006Holmes et al, , 2010Seigneur and Lohman, 2008;Auzmendi-Murua et al, 2014;Gratz et al, 2015;Steffen et al, 2015;Shah et al, 2016;Horowitz et al, 2017). The wet and dry deposition (direct or uptake by sea-salt aerosol) represents a major input of RGM and Hg P to the sea and ocean due to their special and unique characteristics (i.e., high reactivity and water solubility) (Lindberg and Stratton, 1998;Landis et al, 2002;Mason and Sheu, 2002;Holmes et al, 2009).…”

Atmospheric speciated mercury concentrations on an island between China and Korea: sources and transport pathways

“…A trailing effect is often observed, especially with a limited number of measurements or short sampling period, since PSCF evenly distributes weight along the path of trajectories so that PSCF results often identify areas upwind and downwind of real sources as a source area . However, it should be also noted that the marine boundary layer provides good conditions for active Hg oxidation reactions due to an abundance of oxidants (Auzmendi-Murua et al, 2014); therefore, the possibility of areas over the ocean being a GOM source should not be excluded. It should be noted that different temporal resolutions for trajectories (hourly) and concentrations (every 12 h) were used for GOM and PBM.…”

Response to anonymous referee #4

“…and proposed Y ≡ OH and Br as effective radicals to carry out the second stage of oxidation to Hg II . The Cl atom can also oxidize Hg 0 to produce HgCl (Balabanov and Peterson, 2003;Donohoue et al, 2005). found that a broad range of radical oxidants could oxidize HgBr and HgCl including Y ≡ NO 2 and HO 2 , the most abundant atmospheric radicals, as well as Y ≡ BrO, ClO, and Cl.…”