BrHgO^• + C₂H₄ and BrHgO^• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Prereactive Complexes and Bifurcation

Abstract

Models suggest BrHgONO to be the major Hg­(II) species formed in the global oxidation of Hg(0), and BrHgONO undergoes rapid photolysis to produce the thermally stable radical BrHgO•. We previously used quantum chemistry to demonstrate that BrHgO• can, like •OH radicals, readily abstract hydrogen atoms from sp3-hybridized carbon atoms as well as add to NO and NO2. In the present work, we reveal that BrHgO• can also add to C2H4 to form BrHgOCH2CH2 •, although this addition appears to proceed with a lower rate constant than the analogous addition of •OH to C2H4. Additionally, BrHgO• can readily react with HCHO in two different ways: either by addition to carbon or by abstraction of a hydrogen atom. The minimum energy path for the BrHgO• + HCHO reaction bifurcates, forming two prereactive complexes, each of which passes over a separate transition state to form different products. Rate constants computed using Master Equation simulations indicate that hydrogen abstraction dominates over addition at atmospheric temperatures (200 K ≤ T ≤ 333 K) and pressures (0.01 atm ≤ P ≤ 1 atm). Subsequently, we compute the atmospheric fate of BrHgO• in a variety of air masses and find that BrHgOH formation via hydrogen abstraction will be the predominant fate (∼70–99%), with major competition (∼20%) coming from addition to NO and NO2 in polluted urban regions and stratospheric air. Given the absence of either field data on the identity of Hg­(II) compounds or experimental data on the kinetics of BrHgO• reactions, the present manuscript should provide guidance to a range of scientists studying atmospheric mercury.

BrHgO• + CO: Analogue of OH + CO and Reduction Path for Hg(II) in the Atmosphere

“…BrHg • produced in the reaction BrHgO • + CO → BrHg • + CO 2 can dissociate in competition with bimolecular reactions to make Hg­(II) compounds. , The larger the fraction of BrHgO • reacting with CO, the longer the residence time of mercury in the atmosphere and the greater the extent of mercury deposition as Hg(0) rather than Hg­(II) compounds. Previous work by Lam et al mapped out most competing types of reactions of BrHgO • . , Scheme depicts their results together with the findings of this work. BrHgO • is very effective at abstracting hydrogen atoms from sp-hybridized carbon atoms, and this reaction dominated the fate of BrHgO • in the analysis of Lam et al The model reaction was computed to proceed with a pseudo-first-order rate constant (at [CH 4 ] = 1.85 ppmv) of 11 s –1 at 298 K and 1 atm.…”

“…Previous work by Lam et al mapped out most competing types of reactions of BrHgO • . 22,24 Scheme 1 depicts their results together with the findings of this work. BrHgO • is very effective at abstracting hydrogen atoms from sp 3 hybridized carbon atoms, and this reaction dominated the fate of BrHgO • in the analysis of Lam et al The model reaction…”

“…BrHgO • is stable with respect to thermal dissociation. 23 Lam et al 22,24 further showed that the reactivity of BrHgO • in the atmosphere mimics that of the ubiquitous OH radical. Specifically, they found that, similar to OH, BrHgO • efficiently abstracts hydrogen atoms from alkanes, adds to sp 2 -hybridized carbon atoms of alkenes, and forms bonds with NO and NO 2 .…”

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury

“…Only for syn ‐HgBrONO, recent computational work indicated that dissociation might occur rapidly in the lower atmosphere giving rise to HgBrO. This radical would further react with atmospheric trace gases, such as NO, NO 2 , and volatile organic compounds, to finally yield HgBrOH . However, in order to obtain a comprehensive and quantitative description of the atmospheric Hg chemical mechanism, the spectral properties and photodissociation reaction products of syn ‐HgBrONO, HgBrOOH, HgBrO, and HgBrOH need to be known, which are currently unexplored.…”

Exchange Reactions Alter Molecular Speciation of Gaseous Oxidized Mercury

“…We now know that the oxidation of GEM is initiated by free radicals, most likely by atomic bromine, as shown in experimental, , theoretical, − and modeling , studies. The resulting HgBr radical is relatively short-lived and must be stabilized via reactions with atmospheric species that are sufficiently reactive yet abundant, such as NO 2 and HO 2 , forming gaseous oxidized mercury (GOM) molecules like BrHgONO and BrHgOOH. ,− Recently, the first experimental study of the HgBr + NO 2 reaction kinetics has been reported, and future experiments must focus on identifying reaction intermediates and products . Meanwhile, the major focus of field studies has been to improve the limit of detection, accuracy, and temporal resolution of methods for the quantification of different forms of mercury in the atmosphere .…”