Global
models suggest BrHgONO to be the major Hg(II) species initially
formed in atmospheric oxidation of Hg(0) in most of the atmosphere,
but its atmospheric fate has not been previously investigated. In
the present work, we use quantum chemistry to predict that BrHgONO
photolysis produces the thermally stable radical BrHgO•. Subsequently, BrHgO• may react with NO2 to form thermally stable BrHgONO2, or with NO to re-form
BrHgONO. Additionally, BrHgO• abstracts hydrogen
atoms from CH4 and C2H6 with higher
rate constants than does •OH, producing a stable
BrHgOH molecule. Because BrHgO• can abstract hydrogen
atoms from sp3-hybridized carbons on many organic compounds,
we expect production of BrHgOH to dominate globally, although formation
of BrHgONO and BrHgONO2 may compete in urban regions. In
the absence of experimental data on the kinetics and fate of BrHgONO
and BrHgO•, we aim to guide modelers and other scientists
in their search for Hg(II) compounds in the atmosphere.
We use computational chemistry to determine the rate constants and product yields for the reactions of BrHg˙ with the atmospherically abundant radicals NO and HOO. The reactants, products, and well-defined transition states are characterized using CCSD(T) with large basis sets. The potential energy profiles for the barrierless addition of HOO and NO to BrHg˙ are characterized using CASPT2 and RHF-CCSDT, and the rate constants are computed as a function of temperature and pressure using variational transition state theory and master equation simulations. The calculated rate constant for the addition of NO to BrHg˙ is larger than that for the addition of HOO by a factor of up to two under atmospheric conditions. For the reaction of HOO with BrHg˙ the addition reaction entirely dominates competing HOO + BrHg˙ reaction channels. The addition of NO to BrHg˙ initially produces both BrHgNO and BrHgONO, but after a few seconds under atmospheric conditions the sole product is syn-BrHgONO. A previously unsuspected reaction channel for BrHg˙ + NO competes with the addition to yield Hg + BrNO. This reaction reduces the mercury oxidation state in BrHg˙ from Hg(i) to Hg(0) and slows the atmospheric oxidation of Hg(0). While the rate constant for this reduction channel is not well-constrained by the present calculations, it may be as much as 18% as large as the oxidation channel under some atmospheric conditions. As no experimental kinetic or product yield data are available for the reactions studied here, this work will provide guidance for atmospheric modelers and experimental kineticists.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.