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

Abstract

We present results of the first study of the reaction BrHgO • + CO → BrHg • + CO 2 , which constitutes a potentially important mercury reduction reaction in the atmosphere. We characterized the potential energy surface with CCSD(T)/CBS energies (with corrections for relativistic effects) at MP2 geometries. Master equation simulations were used to reveal the factors controlling the overall rate constant. Much of the potential energy surface mimics that for the ubiquitous OH + CO → H + CO 2 reaction, including the entrance channel and binding energies of intermediates. However, the BrHgOCO intermediate is much less stable than HOCO with respect to loss of CO 2 . This leads to ultrafast dissociation of BrHgOCO and prevents its stabilization in air (unlike HOCO). Because of the relatively high rate constant for BrHgO • + CO and the high abundance of CO throughout the troposphere, this reaction could dominate the atmospheric fate of BrHgO • . The BrHg • product of this reaction can dissociate to form Hg(0), and Hg(0) is transferred to ecosystems much more slowly than Hg(II) compounds. Therefore, this reaction could significantly slow the transfer of neurotoxic mercury from the atmosphere to ecosystems.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere

“…Furthermore, a recent experimental study has reported the reaction rate constant of HgBr + NO 2 to HgBrONO to be 3–11 times lower than that predicted by theory ( 35 ), and incorporated in our model. Also, a new theoretical study, not included in this work, proposes the reduction of HgBrO by reaction with CO (HgBrO + CO → HgBr + CO 2 ) ( 36 ). All of this implies that the model underestimation of Hg II observations would be even larger with the inclusion of the results from these two very recent studies.…”

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

“…1 Preliminary theoretical calculations by Saiz-Lopez et al 5 show that BrHg I may react rapidly with ozone to produce a BrHg II O radical, which can then be stabilized to nonradical Hg II forms by subsequent reactions. [2][3][4]20 The oxidation of Hg 0 by OH has been included in many models, 21−24 but its atmospheric relevance has been questioned because of the low stability of HOHg I . 17, 25 Dibble et al 6 recalculated the stability of HOHg I and found the OHinitiated oxidation pathway to be potentially more important than previously thought.…”

“…d We assume that the BrHg I +Z rate coefficients hold for HOHg I +Z and ClHg I +Z because of the similar bond energies and reactions pathways for the three species, 6,19 and that the BrHg II O+Z rate coefficients hold for HOHg II O+Z and ClHg II O+Z. e Khiri et al 4 calculated the range for the rate coefficient of the BrHg II O+CO → BrHg I +CO 2 reaction at two temperatures: (9.4−52) × 10 −12 cm 3 molec −1 s −1 at 298 K and (3.8−29) × 10 −12 cm 3 molec −1 s −1 at 220 K. We use the mean values at each temperature to determine the temperature-dependent rate coefficient. f We assume that the experimentally determined value of k 0 for the BrHg I +NO 2 reaction 53 holds for this set of reactions too.…”

Reproducing Arctic springtime tropospheric ozone and mercury depletion events in an outdoor mesocosm sea ice facility

“…General reaction schemes involved in bromine explosion events, ozone depletion events, and mercury depletion events in the Arctic during polar sunrise. The photochemical activation of gas-phase reactive bromine species (Br and BrO) produced from multi-phase reactions on the surface of the condensed phase causes the depletion of ozone and gaseous elemental mercury in the boundary layer air (based on Abbatt et al, 2012;Aguzzi and Rossi, 2002;Khiri et al, 2020;Saiz-Lopez et al, 2018Simpson et al, 2007bSimpson et al, , 2015Wang et al, , 2019Wang and Pratt, 2017). uary 2020), the artificial seawater had a pH of 7.8, a dissolved inorganic carbon (DIC) concentration of 2500 µmol kg −1 , and total alkalinity (TA) of 2544 µmol kg −1 .…”