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

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 center dot. We previously used quantum chemistry to demonstrate that BrHgO center dot can, like (OH)-O-center dot radicals, readily abstract hydrogen atoms from sp(3)-hybridized carbon atoms as well as add to NO and NO2. In the present work, we reveal that BrHgO center dot can also add to C2H4 to form BrHgOCH2CH2 center dot, although this addition appears to proceed with a lower rate constant than the analogous addition of (OH)-O-center dot to C2H4. Additionally, BrHgO center dot 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 center dot + 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 center dot in a variety of air masses and find that BrHgOH formation via hydrogen abstraction will be the predominant fate (similar to 70-99%), with major competition (similar to 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 center dot reactions, the present manuscript should provide guidance to a range of scientists studying atmospheric mercury.