Atmospheric Chemistry of HOHg(II)O• Mimics That of a Hydroxyl Radical

Abstract

HOHg^(II)O^•, formed from HOHg^(I)• + O₃, is a key intermediate in the OH-initiated oxidation of Hg⁽⁰⁾ in the atmosphere. As no experimental data are available for HOHg^(II)O^•, we use computational chemistry (CCSD(T)//M06-2X/AVTZ) to characterize its reactions with atmospheric trace gases (NO, NO₂, CH₄, C₂H₄, CH₂O and CO). In summary, HOHg^(II)O^•, like the analogous BrHg^(II)O^• radical, largely mimics the reactivity of ^•OH in reactions with NO_x, alkanes, alkenes, and aldehydes. The rate constant for its reaction with methane (HOHg^(II)O^• + CH₄ → Hg^(II)(OH)₂ + ^•CH₃) is about four times higher than that of •OH at 298 K. All of these reactions maintain mercury as Hg^(II), except for HOHg^(II)O^• + CO → HOHg^(I)• + CO₂. Considering only the six reactions studied here, we find that reduction by CO dominates the fate of HOHg^(II)O^• (79-93%) in many air masses (in the stratosphere and at ground level in rural, marine, and polluted urban regions) with only modest competition from HOHg^(II)O^• + CH₄ (<15%). We expect that this work will help global modeling of atmospheric mercury chemistry.