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

HOHg (II) O • , formed from HOHg (I)• + O 3 , 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, NO 2 , CH 4 , C 2 H 4 , CH 2 O 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 • + CH 4 → Hg (II) (OH) 2 + • CH 3 ) 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)• + CO 2 . 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 • + CH 4 (<15%). We expect that this work will help global modeling of atmospheric mercury chemistry.