|Abstract Title:||Together, Not Separately, OH and Ozone Oxidize Gaseous Elemental Mercury|
|Presenter Name:||Theodore Dibble|
|Company/Institution:||SUNY College of Environmental Science and Forestry|
|Session:||Atmospheric Hg cycling: Source and Emissions|
|Day and Session:||Wednesday 27th July - Session Four|
|Start Time:||16:00 UTC|
|Co-Authors:||Theodore Dibble,Pedro Castro,Ivan ?ernu? k,Vladimir Kell|
Abstract Information :
To understand the biogeochemical cycling of mercury, it is crucial to understand its oxidation-reduction chemistry in the atmosphere. In 2005, Calvert and Lindberg showed the weakness of our understanding of OH-initiated oxidation of gaseous elemental mercury (GEM). They also presented strong arguments against the occurrence of ozone-initiated oxidation. Dibble et al. (2020) quantified the short lifetime of the ?HgOH intermediate of OH-initiated oxidation (~10 ms at 1 atm and 298 K) and the low probability of its oxidation to Hg(II). Nevertheless, several studies, notably, Gabay et al. (2020), suggest that OH and/or ozone dominate GEM oxidation in the continental boundary layer.
Notably, the mechanism of Dibble et al. (2020) omitted a previously undocumented reaction of ?HgOH:
?HgOH + O3 ? HOHgO? + O2
We use quantum chemistry to show that this reaction proceeds without a barrier, implying a high rate constant. Together with the high abundance of ozone, this enables efficient conversion of ?HgOH to Hg(II). The inclusion of this reaction in a global model suggests that OH is competitive with Br in initiating GEM oxidation (Shah et al., 2021). In this manner, OH and ozone, together, but not separately, can efficiently oxidize GEM to Hg(II).
The analogous reaction of HgBr with ozone does not dramatically increase its (already high) efficiency of forming Hg(II). However, HgBr, unlike ?HgOH, can react with ozone to reduce mercury to GEM:
HgBr + O3 ? Hg + BrO + O2
This occurs because the odd electron in HgBr is almost equally associated with Br as with Hg. This reaction could reduce the efficiency of Br-initiated oxidation, but is not included in any atmospheric models.
We are attempting to determine rate constants for oxidation of ?HgOH/HgBr by ozone as well as those for GEM formation from ozone + HgBr.