Oxidation and Reduction of Atmospheric Mercury

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The Environmental Chemical Sciences Program in the Division of Chemistry funds Professor Theodore Dibble at SUNY College of Environmental Science and Forestry for studies of the chemistry of mercury. Mercury is a neurotoxin that accumulates to toxic levels in many fish species, which is why health agencies recommend limits on fish consumption. In order to determine how much to limit mercury emissions from human activities, scientists work to understand how mercury is transferred between air and soil to the water in which fish live. Mercury enters soil and water from the air most efficiently when it is in the form of mercury compounds. However, mercury mostly enters the air as atoms of mercury. Once in the air, atoms of mercury can react to form mercury compounds, but those compounds may break up to re-form mercury atoms. To understand the transfer of mercury from air to soil and water, we need to know both the reactions by which mercury atoms form compounds and how those compounds break chemical bonds to regenerate atoms of mercury. Professor Dibble studies the reactions of mercury in the gas phase by performing calculations which yield information on the energetics and rates of chemical reactions. in addition to the potential societal impacts in environment, this project provides research opportunities for a graduate student, a postdoc, and several undergraduate students. The first hypothesis of this project is that the reaction of HOHg radical (formed by the reaction of Hg with OH) with ozone is fast, enabling OH radical to initiate oxidation of Hg(0) to Hg(II) throughout most of the atmosphere. The second hypothesis is that direct reaction of Hg with ozone does not initiate Hg(0) oxidation in the gaseous atmosphere, contrary to what is assumed in many atmospheric models. The third hypothesis is that while photolysis of many Hg(II) compounds is fast, it often leaves mercury in the Hg(II) oxidation state rather than reducing it to Hg(0) or Hg(I). No experimental data exist on most of the compounds or reactions important to this project. This research uses quantum chemistry and computational kinetics to test these hypotheses. Specifically, the team determines the thermodynamics and rate constants for key reactions of Hg-containing radicals, and computes absorption cross-sections and photolysis pathways for stable Hg(II) compounds. These results are incorporated into models of atmospheric mercury.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.