The Kinetics and Mechanism of the Halogen Initiated Oxidation of Gas Phase Elemental Mercury and its Role in Atmospheric and Combustion Chemistry.

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Professor Anthony Hynes of the University of Miami is being funded by the Environmental Chemical Sciences Program of the Division of Chemistry to study the chemical processes that oxidize gas phase mercury atoms. Their typical background concentrations in the atmosphere are very low and they pose no threat to human health. However, if the mercury atoms are oxidized, they form mercuric compounds that are much more soluble and much more rapidly incorporated into terrestrial ecosystems. The mechanisms of the atmospheric cycling of mercury are one of the most poorly understood components of atmospheric chemistry. The chemical mechanisms responsible for oxidation of elemental mercury and even the actual identity of the compounds that make up oxidized mercury are not well known. This research provides information needed to better model the chemistry of atmospheric mercury and use the model to develop sound emission control strategies. In addition to engaging graduate students in research, the PI participates in educational outreach programs at his university which are aimed at low-income first-generation college-bound students. The project involves a series of measurements of the rate coefficients, and, where possible, the mechanisms of the halogen-initiated oxidation of gas phase elemental mercury. The mercurous halides that initially form, HgCl and HgBr, are weakly bound and the hence the rates of further reactions to form stable mercuric compounds are critical. The reactions of the mercurous halides with halogen atoms and NO2 to produce stable mercuric compounds have been proposed as potential routes to oxidized mercury formation in the atmosphere. Professor Hynes and his students measure the rate coefficients for these reactions and determine the relative importance of oxidation and reduction channels. The experimental approach utilizes the Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence (PLP-PLIF) technique. This technique allows the rates and mechanisms of the reactions to be measured over the range of tropospheric temperatures and pressures, allowing them to be used in models without the need for extrapolation.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.