Mercury is a neurotoxin which is transported globally via the atmosphere. While Hg(0) tends to underdo dry deposition to vegetation, Hg(II) tends to undergo wet deposition plus dry deposition to oceans. As a result, the oxidation state of mercury strongly influences when and where it enters ecosystems. Consequently, our ability to accurately model the deposition of mercury to ecosystems relies heavily on our uncertain understanding of its atmospheric redox chemistry. My entry into this field came with the realization that models were likely omitting key reactions. Mechanisms of Br-initiated oxidation of Hg(0) included BrHg(I) + Z → BrHg(II)Z (Z = OH or Br). But modelers had not considered that radicals more abundant than OH or Br (e.g., NO, NO2, HOO, and BrO) might react with BrHg(I) via the same mechanism to form Hg(II). I only realized years later that this idea was suggested in the abstract of a well-known 2005 paper! As one might expect, inclusion of these reactions into models increased the predicted rate of oxidation of Hg(0) to Hg(II): by a factor of ~100 near ground level in the southeastern US and by a factor of three (3) in a global model. This latter result spurred a search for reduction mechanisms, in order to increase modeled [Hg(0)] to levels matching field data. Experiments on rainwater suggested that photoreduction in aqueous aerosol did not proceed rapidly enough to explain this. Photoreduction of gas-phase Hg(II) was suggested, but this led to overprediction of [Hg(0)] and the desire to find more oxidation reactions. In 2020, our group suggested that OH-initiated oxidation of Hg(0) accounted for &lt; 1% of Hg(0) oxidation, globally. This arose from the fact that the HO-Hg(I) bond is 3.5 kcal mol-1 weaker than the Br-Hg(I) bond, causing HOHg(I) to fall apart to HO + Hg(0) before it could react with radicals. However, in 2021 I contributed to a paper (Shah et al.) which concluded that OH caused about one-third of net global oxidation of Hg(0). The difference between those papers was the discovery that ozone could oxidize HOHg(I) (and BrHg(I)) to Hg(II) with a high rate constant (generating HOHg(II)O + O2). The recent measurement of the rate constant for BrHg(I) + O3 → BrHg(II)O + O2 by Gomez Martin et al. (2022) suggests that our 2021 paper underestimated the contributions of OH to Hg(0) oxidation.

Mercury redox chemistry: Computational chemistry interacting with experiment and models

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.