Evaluating key uncertainties in Hg models
- Iodine as a Hg oxidant: We collaborated with theoretical chemists to calculate reaction rate coefficients and absorption cross sections involved in the reactions of iodine and Hg compounds. We then included these new reactions in a chemistry-climate model to evaluate the role of iodine in Hg oxidation. Iodine was found to have a small but significant direct contribution to Hg oxidation, mainly confined to the lowermost stratosphere. In addition to these direct impacts, iodine indirectly impacts Hg oxidation by altering the concentrations of important Hg oxidants (atomic Br, OH, and O3). This work, which presents the first global atmospheric model to include Hg–I chemistry, contributes to the recent debate in the scientific literature on the relevance of iodine as a Hg oxidant.
- Multi-model intercomparison of Hg: As part of an international team of scientists, we designed and are conducting a multi-model intercomparison study of Hg cycling, with the aim to produce relevant modeling products for policymakers involved in environmental agreements like the Minamata Convention. This effort, the Multi-Compartment Hg Modeling and Analysis Project (MCHgMAP), has integrated atmospheric, ocean, and terrestrial modeling teams, as well as experts in Hg emissions and measurements. MCHgMAP will be the first multi-model effort of its kind for Hg, aiming to analyze the drivers of Hg trends for the 2010–2020 periods through a suite of sensitivity simulations while accounting for the different representations of the Hg cycle in current models. We contributed modeling results to the Open-Ended Scientific Group report submitted to the Minamata Convention effectiveness evaluation.
New constraints for Hg chemistry based on field observations
- Isotopic constraints: We explored the use of atmospheric Hg isotopic measurements of mass-independent fractionation as a constraint on Hg redox chemistry through atmospheric model simulations.
- Oxidized Hg speciation: We conducted model simulations to interpret novel measurements of oxidized Hg speciation. The existing Hg chemistry schemes are able to reproduce total concentrations of oxidized Hg in measurements, but do not explain the oxidized species fractions that are measured in polar field campaigns.
Temporal trends in atmospheric chemical lifetime of Hg
- Model development: Existing atmospheric Hg models are mostly chemistry-transport models, which means they are run with external meteorology and cannot internally represent climatic changes in different periods. The Whole Atmosphere Community Climate Model (WACCM), was developed to include a state-of-the-art Hg cycle, including interactive dynamics, emissions, deposition, and chemistry. WACCM, as a chemistry-climate model, is a new tool to simulate Hg cycling in different time periods.
- Changes to Hg cycling between preindustrial and present-day periods: Using WACCM, we studied Hg chemistry in the preindustrial (1850) and present-day (2010-2019) atmospheres. Our results suggest that in the Northern Hemisphere Hg(0) is oxidized 16% quicker in the present-day atmosphere compared to the preindustrial period due to increases in two of its oxidants (O3 and OH radicals). On the other hand, in the Southern Hemisphere the oxidation of Hg(0) slows by 20% due to decreases in a different oxidant, the Br atom. These regional oxidation changes shift Hg deposition patterns, increasing Hg delivery to tropical and subtropical oceans where tuna fisheries are located, raising the risk of human Hg exposure through fish consumption.
