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We present a new chemical mechanism for Hg(0)/ Hg(I) / Hg(II) atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg(0) by Br atoms and OH radicals, with subsequent oxidation of Hg(I) by ozone and radicals, re-speciation of gaseous Hg(II) in aerosols and cloud droplets, and speciated Hg(II) photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg(0) concentrations and Hg(II) wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg(0) to Hg(II). Ozone is the principal Hg(I) oxidant, enabling the efficient oxidation of Hg(0) to Hg(II) by OH. BrHgOH and Hg(OH)2 are the initial Hg(II) products of Hg0 oxidation, re-speciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg(II) to Hg(0) takes place largely through photolysis of aqueous Hg(II)-organic complexes. 71% of model Hg(II) deposition is to the oceans. Major mechanism uncertainties for atmospheric Hg chemistry modeling include the concentrations of Br atoms, the stability and reactions of Hg(I), and the speciation of Hg(II) in aerosols and clouds with implications for photoreduction. 

Our understanding of the processes that control the burden and budget of tropospheric ozone has changed dramatically over the last 60 years. Models are the key tools used to understand these changes, and these underscore that there are many processes important in controlling the tropospheric ozone budget. In this critical review, we assess our evolving understanding of these processes, both physical and chemical. We review model simulations from the International Global Atmospheric Chemistry Atmospheric Chemistry and Climate Model Intercomparison Project and Chemistry Climate Modelling Initiative to assess the changes in the tropospheric ozone burden and its budget from 1850 to 2010. Analysis of these data indicates that there has been significant growth in the ozone burden from 1850 to 2000 (approximately 43 ± 9%) but smaller growth between 1960 and 2000 (approximately 16 ± 10%) and that the models simulate burdens of ozone well within recent satellite estimates. The Chemistry Climate Modelling Initiative model ozone budgets indicate that the net chemical production of ozone in the troposphere plateaued in the 1990s and has not changed since then inspite of increases in the burden. There has been a shift in net ozone production in the troposphere being greatest in the northern mid and high latitudes to the northern tropics, driven by the regional evolution of precursor emissions. An analysis of the evolution of tropospheric ozone through the 21st century, as simulated by Climate Model Intercomparison Project Phase 5 models, reveals a large source of uncertainty associated with models themselves (i.e., in the way that they simulate the chemical and physical processes that control tropospheric ozone). This structural uncertainty is greatest in the near term (two to three decades), but emissions scenarios dominate uncertainty in the longer term (2050–2100) evolution of tropospheric ozone. This intrinsic model uncertainty prevents robust predictions of near-term changes in the tropospheric ozone burden, and we review how progress can be made to reduce this limitation.