In this study, we propose the first complete mechanism of photochemical and thermal conversion between Hg(0) and the Hg(I) and Hg(II) species involved in the global atmospheric mercury cycle.

Mercury is a global pollutant transported through the atmosphere as elemental mercury Hg(0) and its oxidized forms Hg (I) and Hg (II). The exact oxidation mechanism of Hg(0) to its oxidized and soluble oxides, which are eventually deposited on terrestrial and aquatic ecosystems, is still unknown.

In this work, we have studied the photolysis of Hg(I) and Hg(II) by solar radiation in gas phase. However, it was unknown whether both Hg(II) photolysis and its competition in thermal reactions lead to other stable species of Hg(II), Hg(I) or Hg (0). In this study, led by the Department of Atmospheric Chemistry and Climate of the IQFR and the University of Valencia, we show that all oxidized forms of mercury are quickly photolyzed to Hg(0). These results are based on non-adiabatic dynamic mechano-quantum simulations. We have design a new quantitative mechanism of photochemical and thermal conversion between Hg(0) and the Hg(I) and Hg(II) forms. These results reveal that in the global atmospheric Hg cycle, photo-reduction clearly competes with thermal oxidation, being Hg(0) the final destination of Hg in the atmosphere. This significantly increases the residence time of this metal in the environment.

Antonio Francés-Monerris, Javier Carmona-García, A. Ulises Acuña, Juan Z. Dávalos, Carlos A. Cuevas, Douglas E. Kinnison, Joseph S. Francisco, Alfonso Saiz-Lopez and Daniel Roca-Sanjuán. "Photodissociation mechanisms of major Hg(II) species in the atmospheric chemical cycle of mercury”. Angew. Chem. Int. Ed. 2020, 59, 2-8 DOI: 10.1002/anie.201915656

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