(442f) Elucidating the Effect of Photons and Aerosols on the Physical and Chemical Transformations of Atmospheric Mercury

Authors: 
Tacey, S., University of Wisconsin-Madison
Xu, L., University of Wisconsin-Madison
Szilvási, T., University of Wisconsin-Madison
Schauer, J., University of Wisconsin-Madison
Mavrikakis, M., University of Wisconsin-Madison
The deposition mechanism for mercury from the atmosphere to the Earth’s surface largely depends on the chemical form of mercury [1]. Atmospheric mercury exists in three primary forms: Elemental gaseous (Hg(0)), oxidized gaseous (Hg(II)), and particulate-bound (Hg(p)) mercury [2]. Compared to Hg(II) and Hg(p), which deposit to the Earth’s surface shortly after emission (within ~100 km), Hg(0) travels globally and remains in the atmosphere for months to years [3]. Therefore, the transformation of mercury between these chemical forms largely dictates the time and length scales for atmospheric mercury transport before eventual deposition. For example, the reduction of Hg(II) to Hg(0) has been shown to occur in the presence of aerosols and photons, which leads to the widespread accumulation of mercury in the environment [4]. Furthermore, Hg(II) species can partition between the gas and particle phase [1], which dictates the mechanism for deposition. By understanding the role of aerosols and photons in the physical and chemical transformations of mercury in the atmosphere, better atmospheric mercury transport models can be achieved.

In this presentation, we show that quantum chemical calculations can be used to understand the physical and chemical transformations of atmospheric mercury. First, we present a computational model to predict the partitioning coefficient for Hg(II) species in the presence of various aerosol surfaces [5]. Second, we show a framework to elucidate the combined role of photons and iron-oxide aerosols in the reduction of Hg(II) species to Hg(0), as this mechanism remains unclear in the literature [6]. We develop the reaction energy surface for the reduction of HgCl2 to Hg(0) to determine potential roles of photons in the chemistry. Through both models, we show how quantum chemical computations can further understandings of atmospheric processes, as well as provide models that can be extended to other pollutants and aerosols.

References
1. A.P. Rutter & J.J. Schauer. Environ. Sci. Technol., 41, 3934–3939 (2007).
2. S.A. Tacey, L. Xu, M. Mavrikakis, & J.J. Schauer. J. Phys. Chem. A, 120, 2106–2113 (2016).
3. W.H. Schroeder & J. Munthe. Atmos. Environ., 32, 809–822 (1998).
4. Y. Tong et al. Environ. Sci. Process. Impacts, 15, 1883–1888 (2013).
5. S.A. Tacey, L. Xu, T. Szilvási, J.J. Schauer, & M. Mavrikakis. (Submitted)
6. S.A. Tacey, T. Szilvási, L. Xu, J.J. Schauer, & M. Mavrikakis. (Submitted)