The global biogeochemical cycle of mercury: Insights from modeling atmospheric chemistry and all-time emissions from human activity

Abstract

Methylmercury, a neurotoxin formed from inorganic mercury (Hg), bioaccumulates in aquatic food chains and adversely affects human health on a global scale through fish consumption. A global treaty, the Minamata Convention, aims to reduce human and wildlife Hg exposure by tackling anthropogenic emissions sources, but major uncertainties remain in predicting and assessing its impact. First, prior best understanding of anthropogenic atmospheric emissions suggested global emissions increased sharply from 1990 to present, in conflict with observed declining trends in atmospheric Hg concentrations over the same period. Historical atmospheric Hg deposition in lake sediment records since 1850 also appeared to conflict with historical emissions estimates that showed peak emissions occurred in the late 19th century. Second, Hg is transported near-globally through the atmosphere, but the chemistry driving the atmospheric lifetime of Hg and where it is deposited is not well- constrained due to measurement limitations. Therefore, we must turn to theoretical chemistry and modeling tools. This dissertation aims to improve our understanding of the global Hg biogeochemical cycle and how anthropogenic emissions impact past, present, and future environmental Hg by using multiple global Hg modeling tools (7-box biogeochemical model; 3-D chemical transport model coupled to 3-D ocean general circulation model and 2-D land surface) evaluated against a suite of available observations. The objectives of this work are: (1) quantify emissions and releases of Hg from intentional use in commercial products and its impact on global environmental Hg loading from year 1850 to 2010; (2) develop a state-of-the-science mechanism for atmospheric Hg redox chemistry and evaluate its effects on the global Hg budget and deposition patterns. The first dissertation chapter concludes that releases of Hg from commercial products more than double previous estimates of total anthropogenic Hg sources to the environment from 1850 to 2010 and that including this source improves our ability to reproduce historical and recent trends in atmospheric Hg, with peak emissions occurring in 1970 followed by sharp declines. Secondly, the state-of-the-science theoretical understanding of atmospheric Hg oxidation and reduction chemistry implies reduction takes place, improves modeled representation of low wet deposition over China, and suggests greatest deposition of Hg(II) occurs over the tropical oceans.