Table of Contents

  • In December 2004, in recognition of an increasing body of scientific evidence suggesting the potential importance of intercontinental flows of air pollutants, the Convention on Longrange Transboundary Air Pollution (LRTAP Convention) created the Task Force on Hemispheric Transport of Air Pollution (TF HTAP). Under the leadership of the European Union and the United States, the TF HTAP was charged with improving the understanding of the intercontinental transport of air pollutants across the Northern Hemisphere for consideration by the Convention. Parties to the Convention were encouraged to designate experts to participate, and the task force chairs were encouraged to invite relevant experts to participate from countries outside the Convention.
  • While mercury (Hg) is globally distributed mainly through the atmosphere, it differs from other major atmospheric pollutants (e.g. ozone, particulates) in that its environmental impact is not directly related to the atmospheric burden. While the major redistribution of Hg is via the atmosphere, its primary environmental and health impact is in aquatic systems, and for aquatic organisms and their consumers, as this is the location where the inorganic Hg deposited directly or indirectly from the atmosphere is converted into the highly toxic and bioaccumulative methylmercury (MeHg) (Figure 1.1). Consumption of aquatic organisms with elevated MeHg concentrations is the primary route of exposure for humans [Mahaffey et al., 2004; Sunderland, 2007] and for freshwater and marine fish-eating wildlife [Braune et al., 2006; García-Hernández et al., 2007; Kemper et al., 1994; Landers et al., 2008] (Chapter 5). In terms of relative toxicity, MeHg is orders of magnitude more toxic than the inorganic forms (ionic Hg (HgII) and elemental Hg (Hg0)) [Clarkson and Magos, 2006]. Because Hg is globally distributed, due to its relatively long residence time in the atmosphere, fish in remote regions may be impacted by regional and global sources. For example, in the United States, 48 states have Hg consumption advisories, [U.S. EPA, 2006] and many are associated with water bodies located in areas with no apparent land-based Hg contamination or local anthropogenic Hg source. However, while long range transport (LRT) is important, there are locations where Hg0 is efficiently oxidized and deposited, and for these regions, regional inputs are more important and local hotspots of Hg accumulation can be found. Since MeHg is bioconcentrated in organisms and biomagnified in aquatic food webs, large fish and those with high trophic stature tend to have higher concentrations. Thus, marine and freshwater advisories often target specific fish species and are size based [Burger and Gochfeld, 2004; Chen et al., 2008]. Since the developing human nervous system is sensitive to MeHg, young children and children of women who consume fish during pregnancy are potentially at risk [Clarkson et al., 2003; NRC, 2000; WHO, 1990], and are therefore the target of most advisories.
  • A large number of activities have been carried out in different regions of the world aiming to assess the level of mercury (Hg) in ambient air and precipitation, its variation over time, and how it varies with changing meteorological conditions. Recent studies have highlighted that in fast developing countries (i.e., China, India), Hg emissions are increasing in a dramatic fashion due primarily to a sharp increase in energy production from the combustion of coal [Mukherjee et al., 2009; Pacyna et al., 2010; Pan et al., 2007; Streets et al., 2005; Streets et al., 2009; Wu et al., 2006].
  • This chapter reviews global mercury emission estimates published in recent years, and describes relative contributions from major anthropogenic and natural sources as well as from natural ecosystem-driven processes (re-emissions/recycling). The assessment of historical and current emissions for both anthropogenic and natural source categories is supplemented by scenario evaluations for the target years of 2025 and 2050.
  • Chemical transport modelling is a universal tool of mercury pollution assessment. Its role is particularly important for the evaluation of atmospheric mercury dispersion over long distances, taking into account the limited coverage of existing monitoring networks (see Chapter B2). In particular, model simulations provide estimates of mercury ambient concentration and deposition on global and regional scales, evaluation of intercontinental transport and forecasts of future pollution changes. Thus, application of chemical transport models can supplement direct measurements, giving more comprehensive and detailed information on mercury pollution.
  • Both humans and wildlife are adversely affected by exposures to multiple chemical forms of mercury. For all three forms of mercury (divalent mercury — HgII, elemental mercury — Hg0, and methylmercury — MeHg) the severity of health impacts varies with the intensity and duration of exposure (i.e., the dose). Adverse human health effects range from those detectable only with specialized testing protocols to gross, clinically evident abnormalities, as well as death [Clarkson and Magos, 2006]. High levels of MeHg exposure cause a variety of negative health effects in humans and wildlife, including kidney and liver failure, endocrine disruption, reproductive abnormalities, neurodevelopmental delays and compromised cardiovascular health in adults [Clarkson and Magos, 2006; Mergler et al., 2007; Sheuhammer et al., 2007; Tan et al., 2009].
  • The previous chapters of this report give an overview of mercury (Hg) cycling in the environment (Chapter B1), a review of the environmental measurements of mercury to date (Chapter B2), an overview of the various types of mercury emissions to the atmosphere and their magnitude (Chapter B3), the results of the HTAP intercomparison of global and hemispheric mercury models (Chapter B4), and a summary of known environmental and health impacts of mercury, especially methylmercury (MeHg) (Chapter B5). Even though the authors of these chapters approached the problem of mercury in the environment from quite different starting points and perspectives, their findings, and associated recommendations, show a remarkable consensus in their identification of the major issues confronting the mercury research community. The research to date, described in the previous chapters shows that mercury transport on a hemispherical scale hinges on the magnitude or rate of the emission, reaction, deposition and re-emission of mercury. In addition, new research has begun to demonstrate the importance of large-scale oceanographic transport for inter-hemispheric transport (Chapter B5).