Sufficiency and Effectiveness Review of the 1998 Protocol on Heavy Metals, UN/ECE Convention on Long-range Transboundary Air Pollution Best Available Scientific Information on the Effects of Deposition of Heavy Metals from Long-Range Atmospheric Transport

Executive Summary This chapter describes atmospheric transport, ambient concentrations and depositions of heavy metals, observed and modelled concentrations and temporal trends of heavy metals in environmental media and biota and comparisons, as appropriate, with effects indicators of significance. It provides, moreover, an assessment of effects on ecosystem and human health from the deposition of heavy metals from long-range atmospheric transport with a focus on cadmium, lead and mercury. Heavy metals are a natural component of the earth's crust and as such they cycle throughout the environment in varying concentrations as a result of natural processes. In the post-industrial age, however, human activities have unlocked previously sequestered heavy metals from stable matrices, and released them into the atmosphere where to some extent they are transported across national boundaries. This has resulted in an enrichment of heavy metals in environments that are far from emission sources. While atmospheric deposition contributes to this enrichment, and potential harmful effects to the environment, other sources and site-dependent chemical and physical factors are also important for ecosystem accumulation. Mercury. Of the three metals that are the focus of this review, mercury displays the greatest potential for long-range atmospheric transport. Mercury is emitted primarily in the gaseous elemental state (Hg(0)) and various compounds of ionic mercury [Hg(II)]. The Hg(II) compounds tend to deposit closer to the emissions sources, but elemental mercury can remain in the atmosphere for as long as 2 years and is easily transported over long distances. Concentrations of atmospheric mercury at remote locations in the UNECE region are fairly consistent (about 1.5 ng/m 3). Long-term data sets are scarce, however; data from stations located around the North Atlantic suggest that levels reached a peak in the 1980s, decreased by more than 50% by the late 1990s, and then stabilized. These findings are generally reflected in sediment, bog and glacier cores. Within Europe, EMEP modelling suggests, in general a 2-fold decrease in deposition between 1990 and 2003 as a result of reduced emissions, and that up to 50% of anthropogenic mercury deposition are derived from transboundary European sources, whereas between 25% and 60% may be derived from intercontinental sources. North American models suggest an even greater contribution from intercontinental sources to some regions of North America. Recent results from the International Cooperative Programme on Integrated Monitoring (ICP-IM) suggest that remote forest catchments of northern and central Europe continue to accumulate deposited mercury. Studies in Scandinavia …