An update on the natural sources and sinks of atmospheric mercury

This paper summarizes recent advances in the understanding of the exchange of Hg between the atmosphere and natural terrestrial surfaces including substrates (soil, rocks, litter-covered surfaces and weathered lithological material) and foliage. Terrestrial landscapes may act as new sources of atmospheric Hg, and as repositories or temporary residences for anthropogenically and naturally derived atmospheric Hg. The role of terrestrial surfaces as sources and sinks of atmospheric Hg must be quantified in order to develop regional and global Hg mass balances, and to assess the efficacy of regulatory controls on anthropogenic point sources in reduction of human Hg exposure. Continued field research has allowed for refinement of emission estimates for geothermal and volcanic, and Hg mineralized areas in the western USA to similar to 1.2-3.0, and 10-20 Mg/a, respectively. The emission estimate for areas of Hg mineralization in the western USA includes only identified Hg deposits and occurrences, and since other areas of geologic Hg enrichment such as Au and Ag deposits are not considered, the range in values is most likely an underestimate. Laboratory and field measurements have improved understanding of air-surface Hg exchange associated with soils with low or natural background concentrations of Hg (< 100 ppb), litter-covered forest floors, and foliar surfaces, all of which have large spatial coverage. Deposition of atmospheric Hg and re-emission are important processes occurring at these surfaces on diel and seasonal time scales. Foliage is a significant sink for atmospheric elemental Hg, however, the net flux associated with low Hg containing soils is uncertain. Mass balances developed for soil-air exchange using measured fluxes and estimated deposition indicate that over a year background soils may exhibit no net flux. This suggests that the residence time for elemental Hg in the air is on the order of hours to weeks. Short term exchange would result in a homogenous air Hg concentration due to constant mixing and in an apparent calculated residence time that is most likely too long (one year). Recycling of atmospheric Hg between natural background soils and foliar surfaces also provides a mechanism for long-term atmospheric contamination and continued deposition in pristine ecosystems well after anthropogenic sources are controlled. (c) 2008 Elsevier Ltd. All rights reserved.