Atmospheric mercury-a review

An attempt is made to assess present knowledge about atmospheric mercury: its occurrence in air and precipitation, chemical transformations taking place in the atmosphere, and mercury fluxes to and from the Earth’s surface. Tentative budgets are estimated for mercury in the global atmosphere and in the atmosphere over Europe and Sweden. Major features revealed by this include the following: For the global atmosphere, current anthropogenic emissions are comparable to emissions by natural processes (preindustrial). The present background fluxes are probably significantly augmented by anthropogenic emissions during the industrial era. A dominant fraction (≥80%) of the total mercury in the atmosphere consists of a volatile gaseous mercury form, presumably elemental mercury, Hg⁰. This mercury has an atmospheric residence time of at least a few months, maybe even one or two years, and is uniformly distributed throughout the troposphere (1-2 ng m^-3).

The volatile mercury vapour (Hg⁰) is oxidized in the atmosphere to unknown forms that are soluble and can be scavenged by precipitation or dry deposited at the surface. The oxidation process is not known but photochemical oxidants (including ozone) are likely to be important. The atmospheric residence time of the water soluble (non-volatile) mercury is in the range of a few days to a few weeks, corresponding to a characteristic transport distance of up to a few thousand kilometers.

Even if a dominant fraction of the mercury emitted from an individual source, such as a chlor-alkali plant, is dispersed regionally or globally, a small fraction (< 10%) is deposited locally. Increases by a factor of 10 to 100, above background deposition rates, have been measured within the nearest km of such plants. At a distance of 10-50 km, the deposition normally approaches the background value. Around a large Swedish smelter, mercury levels in lake sediments are significantly augmented even beyond 50 km from the plant.

The contribution to current mercury deposition in Sweden from anthropogenic mercury emissions in other European countries is most likely larger than the contributions from current Swedish emissions.

Measurements of mercury in lake sediments and peat bogs show that in southern Scandinavia. the rate of mercury deposition has increased by a factor of about 5 during the last hundred years. The increase in northern Scandinavia is significantly less, at most a factor of two. These increases are caused, most likely, by anthropogenic emissions into the atmosphere mainly within the European region.