GKSS Natural and Anthropogenic Mercury Sources and Their Impact on the Air-Surface Exchange of Mercury on Regional and Global Scales Authors: R. Ebinghaus R. M. Tripathi D. Wallschlager S. E. Lindberg DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. KS002458351 R: KS DE012844914 *05012844914* Natural and Anthropogenic Mercury Sources and Their Impact on the Air-Surface Exchange of Mercury on Regional and Global Scales R. Ebingiiaus , R.M. T ripathi , D. Wali .sciii.ager , and S.E. L indberg 1 Introduction Mercury is outstanding among the global environmental pollutants of continuing concern. Especially in the last decade of the 20th century, environmental scientists, legislators, politicians and the public have become aware of mercury pollution in the global environment. It has often been suggested that anthro­ pogenic emissions are leading to a general increase in mercury on local, regional, and global scales (Lindqvist et al. 1991; Expert Panel 1994). Mercury is emitted into the atmosphere from a number of natural as well as anthropogenic sources. In contrast with most of the other heavy metals, mercury and manyof its compounds behave exceptionally in the environment due to their volatility and capability for methylation. Long-range atmospheric transport of mercury, its transformation to more toxic methylmercury compounds, and their bioaccumulation in the aquatic foodchain have motivated intensive research on mercury as a pollutant of global concern. Mercury takes part in a number of complex environmental cycles, and special interest is focused on the aquatic-biological and the atmospheric cycles. Environmental cycling of mercurycan be described as a series of processes where chemical and physical transformations are the governing factors for the distribution of mercury in and between different compartments of the environment. Mercury can exist in a large number of different physical and chemical forms with a wide range of physical, chemical, and ecotoxicological properties and consequently with fundamental importance for the environmental behavior. The three most important chemical forms known to occur in the environment are: elemental mercury [Hg(o)], which has a high vapor pressure and a relatively low solubility in water; divalent inorganic mercury (Hg2+), which can be far more soluble and has a strong affinity for many inorganic and organic ligands, especially those containing sulphur; and methylmercury (CH3Hg+), which is strongly accumulated by living organisms. Conversions between these different forms provide the basis of mercury’s complex distribution pattern on local, regional, and global scales. This chapter was prepared while R.M.T. and S.E.L. were visiting scientists at the Institute of Physical and Chemical Analysis at GKSS Research Centre, Geesthacht, Germany. Publication number 4745, Environ. Sci. Div. Oak Ridge National Lab. Environmental Science Mercury Contaminated Sites (ed. by R. Ebinghaus el al.) <0 Springer-Verlag Berlin Heidelberg 1999 4 R. Ebinghaus et al. Extensive information exists on environmental and health effects of mercury and its behavior in the environment (e.g., Wheatley and Wyzga 1997). Much less information is available on the fluxes of the element and its compounds to the air, water, and soils. This chapter summarizes present knowledge on natural and anthropogenic mercury fluxes to the atmosphere from various sources and their relative importance on regional and global scales. 2 The Atmospheric Mercury Cycle The atmospheric cycle of mercury is determined by natural and anthropogenic emissions, a complex atmospheric chemistry, and wet and dry deposition processes. Atmospheric chemistry and especially the deposition of mercury are strongly linked to the speciation of mercury released into the atmosphere by different types of sources. The linkage between the atmospheric and biological cycles is manifested in the deposition of atmospheric mercury species. Schroeder and Lane (1988) illus­ trated (Fig. 1) the most important processes in the emission and deposition cycle of atmospheric mercury. The deposition pathway is dominated by the flux of emitted Hg(II) compounds (formal reactive gaseous mercury or RGM), the oxidation of elemental mercury vapor to Hg(II), and subsequent wet and/or dry deposition. Mercury species attached to particles can be removed from the atmosphere by precipitation or dry deposition (Expert Panel 1994), but these fluxes are generally less important. Once deposited, the formation of volatile gaseous mercury, especially the formation of highly toxic methylmercury, its enrichment in organisms and nutritional chains, and finally destruction (demethylation) of methylmercury are the main features of the biological cycle of mercury. 2.1 Speciation of Emissions On a global scale, the atmospheric mercury cycle is dominated by elemental mercury vapor (generally > 95% of total airborne Hg). However, the emission speciation of mercury is determined by the source characteristics and conse­ quently shows large regional variability. To characterize the main emission pathways, the Expert Panel on Mercury Atmospheric Processes (1994) defined three different terms for mercury emissions: 1. Anthropogenic mercury emissions: the mobilization or release of geologically bound mercury by man’s activities, with mass transfer of mercury to the atmosphere. Natural and Anthropogenic Mercury Sources 5 Dry Transformations Air Wet Concentrations Transformations Transport and Scavenging Diffusion Total Emissions Scavenging Dry Wet Anthropogenic Natural Re-emission Deposition Deposition \ Adapted from Schroeder, W.H. and Lane, DA, 1988. Fig. 1. Mercury emissions-to-deposition cycle, (After Schroeder and Lane 1988) 2. Natural mercury emissions: the mobilization or release of geologically bound mercury by natural biotic and abiotic processes, with mass transfer of mercury to the atmosphere. 3. Reemission of mercury: the mass transfer of mercury to the atmosphere by biotic or abiotic processes drawing on a pool of mercury that was deposited to the Earth’s surface after initial mobilization by either anthropogenic or natural activities. Together, these last two pathways are also designated mercury emissions from natural surfaces, and they represent large uncontrolled area source emissions which must be taken into account by global models. Speciation of atmospheric mercury originated from these source types is discussed in the following sections. 2.1.1 Anthropogenic Mercury Emission Speciation Fuel combustion, waste incineration, industrial processes, and metal ore roasting, refining, and processing are the most important point source categories for anthropogenic mercury emissions into the atmosphere on a worldwide basis. Besides elemental mercury an important and variable fraction can be emitted as reactive gaseous mercury (RGM) or particulate Hg(II) (Expert Panel 1994). For Europe, Pacyna (1993) estimated the anthropogenic mercury emissions for the above mentioned species summarized in Table 1. It is evident from the table that 6 R. Ebinghaus et al. the major species is Hg(o). The relative distribution between elemental mercury vapor and gaseous or particulate Hg(II) varies from country to country, however. The major portion of mercury emissions from combustion of fuels is in the gaseous phase. In the combustion zone, mercury present in coal or other fossil fuels evaporates in elemental form and then most likely a portion of it is oxidized in the flue gases (Prestbo et al. 1995). Emitted mercury species into the environment depend upon the nature of the source of emission. Mercury emitted from high temperature processes such as coal combustion and pyrite roasters will probably be converted to the elemental form, Hg(o). However, in flue gases, where the temperature drops, Hg(o) may be oxidized by HC1 and 02 in presence of soot or other surfaces (Hall et al. 1991; Prestbo et al. 1995). In modern combustion plants equipped with flue gas cleaning facilities such as wet scrubbers, the oxidized and particulate forms should be removed easily. Hence, the primary emission will be Hg(o). Table 2 summarizes the speciation of mercury emissions from flue gases and other industrial«emissions. 2.1.2 Speciation of Natural Mercury Emission and Re-Emission The mercury that evades from natural sources is generally entirely in the elemental form (S.E. Lindberg et al. 1979, 1995, 1998). Natural sources are, for example, the evasion from surface waters, from soils, from minerals, and from vegetation located in terrestrial and wetland systems. Volcanism, erosion, and exhalation from natural geothermal and other geological crevices also mainly emit elemental mercury. Global volcanic emissions were estimated from the Hg/S ratio and account for approximately 20 to 90 t year-1, which is about 1-5% of the annual emissions from human activities (Fitzgerald 1996). Generally speaking, the distinction between natural and “quasinatural reemissions” of mercury (that which was formerly deposited from the Table 1. Anthropogenic Emission (t year ') of mercury and its species in Europe. (After Pacyna 1993) Country 1lg(o) gas Hg(ll) (gas) Hg (particles) Hg (total) Belgium 5.3 2.2 1.4 8.9 Czechoslovakia 7.8 4.5 2.7 15.0 Denmark 2.1 1.9 0.8 4.8 Finland 3.1 0.8 0.3 4.1 France 15.3 9.0 5.6 29.9 GDR 203.0 99.0 28.0 330.0 FR Germany 38.0 20.0 7.0 65.0 Netherlands 3.0 3.8 1.4 8.2 Norway 1.4 0.4 0.2 2.0 Poland 23.3 13.1 8.3 44.7 Sweden 5.6 1.4 0.5 7.5 Soviet Union 45.0 25.7 17.0 87.7 United Kingdom 21.0 14.0 5.0 40.0 Natural and Anthropogenic Mercury Sources 7 Table 2. Speciation of mercury in flue gases and other industrial emissions. (After Munthe 1993) Process % Hg(o) % Hg(II) (g) % Hg(I!)(s) Reference Chlorine alkali 50-90 10-50 Leavander (1987) Coal combustion 50 30 20 Brosset (1983) Bergstrom (1983) Roasting of sulphide ores 80-90 10-20 Leavander (1987) Pyrite burning 100 Leavander (1987) Waste incineration 20 60 20 Bergstrom (1986) Vogg et al.