Atmospheric mercury residence times

The average residence times for mercury in the atmosphere, terrestrial soils, oceans, and oceanic sediments are approximately 1 yr, 1000 yr, 3200 yr, and 2.5 x 10 yr, respectively. (See Bergan et al. (1999) for more details on atmospheric residence times.)... 

Due to its chemical inertness, vaporizable nature (enthalpy of vaporization = 59.15 kJ/mol), and low water solubility (at 20°C, 2 x 10 6 g/g), elemental mercury vapor has over one year of residence time, long-range transport, and global distribution in the atmosphere [3-8]. 

Elemental mercury, a monatomic gas, is the dominant atmospheric form and has a long residence time in the troposphere (>1 yr Fitzgerald et al., 1981 Slemr et al., 1981 Lindqvist et al., 1991 Lamborg et al., 2000, 2002a). Such longevity allows emissions of mercury to the atmosphere from natural and... 

There remains an intriguing inconsistency between experiments related to the mechanisms for mercury removal. Many lab, field, and model efforts indicate that the lifetime of mercury in the atmosphere must be 1 -2 yr, but there exist a number of plausible removal mechanisms (such as foliar mercury uptake followed by litterfall) that suggest the flux from the atmosphere is more consistent with lifetimes that are less than 1 yr. The likely resolution of this problem hes in the observation that majority of the Earth s surface is covered by areas that are not temperate or boreal forests, including the open ocean and tropical regions. The deposition to the ocean is consistent with an atmospheric residence time in excess of 1 yr, while the mercury cychng within tropical forests is understudied. 

Biological, chemical, and physical effects of airborne metals are a direct function of particle size, concentration, and composition. The major parameter governing the significance of natural and anthropogenic emissions of environmentally important metals is particle size. Metals associated with fine particulates are of concern particles larger than about 3-fjim aerodynamic equivalent diameter are minimally respirable, are ineffective in atmospheric interactions, and have a short air residence time. Seventeen environmentally important metals are identified arsenic, beryllium, cadmium, chromium, copper, iron, mercury, magnesium, manganese, nickel, lead, antimony, selenium, tin, vanadium, and zinc. This report reviews the major sources of these metals with emphasis on fine particulate emissions. 

This ionic mercury (Hgll) adheres to aerosols and thus has a short (days to weeks) residence time in the atmosphere rainfall delivers it to the local soils and rivers. Ionic mercury is readily methylated (eqn. 5.24) by both abiotic and biotic pathways. However, most scientists now agree that methylation by anaerobic sulphate reducing bacteria (SRB) is most important. 

Hg has an average residence time in the atmosphere of between 6 months and 2 years, and will thus be distributed fairly evenly in the troposphere. Oxidized mercury (Hg(II)) may be deposited relatively quickly by wet and dry deposition processes, leading to a residence time of hours to months (Lindqvist and Rodhe 1985). However, the atmospheric residence time for some... 

Elemental mercury is volatile and sparingly soluble in water [8]. Due to the high vapor pressure of Hg° it is the dominant form of mercury in the atmosphere and has a long residence time (1-2 years) [9]. In the ionic form, mercury can... 

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