Processes Affecting Atmospheric Transport and Fate

While atmospheric concentrations of mercury are low ( 1-2 ng m ), the total mass of mercury stored in the atmosphere is quite large. Fitzgerald [13] estimated the mass of mercury in the atmosphere to be 5000 metric tons. Current research using core samples from sediment, peat bog and glacial ice as a record of metal deposition indicate that mean amounts of total mercury in the atmosphere have been increasing over the past 100 years with some research indicating a recent 

In temperate regions, mercury speciation processes are important as they facilitate the movement of mercury to Polar Regions in a process referred to as the Grasshopper Effect (Fig. 2). Elemental mercury transport has erroneously 

PROCESSES AEEECTINC AQUATIC TRANSPORT AND FATE 4.1. Mercury Oxidation and Reduction 

Several researchers have also suggested that bacteria mediate mercury reduction [54,55]. SicUiano et al. [56] recently examined the role of microbial reduction and oxidation processes in regulating DGM diel (over a 24h period) patterns in freshwater lakes. The authors demonstrate that photochemi-cally produced hydrogen peroxide regulates microbial oxidation processes and may account for the diel patterns observed in DGM data. Overall, the mechanisms responsible for mercury reduction and the relative contributions of biotic and abiotic processes are still unclear but solar radiation appears to be a common instigator of photo-reduction. 

Photo-oxidation is the reversal of photo-reduction, that is, the transformation of DGM into inorganic mercury. As with photo-reduction, both abiotic [45,57] and biotic [56] mechanisms for photo-oxidation have been proposed. Lalonde et al. [57], who discovered that DGM can be photo-oxidized, claimed that chloride ions stabilize Hg(l) in solution and decrease the Hg(I)/Hg(0) potential such that electron transfer to semiquinones may take place. They determined that photo-oxidation of Hg(0) follows pseudo-first-order kinetics with a rate constant of 0.25 h for freshwater and 0.6 h for saline waters. 

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