The ELOISE and IMPACTS projects have looked at a small number of contaminants following this pathway. For mercury, the majority of sources are located outside coastal zones, and moreover whilst contributing to both fluvial and atmospheric transport are generally unconnected with watersheds. Unlike most metals whose natural release is limited to erosion and leaching processes, the semi-volatility of mercury adds to the complexity of its biogeochemical cycling, lending it more in common with certain POPs than other metals. The lesser tendency of some mercury forms to precipitation scavenging from the atmosphere also gives candidacy for longer tropospheric lifetimes. Other forms, principally HgII, are soluble, have propensity to scavenging, and have a higher dry deposition velocity. There is thus an interplay between geological mercury sources, industrial exploitation, and the chemical and physical qualities of the contaminant which will influence coastal flows. Atmospheric transport and deposition of these forms of mercury to the coastal zone has been studied within the MAMCS project, indicating the significant role of meteorology in defining source regions for mercury supplied to the Mediterranean. Atmospheric transport is less site specific than the riverine pathway, with the scale of source-receptor relationships being regional-global. With coastal mercury flows having dependence on regional/global emission patterns, evaluation and management can be considered as rather independent of the traditional catchment concepts of water management as used in some European countries for several decades. The focus on riverine catchment–coast interactions for fluvial contaminants might need supplementing with the concept of airshed-coast interactions. Such scales are often the domain of multilateral agreements, rather than local managers. When it is considered that inventorying of the contributions of various economic sources to total anthropogenic mercury emissions in Europe, as undertaken by the ELOISE projects MAMCS, MOE and MERCYMS, indicates that coal combustion (power plants and residential heat furnaces) generates more than half of the European emissions (Pacyna et al., 2001), the restricted role for local management is emphasised. Defining the interplay between atmospheric and riverine supply forms one aspect of the current project MERCYMS on Mediterranean mercury cycling, specifically the current construction of inputs databases.
Troposphere
The lowest layer of the atmosphere. The troposphere has distinctive winds and cloud formations, and it has a very marked drop in temperature with altitude. It is 10 - 16 km from the surface of the Earth.
See Glossary for a complete list of all terms.
|
Focusing on the volatility of mercury, a characteristic feature of the environmental behaviour of certain persistent organic pollutants (POPs) are their tendency for undergoing reversible atmospheric deposition to various terrestrial and aquatic surfaces, as well as the propensity for reversible deposition between the aquatic and the sediment compartments. To model this, and to seek out source-recptor relationships, the POPCYCLING project built a multi-media model based on the fugacity of compounds (Wania et al., 2000).
The results lend support to the view that for marine waters, deposition across the air-sea interface is the main route of supply of persistent organic pollutants (POPs). Table 3 below provides estimates of the split between riverine supply and atmospheric supply to the Baltic Sea (Breivik & Wania, 2002). Evaluation within POPCYCLING has aided the description of the principle processes involved in sea-air exchange of POPs, as did the AIRWIN project i.e. diffusive vapour exchange, precipitation scavenging of vapours and particles, direct particle dry deposition, aerosol-vapour partitioning, and partitioning and sedimentation in the water column (Wania et al., 1998). Following AIRWIN, trophic status also has a role to play in determining chemical partitioning with implications for bioaccumulation. The pathways to bioaccumulation were the subject of the BIOCET and the FAMIZ projects, potential influence on reproduction of cetaceans the topic of the latter. The possible influence of POP’s on eagles and seals was given some attention in POPCYCLING (Koistinen
et al., 1997a & b). There is some suggestion from these projects of absorption of organic compounds in open waters, and conversely of volatilisation nearer to the coast. Whilst the exact parameterisation of such features will inevitably be case-specific, a valuable generic benefit is the understanding of process interactions which will apply to other substances of varying solubility.
|
