The Unpredictable Pathway and Temporal Trend of POPs

Predicting the possible consequences of human activity and climate change on the pathway and environmental fate of POPs in the Southern Hemisphere is much more challenging than for other persistent contaminants. POPs include a large number of chemical compounds with a wide range of physico-chemical properties, and they are released by many different human activities from all continents. The extent to which POPs are associated with aerosols generally plays an essential role in their atmospheric transport to polar regions. The association with particles may reduce or slow the transport of POPs through their temporary or permanent deposition on the ground. However, the association with small particles may also protect an organochlorine compound from oxidation during its transit to the south. Temperature changes may alter partitioning between gaseous and particulate phases, especially of compounds with higher log values of the octanol-air partition coefficient (Koa). In the Arctic atmosphere, for instance, over 70 % of DDT is bound to aerosols in winter (air temperature=-30 °C), while DDT occurs almost exclusively in the gaseous phase in summer (temperature=0 °C; Bidle-man et al. 2003). An increase in air temperature of only a few degrees can increase the volatility of many POPs and, consequently, their atmospheric transport potential. One expected effect of global warming is therefore increased atmospheric cycling of POPs. A fraction of the POP atmospheric burden is lost during transport through photolytic oxidation by OH, O3 or NO3. Although photolytic reactions are scarcely affected by temperature, global warming is predicted to increase cloud cover (IPCC 2001), which is already higher over the Southern Ocean than elsewhere in the Southern Hemisphere (more than 85 % cloud cover throughout the year along the 60° S parallel; King and Turner 1997). A further increase in cloud cover promoted by global warming would probably significantly reduce hydroxyl radical concentrations and POP removal from the atmosphere.

It is even more difficult to forecast the environmental fate of POPs in the Southern Hemisphere because some of their chemical properties, such as volatility, phase partitioning and degradation kinetics, are affected not only by changes in temperature or other climatic and meteorological factors but also by environmental characteristics of the ocean. Recent findings (e.g. Li et al. 2002) emphasise the importance of atmosphere-ocean coupling in the transfer of POPs from release points in tropical and temperate regions to polar regions. Models which combine the transport of semi-volatile POPs in air and seawater and consider the continuous exchange between the two compartments show an overall accelerated transport of POPs to remote regions with respect to those which treat air and water separately (Beyer and Matthies 2001). Other models (e.g. Dachs et al. 2002) suggest that levels of primary and secondary productivity in surface ocean waters may contribute to the deposition of POPs at mid-high latitudes.

Available data show that the main sources of POPs in the Southern Hemisphere are urbanised areas, those with intensive agriculture, and tropical or subtropical regions where spraying is used for disease vector control. In the Northern Hemisphere the emission of most POPs of environmental and tox-icological concern peaked in the 1970s and 1980s and thereafter generally declined or ceased. In contrast, in the Southern Hemisphere the demand and use of many POPs were still increasing in the 1990s. In several developing countries, considerable quantities of PCBs are used in older electrical devices and deposited as landfill. There is evidence (e.g. Kallenborn et al. 1998) that air masses with high PCB concentrations from South America may reach the sub-Antarctic islands, the Antarctic Peninsula and the northern Weddell Sea. South America has historically been among the heaviest users of DDT, toxaphene and lindane. In general, more volatile POPs such as HCHs and HCB are dominant in the polar atmosphere; they are also deposited in the Bellingshausen, Scotia and Weddell Seas through the global distillation process. Much higher concentrations of HCB, for instance, were measured in preen oil from seabirds nesting in the sub-Antarctic islands than in samples from related species of seabirds from the Northern Hemisphere (van den Brink 1997). Concentrations of HCB in fish species from the Antarctic Peninsula were as high as those in Limanda limanda from the North Sea (Weber and Goerke 1996). These data suggest that during the next decade marine and terrestrial ecosystems in the Antarctic Peninsula will probably still be affected by enhanced warming and deposition of persistent atmospheric contaminants.

Biomass burning in South America and southern Africa produces large amounts of hydrocarbons, and there is evidence that emitted trace gases may affect tropospheric concentrations of O3 in southern areas of the Pacific and Atlantic oceans (Thompson et al. 1996; Schultz et al. 1999). Natural and human-related combustion processes produce PAHs which can be transported over long distances and, as in the case of many other POPs, an increase in air temperature may shift the equilibrium from particulate to vapour phases for compounds such as pyrene, fluoranthene, phenanthrene and anthracene. Moreover, it seems likely that warmer temperatures will increase the occurrence of forest fires, especially in the interior of Southern Hemisphere continents.

The above scenario suggests that deposition of some POPs in Antarctica and the Southern Ocean will increase in the near future. As discussed in the preceding chapter, the melting of sea ice and snow cover during the austral summer enhances the transfer of POPs from surface seawater to lipids of planktonic organisms. The chemicals are transferred through krill to organisms at higher trophic levels, such as seabirds and marine mammals. The potential toxicological effects for these animals are further exacerbated by starvation cycles during which their fat store is reduced.

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