| dc.description.abstract | The global distribution of persistent organic contaminants is controlled by (1) the point of discharge into the global environment, (2) the movements of atmosphere and oceans, (3) the rate of exchange processes between the atmosphere and the Earth's surface and (4) the rate of chemical loss from various environmental phases. The strong temperature dependence of atmosphere-surface exchange favours evaporation from low and warmer latitudes and deposition in high and colder latitudes. As different chemicals have different volatilities this results in chemical fractionation along temperature, and thus latitudinal and climatic gradients. Relatively volatile chemicals with sub-cooled liquid vapour pressure PL at 25 °C from 10-2 to 1 Pa preferentially condense in polar climates, while less volatile chemicals with PL between 10-2 and 10-4 Pa tend to accumulate in mid latitudes. Poleward atmospheric transport of these chemicals takes place in several steps facilitated by the seasonal fluctuation of temperature. Different chemicals will move with different meridional transport velocities depending on the capacity of the Earth's surface to retain them. These large scale distribution processes are best understood by viewing the global environment as a complex chromatographic system. The interpretation of global distribution patterns is complicated by the large and often unknown changes in chemical emission that have occurred in time and space. | |
