The lead content of atmospheric aerosols above the eastern channel : seasonal variability and solubility in a coastal seawater

In order to describe the seasonal variability of the atmospheric lead concentrations above the Eastern Channel, aerosols samples have been collected by bulk filtration at the Cape Gris-Nez (Dover Strait) during one year. A statistical analysis of the obtained time-series from lead concentrations and rain amounts illustrates the geochemical implications of a scavenging process: when precipitations become lower than the average rain events, the lead concentrations become higher than the mean values and reciprocally. On a global scale, the lead concentrations over the Eastern Channel and the Southern Bight of the North Sea have decreased by about one order of magnitude since fifteen years. Lead concentrations have been compared to the origin of the collected air masses. The use of the back-trajectories permits us to classify the origins of the air masses among five geographical sectors relevant to the position of our sampling site: Atlantic Ocean and English Channel; North Sea; British Islands; Southern and Western Europe; Scandinavia and Central Europe. Continental sectors show greater lead concentrations than marine and semi-continental sectors, by a factor of about 3 to 7. These results are in a good agreement with the assumption according to which the atmospheric lead is mainly associated with emissions from heavy industrialised and urbanised areas, especially from automotive origin. Under our experimental conditions, the solubility in a coastal seawater for ‘continental’ lead is different from the one of ‘maritime’ lead. These dissolution experiments are developed in order to ultimately answer to the questions ‘how does the dry fall-out scatter into the upper layer of the seawater mass, regarding as a chemical system?’ and more particularly ‘what is the part of the heavy metals transported via the atmosphere which is rapidly transferred into the dissolved phase, i. e. under a labile form potentially available for the interfacial organisms?’