Atmospheric trace metal biogeochemistry and fluxes to shelf seas
The total concentrations of particulate trace metals (Al. Cd, Co, Cu, Fe. Mn, Na. Ni. Pb, Zn) with analytical quality assurance, have been determined in atmospheric aerosols at two coastal sites, and during cruises in the Celtic Sea. Sampling at a site on the western English Channel covered 19 months and represents one of the most comprehensive time series of trace metals in atmospheric aerosols. Aerosol concentrations of Cd. Cu, Pb and Zn for the English Channel were lower than previously reported and Al. Co and Mn concentrations were similar to literature values. The elements were grouped according to behaviour such that Group 1 elements (Cd, Fe. Mn, Pb and Zn) displayed enhanced concentrations in autumn/winter 1994 and 1995. whereas Group 2 elements (Al, Co, Cu, Na and Ni) had enhanced concentrations during winter 1995 only. This was ascribed to source functions being dependant on wind direction with Group 1 elements being carried mainly by north easterly air masses, whereas Group 2 elements originated mainly with air masses from the south west. Dissolved trace metal (Al, Co, Cu, Ni, Pb) analyses were carried out on rain waters collected at the English Channel site. The rain water analyses showed that the soluble trace metal fraction was in the order Co = Cu > Ni >Pb > Na » Al and wet and dry depositional fluxes accounted for the differential behaviour and the solubility of aerosol trace metals on contact with sea water. Aerosol concentrations are also reported for the north western Mediterranean and the Celtic Sea. In the former location, the aerosol trace metal concentrations were lower than those reported in literature and in the Celtic Sea there was a gradient in the trace metal composition of the aerosol from land to open sea. The solid state speciation of Cd and Pb gave an increasingly matrix-bound fraction in the order north western Mediterranean < western English Channel < Celtic Sea and the Celtic Sea aerosols had the greatest sea water solubilities of all elements, except Ni. Concentrations of Cu, Ni and Pb in English Channel aerosols, rain waters and the fraction partitioning from aerosols on contact with sea water, together with sea water concentrations from the literature, were used to devise a trace metal flux model for the English Channel. The model showed that of the total trace metal fluxes into the English Channel, the atmospheric fluxes were in the order of importance Pb > Cu > Ni. The overall budgets revealed discrepancies in the mass balances, which were identified as sediment-water exchange for Ni and Pb and adsorption onto particles for Cu. These budget calculations provide a basis for the development of more advanced modelling concepts involving coupled atmosphere-ocean interactions.