Trace metal chemistry of the North Sea particulate aerosol; concentrations, sources and sea water fates

Abstract

The long-term averaged concentrations of trace metals in particulate aerosols sampled over large areas of the southern North Sea are generally similar to those at fixed stations in the same region. This averaged North Sea aerosol can therefore be used to define a number of chemical characteristics which constrain the fates of the trace metals in sea water. Anthropogenic processes are the most important source of particulate trace metals to the North Sea atmosphere, and the anthropogenic fraction of the North Sea aerosol approximates in composition to that of the average anthropogenic European trace metal emissions. Solid state speciation data indicate that large fractions of the total Cu(45%), Pb(77%) and Zn(77%) in the aerosol are in exchangeable “potentially environmentally mobile” (PEM) associations. The actual environmental mobility of the trace metals was then assessed experimentally in terms of “dry” (direct deposition to sea water) and “wet” (rain water scavenging) aerosol deposition modes to the sea surface. The data revealed the following trends; (a) similar amounts of the PEM Zn in the aerosol are soluble in both sea water (average 98%) and rain water (average 95%); (b) the PEM Cu in the aerosol which is soluble in rain water (average 98%) is greater than that soluble in sea water (80%); and (c) on average only ∼66% of the PEM Pb is soluble in sea water, but ∼95% is soluble in rain water. Clearly, precipitation scavenging, at the average pH for North Sea rain water (pH 4.0), initially releases most of the PEM fractions of Cu, Zn and Pb into the dissolved trace metal pool, but sea water is less corrosive with respect to the aerosols for Cu and particularly for Pb.