Atmospheric input of trace metals to the western Mediterranean: uncertainties in modelling dry deposition from cascade impactor data

The mass-particle size distributions (MSDs) of Na, Al, Cd, and Pb were determined from 17, 1-5 day, high-volume cascade impactor samples collected throughout the Western Mediterranean atmosphere between 1980 and 1983. As expected, the mass median diameter (MMD) was the largest for Na, representative of sea-salt aerosol, with a median value of 5.9 mu m. The median value for the MMD for Al, representative of mineral aerosol, was 2.8 mu m. The smallest values of the MMD were found for pollution-derived elements, Cd and Pb: 0.7 mu m. In most cases, the MSDs, for each of the elements, were log-normal. Total dry deposition velocities were calculated from the two-layer deposition model of Slinn and Slinn (1980) using three approaches: (i) by characterizing the distribution as a MMD, (ii) by considering the size distribution as directly given by the cascade impactor, (iii) by fitting the assumed log-normal distribution and dividing it into 100 successive intervals. The first approach appeared to give underestimates. The two other approaches yielded similar results for Cd and Pb, of the order of 0.05 cm s(-1). For these elements, however, more than 20% of the total dry deposition flux was due to particles with diameters of 7.2 mu m or greater (collected by impactor stage 1). For Na and Al, the third approach yielded values at least one order of magnitude higher than when using the two others. This clearly underlines the major role played by large particles in controlling the dry deposition of sea-salt and mineral aerosol particles. Direct measurements of Al dry deposition, made in 1985-1986 on the northwestern coast of Corsica, agree best with the values predicted by the third approach (mean calculated and measured values of 1.8 and 3.0 cm s(-1) respectively). Scanning electron microscopy examinations of samples confirm that mineral aerosol particles with diameters of 10 mu m or greater dominate the dry deposition flux. Preliminary data indicate that for mineral aerosol particles, this flux is, on a yearly basis, about half of the wet deposition flux. However cascade impactor data are not adequate to retrieve true mass-particle size distribution in the size range which controls dry deposition (D > 7 mu m), and this is shown to be the source for one order of magnitude uncertainties in dry deposition calculations.