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

Abstract

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 pm. The median value for the MMD for Al, representative of mineral aerosol, was 2.8 pm.The smallest values of the MMD were found for pollution-derived elements, Cd and Pb: 0.7 pm. 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 K I , For these elements, however, more than 20% of the total dry deposition flux was due to particles with diameters of 7.2 pm 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-I respectively). Scanning electron microscopy examinations of samples confirm that mineral aerosol particles with diameters of 10 pm 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 pm), and this is shown to be the source for one order of magnitude uncertainties in dry deposition calculations.

Overall elemental dry deposition velocities measured around Lake Michigan

“…Reported values for primarily anthropogenic elements are generally smaller than 1 cm s\ whereas deposition velocities for crustal elements often exceed this value (Davidson and Wu, 1990). The values obtained are signi"cantly greater than the values typically used by researchers in conjunction with PM10 concentrations (0.2 cm s\ ) to calculate dry deposition #ux to the Great Lakes (Stranchan and Eisenreich, 1988;Eisenreich and Stranchan, 1992;Ho! et al, 1996).…”

“…For none of the elements were r values highest using "ne particle concentrations. This "nding is because the majority of the #ux is due to particles larger than 2.5 m in size due to their high deposition velocities (Davidson and Wu, 1990;Paode et al, 1998;Dulac et al, 1989;Holsen and Noll, 1992) and indicates that best approach to estimate overall dry deposition is by using either coarse or total particle concentrations. Fig.…”

“…It is common to have one order of magnitude variations in calculated overall deposition velocities due to the di!erences in methods used to establish the associated particle size distribution (Van Aalst, 1988). The number of steps used to discretize the particle's size range has a large in#uence on the computed dry deposition velocity (Dulac et al, 1989) due to the non-linear dependence of the calculated deposition velocity on particle diameter and shape of the size distribution curve. For these reasons, the use of an overall deposition velocity, in combination with measured ambient concentrations, to estimate dry deposition, often greatly underestimates the contribution of coarse particles, particularly those '10 m, to dry deposition (Holsen et al, 1991;Holsen and Noll, 1992).…”

Heavy metals in aerosols over the seas of the Russian Arctic

“…2. The concentrations of many microelements are higher in the finest (-0.5 mm) fractions (Duce et al, 1976;Dulac et al, 1989). Such a small size range is often associated with emissions of high-temperature industrial processes (Brimblecombe, 1996) or with narrowing the aerosol size spectrum during long-range atmospheric transport (Barrie, 1986).…”

Size resolved mass concentration and elemental composition of atmospheric aerosols over the Eastern Mediterranean area

“…Dulac et al, 1987;Bergametti et al, 1989a, b;Dulac et al, 1989;Migon et al, 1991;Migon et al, 1993;Sandroni and Migon, 1997) or on the eastern coast including Turkey and Israel (e.g. Mamane et al, 1980;Ganor et al, 1991;Kubilay et al, 1994;Luria et al, 1996;Maenhaut et al, 1999;Güllü et al, 2000;Yatin et al, 2000;Erduran and Tuncel, 2001).…”