Size distributions of atmospheric trace elements at dye 3, Greenland — I. Distribution characteristics and dry deposition velocities

Abstract

Cascade impactor samples were collected at Dye 3 on the south-central Greenland Ice Sheet during March 1989. The impactor was calibrated in the laboratory, and the resulting collection efficiency curves were used to derive the impactor response for use in a data inversion procedure. The impactor samples were chemically analysed by proton-induced X-ray emission (PIXE), and the chemical concentration data were used with the inversion procedure to generate smooth size distributions for 15 elements. Results show three distinct size distribution categories. The first category includes elements that mainly originate from gas to particle conversion, with a substantial fraction from anthropogenic combustion (S, Pb, Zn, Br and Ni). These elements exhibit a unimodal size distribution with geometric mean aerodynamic diameter close to 0.6 μm, although S and Zn show a weak second mode centered at about 2 μm. Elements in the second category (Ti, Si, Fe, Mn, Ca, K) exhibit bimodal size distributions, with geometric mean diameters for the two modes of 0.6 and 2 μm, respectively. These elements results from a variety of sources, including crustal erosion as well as combustion from natural and anthropogenic sources. For elements in the third category (Al, Cl, Na, Mg), most of the mass occurs in particle sizes above 1 μm. Their size distribution is generally unimodal, with the geometric mean aerodynamic diameter around 2 μm. These elements are most likely to be of crustal and/or marine origin. The best-fit size distributions were used with curves of dry deposition velocity vs aerodynamic particle diameter to estimate the overall dry deposition velocity expected from the entire distribution. The deposition velocities for S, Pb, Zn, Br and Ni are all very low, with values less than about 0.02 cm s⁻¹ if hygroscopic growth in the humid layer is neglected. For the other elements, deposition velocities are in the range 0.2-0.7 cm s⁻¹. For those distributions that are bimodal, the upper mode generally dominates deposition even when most of he airborne mass is associated with the lower modes, as in the case of S and Zn.