Overview of size distribution, concentration, and dry deposition of airborne particulate elements measured worldwide

This article reviews the current knowledge of the size distributions, concentrations, and dry deposition fluxes of airborne particulate elements, with a particular focus on 13 priority toxic metals or metalloids (Sb, As, Be, Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl, and Zn). Reviewed studies comprise diverse emission sources and environments including urban, industrial, suburban, rural, traffic, and coastal sites. The primary modes of the size distributions of some elements are generally consistent among these environments, whereas size distributions of other elements significantly vary with individual samplings and meteorological conditions. Most of the toxic elements (Sb, As, Cd, Pb, Ni, Se, Ag, Tl, and Zn) originate primarily from anthropogenic sources and demonstrate their major peak(s) in the fine mode (<2.5 μm). Fine fraction (PM2.5/PM10) of these elements in urban–industrial sites vary from 0.68 (Ni) to 0.82 (Cd). Size distributions of the other toxic elements (Be, Cr, Cu, and Hg) vary with season and the proximity of the sampling sites to potential sources. Among the 13 toxic elements, Zn, Pb, and Cu are the most abundant, whereas Be, Hg, and Tl are the least abundant. The median concentrations of Zn, Pb, and Cu in PM10 are 24.3, 3.77, and 15.5 ng/m3 in North America; 72.3, 23.8, and 19.3 ng/m3 in Europe; and 425, 192, and 26.2 ng/m3 in Asia, respectively. Ambient concentrations of most toxic metals or metalloids in PM2.5 and PM10 are higher in Asia than Europe and North America. Concentrations of the anthropogenic elements, such as As, Pb, and Zn, tend to be higher in the winter due to increased fossil fuel combustion. Dry deposition flux of any element is higher in urban and industrial areas than in other areas. Generally, the crustal elements deposit at higher rates than those of the anthropogenic elements due to the larger particle sizes of crustal elements. The overall mean dry deposition velocities obtained from measured flux and total concentration of fine, coarse, and variable mode elements are 1.7, 2.0, and 1.6 cm/s, respectively, which are higher than the model estimated size-fractioned values.