Comparative study of elemental mass size distributions in urban atmospheric aerosol

Elemental mass size distributions in aerosols collected at four different urban sites with gradually increasing overall aerosol mass concentration are presented, compared and discussed in the present paper. The aerosol samples were collected with cascade impactor and stacked filter unit samplers, and were analyzed by particle-induced X-ray emission spectrometry and instrumental neutron activation analysis. Typical coarse-mode elements, i.e., Na, Mg, Al, Si, P, Ca, Ti, Fe, Ga, Sr, Zr, Mo and Ba, exhibited unimodal size distributions at all four urban locations studied, and the mass median aerodynamic diameters were increased with aerosol pollution level. Elements typically related to high-temperature or anthropogenic sources, i.e., S, Cl, K, V, Cr, Mn, Ni, Cu, Zn, Ge, As, Se, Br, Rb and Pb, either had a unimodal size distribution with most or their mass in the fine size fraction or clearly showed a bimodal size distribution at the urban background site. However, significant differences between the size distributions of four sampling sites were noted. There was a clear tendency for the accumulation mode to decrease and for the coarse mode to increase with increasing total aerosol mass concentration. A pronounced resuspension of the soil and roadway dust associated with the fine aerosol particles that were deposited on the ground surface previously, and the condensation process of volatile precursor gases on the already existing aerosol particles can explain the observed tendencies. The elemental mass size distributions derived for the polluted urban environments differ from those typically observed for industrial, combustion or automotive sources. A consequence of the diversity in the size distributions on the PM_2.5 speciation concept is also presented.