Scavenging properties of major and trace species in rainfall collected in urban and suburban Tokyo

Sequential rain collections were carried out for 93 rainfall events, with samples of I mm of precipitation. A total of 29 major and trace elements and major anions were determined for each fraction. The scavenging properties of these elements with rainfall depth were well approximated by a power law C = aR(t)(-b), rather than an exponential law C = C-0 exp[- lambda*R-t], where C is the concentration of a trace element in rain, C-0 is the initial concentration, R, is rainfall depth, a and b are empirical constants and, lambda* is the scavenging coefficient. We propose a power exponent b as a scavenging index that is the slope of linear approximation of the log C versus log R, plot and represents degree of scavenging efficiency. Elements such as Na, Mg, Ca, Sr, which are predominantly from natural sources, had large b values and were efficiently scavenged by raindrops, while elements such as Zn, Cu, Se, Sb, which are predominantly from anthropogenic sources, had intermediate b values and were scavenged less efficiently than natural source elements. On the other hand, Sn had a distinctly small b value compared to the other elements. The scavenging index b values correlate well with the aerodynamic diameter of aerosols that bear the respective elements. The size of aerosols bearing Sn could be extremely small, down to 0.05 mu m. (c) 2006 Elsevier Ltd. All rights reserved.