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 1 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=aRt-b, rather than an exponential law C=C0exp[-λ*Rt], where C is the concentration of a trace element in rain, C0 is the initial concentration, Rt is rainfall depth, a and b are empirical constants and λ* 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 Rt 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 μm.