Analyses of Aerosol Events Observed at Four Sun Photometer Sites in Japan during March-April 2002

Abstract

Aerosol-induced atmospheric optical phenomena were analyzed at four sun photometer sites in Japan, viz., Ryori (39°02´N, 141°50´E), Tsukuba (36°03´N, 140°08´E), Yonagunijima (24°28´N, 123deg;01´E), and Minamitorishima (24°18´N, 153°58´E), over the period MarchHApril 2002 to elucidate the causes of such phenomena, and explain regional differences. Using observed optical properties, the phenomena were classified into three kinds of aerosol events: Type A, Type B, and Type C, which, respectively, correspond to relatively clear conditions, typical Kosa conditions, and haze conditions. The frequency of occurrence of these aerosol events was found to be significantly different at the four observation sites in Japan during the observation period. Type B events were found to occur rather frequently in Ryori in northern Japan. The severe Kosa events observed at Ryori on April 10, 2002, which constitute a case study, passed over Northeast China where dust storms have rarely been reported before 2000. The smallest mean aerosol optical depth at 500 nm (τ500) and Angstrom exponent (α) were observed in Minamitorishima. However, according to other studies, the mean τ500 in Minamitorishima was larger than those in the eastern Pacific, which showed that the influence of the Asian outflow on atmospheric optical properties was still significant even 3,000 km away from the Asian continent. The mean τ500 and α, as well as the frequency of occurrence of Type C events, were the largest at our observation site in Yonagunijima. Based on the trajectory analysis obtained during the observation period, the high frequency of Type C events at Yonagunijima was ascribed to the haze from biomass burning in Southeast Asia and/or urban pollution in coastal areas in Southeast China. The frequency of Type C events in Ryori and Tsukuba indicated that haze is not an uncommon occurrence, not only in southern Japan, but also in central and northern Japan, and that, along with Kosa events, they may affect atmospheric optical properties over Japan in the spring.