The Effect of Asian Dust on Urban Airborne PM in Japan

Abstract

An intensive measurement of particulate matter and raindrops was made to assess the effect of Asian dust on urban atmosphere in Japan. A multi-stage particle sampling instrument and a particle counter were operated at the ground monitoring site in Fukuoka which was directly exposed to the outflow of air masses from the Asian continent during the springtime of 2005. The yellowish rainwater and individual raindrops were collected simultaneously in Kyoto, Japan on April, 2001. The bulk and individual dust particles were analyzed simultaneously by ICP-MS and micro-PIXE, respectively. Particle induced X-ray emission (PIXE) was employed in the determination of chemical properties of soluble and insoluble fractions of rainwater. The number concentration of gigantic particles (e.g. larger than 5 μm) was measured to be 10 times higher in an Asian dust storm (hereafter called “ADS”) period than in a non-ADS period. There is an outstanding increase of mass concentration in the range of 3.5–7.7 μm in ADS event. In ADS event, soil fraction accounts 57.9–70.1% of particle mass concentration in coarse particles larger than 3.5 μm. Micro-PIXE analysis enables us to classify individual dust particles into several types. The particles with 3.5–5.1, 5.1–7.7, and 7.7–10.9 μm experienced aging processes by 60.6, 69.2, and 77.2%, respectively. On the basis of the reconstructed elemental maps by micro-PIXE analysis, the chemical transformation of dust particles was also presumed. In compliance with the general expectation, major components in rainwater were of soil-origin such as Si, K, Ca, Ti, and Fe. These soil-related components contributed to as much as 92% of total elemental composition in both soluble and insoluble fractions. By scanning a microbeam, the residual particle cluster in individual raindrops was reconstructed in terms of an elemental map. This visualized elemental map in fact enabled us not only to estimate the chemical mixing state of raindrop residual particles but also to presume the wet scavenging of dust particles by rainfall.