Transport of Asian dust (KOSA) particles; importance of weak KOSA events on the geochemical cycle of soil particles

Lidar measurements and airborne-particle collections were made during KOSARP 87 (‘KOSA’ Research Program of Nagoya University; April-May 1987) at Nagoya (35°N, 137°E) to investigate the vertical distribution of Asian dust (KOSA) particles and the transport of these particles. According to the lidar measurements, the highly concentrated particle layers with large depolarization ratio were frequently in the range from about 2 km to about 6 km.

Electron microscope observations on the morphology of individual particles in the height range from near the ground to about 4400 m suggested that the particle layers contained many soil particles. It is reasonable to consider that KOSA particles were very frequently transported from Asian desert areas to the islands of Japan in the middle troposphere, even when the effect of the KOSA was not detected near the ground. This ‘background KOSA’ has concentrations of about 1.9 ~ 25 μg/m³ at the layer peaks and one order of magnitude smaller than the values of severe KOSA. However, the contribution of the ‘weak KOSA’ to the global budget of soil particles is not negligible since the frequency of occurrence of ‘weak KOSA’ is high. The reaction of soil particles sampled on the vapour-deposited Ca thin-film suggested that some of the particles were coated by water or solution containing SO^2-₄. Such particles can absorb various atmospheric gases, and therefore the KOSA particles can play an important rôle in the geochemical cycle of many chemical constituents, as chemical reaction sites in the atmosphere and as carriers of the chemical products.