Continuous lidar observations of Asian dust in Beijing, Nagasaki and Tsukuba

摘要

Automated Mie-scattering lidars have been operated since March 2001 at Beijing, Nagasaki and Tsukuba to reveal the time and height distribution of Asian dust and its optical properties. The lidars detect backscattering light from clouds and aerosols at 532nm in both parallel and perpendicular polarization channels. They continuously measure profiles every 15 minutes regardless of weather conditions. At first we eliminated clouds using vertical profiles of intensity, then Asian dust was identified by the depolarization ratio. In Beijing, close to the source region of the dust, Asian dust events occurred 15 times in March April, and May 2001. Each event continued for several days. The aerosol depolarization ratio (ADR) frequently reached up to 40 %. In Nagasaki, located western part of the Japan, Asian dust was confirmed near the surface with a delay of a few days from events in Beijing. However, in Tsukuba, there were few surface dust events and passing dust in the free troposphere was confirmed. The ADR in Tsukuba were lower than those in other two observatories. Internal mixing of mineral dust and anthropogenic aeorols, and changing size distribution may contribute the differences of ADR among observatories. In Beijing, ground sampling of mineral dust was simultaneously carried out. Mass concentration by the sampler at the surface and extinction coefficient near the surface derived from lidar observation were compared to estimate the conversion factor from extinction coefficient to mass concentration. Utilizing this factor we estimated the vertical distribution of the mass of Asian dust in Beijing.