The coarse particle aerosols in the free troposphere around Japan

Abstract

Airborne measurements of solar aureole intensities were carried out around Japan with a spectral scanning radiometer. Horizontal and vertical stratifications and volume spectra of tropospheric aerosols were retrieved from the measured solar aureole intensities. The results show that bimodal volume spectra of aerosols with a saddle point radius around 0.5 μm are generally observed in the troposphere, both within and above the planetary boundary layer. A submicrometer particle mode of the spectra, known as the accumulation mode, dominates the coarse particle mode within the planetary boundary layer, especially in summer, whereas coarse particles were observed frequently in the free troposphere. Synoptic analyses with air mass trajectories, isentropic surfaces, and weather charts suggest that the windblown dust particles injected into the atmosphere in arid regions of eastern Asia are the origin of the coarse particles observed in the free troposphere around Japan. Windblown dust particles are elevated to the free troposphere by ascending air motion in a warm front and then transported by the westerlies. The arid regions to the east of the Tibetan Plateau, where disturbances due to the frontogenesis occur frequently, are capable of supplying coarse particles to the free troposphere over the northwestern Pacific Ocean throughout the year.