Sky radiometer measurements of aerosol optical properties over Sapporo, Japan

Abstract

Ground-based sky radiometers were used to measure direct solar irradiance and solar aureole radiance for several years at Sapporo, Tsukuba, and Tokyo, Japan. From these measurements, we computed aerosol optical thickness at 0.5 μm, τ(0.5), and the Ångström exponent, α, and volume size distributions within a column. The optical thickness at Sapporo increased markedly over a short period of time following Asian dust events, and a forest fire in Siberia. The columnar volume size distributions observed during the Asian dust events showed a peak radius of 2.0-3.0 μm. Backward trajectory analyses suggest that the particles producing this springtime event originated in the Loess Plateau and Gobi Desert, and reached Sapporo via southern China. The columnar size distribution during the forest fire case showed an increase in the density of particles with a peak radius ∼0.2 μm. Trajectory analysis clearly linked the atmospheric changes over Sapporo with a forest fire in Siberia. The aerosol optical thickness, τ(0.5), has a clear seasonal cycle at Sapporo, with a vernal maximum and an autumnal minimum. The Ångström exponent, α, has a clear seasonal cycle at both Tokyo and Tsukuba, where early-winter maxima and springtime minima are observed, but at Sapporo the seasonal cycle is weaker, with a summer maximum and a vernal minimum. Aerosols were classified into four types (Types I∼IV) based on τ(0.5), and α data observed at the three sites. Aerosols with a τ(0.5) smaller than the total mean of τ(0.5), but greater than or equal to the total mean of α (τ(0.5) < ̄τ(0.5), α ≥ ̄α) were classified as Type I; aerosols with τ(0.5) ≥ ̄τ(0.5) and α ≥ ̄α were Type II; those with τ(0.5) < ̄τ(0.5) and α < α were Type III; and those with τ(0.5) ≥ ̄τ(0.5), α