Characterization of seasonal variation of tropospheric aerosols in Chiba, Japan

Seasonal variations of tropospheric aerosol properties in Chiba, Japan, are investigated by means of optical monitoring (1999–2005), ground sampling (1998–2004), and wind data. The observation site at Chiba University is located in the urban Chiba area, about 30 km southeast of Tokyo. The aerosol optical thickness (AOT) and Angstrom exponent are derived from the data taken with a sun photometer. The value of AOT at the wavelength of 500 nm exhibits a seasonal variation, showing small values (<0.2) in fall and winter, and large values (∼0.5) in spring and summer. The variation of Angstrom exponent is in the range 0.5–1.8, showing negative correlation with the wind speed. This indicates that the influence of anthropogenic particles from local sources becomes relatively important when the wind is weak near the ground level. The influence of Asian dust particles is also detected during March and April. The seasonal variation is remarkable also in the chemical analysis data. For fine particles with diameters less than 2 μm, elemental carbon dominates in fall and winter, while (NH4)2SO4 is prominent in spring and summer. For coarse particles, the fraction of sea salt particles increases in spring and summer.