Continuous Ground-Based Observation of Aerosol Optical Properties at Tsukuba, Japan: Trend and Climatology

To investigate aerosol optical properties, the Meteorological Research Institute has been continuously measuring scattering and absorption coefficients since January 2002 by using an integrating nephelometer and one- and three-wavelength absorption photometers in dry air conditions at Tsukuba, Japan. We used these optical data to investigate trends of aerosol properties and climatology from 2002 to 2013. The results showed that most aerosol characteristics had seasonal variation and decreasing or increasing trends significant at the 95 % confidence level. From 2002 to 2013, the extinction coefficient at 550 nm and absorption coefficient at 530 nm had statistically significant decreases of -1.5 x 10(-6) and -5.4 x 10(-7) m(-1) year(-1), respectively. In the same period, the scattering coefficient showed a non-significant decrease of -8.8 x 10(-7) m(-1) year(-1). The single scattering albedo (SSA) at 550 nm had a significant increasing trend of 7.4 x 10(-3) year(-1). Asymmetry factors did not show a significant trend. The increasing trend in the extinction Angstrom exponent was significant, whereas the trend in the effective radius was not significant. The increasing trend of 2.1 x 10(-2) year(-1) in the absorption Angstrom exponent from 2006 to 2013 was significant. This tendency suggests a compositional change of light-absorbing aerosol. Frequency distributions of aerosol properties were investigated during 2006-2012. In this period, absorption coefficients were measured by the three-wavelength absorption photometer. The most frequent values of the extinction coefficient at 550 nm, the absorption coefficient at 530 nm, and the SSA at 550nm were 25 x 10(-6), 3.0 x 10(-6) m(-1), and 0.905, respectively. The analysis using the extinction Angstrom exponent showed that aerosol characteristics were dependent on the extinction Angstrom exponent. The aerosol characteristics estimated from optical data were consistent with those derived from radiometer data. Therefore, ground-based monitoring of aerosol optical properties is useful for monitoring aerosol characteristics and interpreting variations in the surface radiation budget.