Aerosol Optical Characteristics in Fukuoka and Beijing Measured by Integrating Nephelometer and Aethalometer: Comparison of Source and Downstream Regions

Abstract

 The aerosol optical characteristics in the East Asian cities of Fukuoka and Beijing were measured from 2010 to 2014. These long-term season-crossing data were compared to understand the differences between the aerosol characteristics at a source and a downstream region. Previously, few long-term, season-crossing observations have been reported. Using a method developed by one of the present authors, the measurement data were analyzed so that the retrieved optical properties can be more accurate than those obtained in previous studies. Using these data, the aerosol characteristics and their frequency distributions were reliably obtained. In Fukuoka, the annual means of the extinction, scattering, and absorption coefficients C_ext (525 nm), C_sca (525 nm), and C_abs (520 nm) were 74.6, 66.1, and 8.1 Mm⁻¹, respectively, whereas those in Beijing were 412.1, 367.2, and 42.4 Mm⁻¹, respectively. The coefficients in Fukuoka were approximately one-fifth of those in Beijing. The single-scattering albedos ω₀ (525 nm) in Fukuoka and Beijing were 0.877 and 0.868, respectively. The asymmetry factors G (525 nm) in the two cities were 0.599 and 0.656, respectively. The extinction Ångström exponents α_ext in the two cities were 1.555 and 0.855, respectively. The absorption Ångström exponents α_abs in the two cities were 1.106 and 0.977, respectively. The fine- and coarse-mode volume fractions in Fukuoka were approximately 80 % and 20 %, and those in Beijing were both approximately 50 %, except in summer.

 C_ext, C_sca, and C_abs showed a seasonal variation in both cities. Some other properties also showed a seasonal variation. In particular, the seasonal variation in α_abs was clear in both cities; it tended to be small in summer and large in winter. The frequency distributions of various parameters were also investigated. The frequency of C_ext > 500 Mm⁻¹ in Fukuoka was very low, and large C_ext values were recorded more frequently in spring than in other seasons. In Beijing, C_ext > 1000 Mm⁻¹ values were recorded more frequently, and the frequency of 10 Mm⁻¹ ≤ C_abs ≤ 60 Mm⁻¹ was high in spring and summer. Furthermore, α_abs < 1.0 values were recorded frequently, which cannot be explained by the simple external mixture of absorbing aerosols.

 To demonstrate the usefulness of the data obtained in this study, the relationships among α_abs, α_ext, the volume size distribution, the imaginary part of the refractive index, and ω₀ were investigated, and two characteristic cases in Beijing (winter) and Fukuoka (spring) were preliminarily analyzed.