Ground-based characterization of aerosol spectral optical properties of haze and Asian dust episodes under Asian continental outflow during winter 2014

Abstract

Abstract. Long-range transported (LRT) haze can affect the regional radiation budget and the air quality in areas downwind of the Asian continental outflow. Because in situ observations of spectral aerosol optical properties of the LRT haze are rare, an intensive characterization of aerosol optical properties is needed. This study characterized the spectral optical properties of the LRT haze and Asian dust originating from the Asian continent. Integrated chemical and optical measurements of aerosol particles were carried out in a downwind area of the Asian continental outflow (Daejeon, South Korea) during winter 2014. High concentrations of PM 10 (particulate matter with a diameter ≤ 10 µm) and light scattering coefficients at 550 nm, σ s,550 , were observed during a long-range transport (LRT) haze episode (PM 10 = 163.9 ± 25.0 µg m −3 ; σ s,550 = 503.4 ± 60.5 Mm −1 ) and Asian dust episode (PM 10 = 211.3 ± 57.5 µg m −3 ; σ s,550 = 560.9 ± 151 Mm −1 ). During the LRT haze episode, no significant change in the relative contribution of PM 2.5 (particulate matter with a diameter ≤ 2.5 µm) chemical components was observed as particles accumulated under stagnant atmospheric conditions (13-17 January 2014), suggesting that the increase in PM 2.5 mass concentration was caused mainly by the accumulation of LRT pollutants. On the other hand, a gradual decrease in Ångström exponent (Å) and a gradual increase in single scattering albedo (ω) and mass scattering efficiency (MSE) were observed during the stagnant period, possibly due to an increase in particle size. These results imply that a change in particle size rather than chemical composition during the stagnant period is the dominant factor affecting the aerosol optical properties. During the Asian dust episode, a low PM 2.5 / PM 10 ratio and Å(450/700) were observed with average values of 0.59 ± 0.06 and 1.08 ± 0.14, respectively, which were higher than those during the LRT haze episode (0.75 ± 0.06 and 1.39 ± 0.05, respectively), indicating that PM 2.5 / PM 10 mass ratios and Å(450/700) can be used as tracers to distinguish aged LRT haze and Asian dust under the Asian continental outflow.