9 Vertical variation of optical properties of mixed Asian dust/pollution plumes according to pathway of air mass transport over East Asia

摘要

We use five years (2009-2013) of multiwavelength Raman lidar measurements at Gwangju, South Korea (35.10A degrees N, 126.53A degrees E) for the identification of changes of optical properties of East Asian dust depending on its transport path over China. Profiles of backscatter and extinction coefficients, lidar ratios, and backscatter-related angstrom ngstrom exponents (wavelength pair 355/532 nm) were measured at Gwangju. Linear particle depolarization ratios were used to identify East Asian dust layers. We used backward trajectory modeling to identify the pathway and the vertical position of dust-laden air masses over China during long-range transport. Most cases of Asian dust events can be described by the emission of dust in desert areas and subsequent transport over highly polluted regions of China. The Asian dust plumes could be categorized into two classes according to the height above ground at which these plumes were transported: (case I) the dust layers passed over China at high altitude levels (> 3 km) until arrival over Gwangju, and (case II) the Asian dust layers were transported near the surface and within the lower troposphere (< 3 km) over industrialized areas before they arrived over Gwangju. We find that the optical characteristics of these mixed Asian dust layers over Gwangju differ depending on their vertical position above ground over China and the change of height above ground during transport. The mean linear particle depolarization ratio was 0.21 +/- 0.06 (at 532 nm), the mean lidar ratios were 52 +/- 7 sr at 355 nm and 53 +/- 8 sr at 532 nm, and the mean angstrom ngstrom exponent was 0.74 +/- 0.31 for case I. In contrast, plumes transported at lower altitudes (case II) showed low depolarization ratios (0.13 +/- 0.04 at 532 nm), and higher lidar ratio (63 +/- 9 sr at 355 nm and 62 +/- 8 sr at 532 nm) and angstrom ngstrom exponents (0.98 +/- 0.51). These numbers show that the optical characteristics of mixed Asian plumes are more similar to optical characteristics of urban pollution. We find a decrease of the linear depolarization ratio of the mixed dust/pollution plume depending on transport time if the pollution layer traveled over China at low heights, i.e., below approximately 3 km above ground. In contrast, we do not find such a trend if the dust plumes traveled at heights above 3 km over China. We need a longer time series of lidar measurements in order to determine in a quantitative way the change of optical properties of dust with transport time.