Aerosol optical properties at the Lulin Atmospheric Background Station in Taiwan and the influences of long-range transport of air pollutants

The Lulin Atmospheric Background Station (LABS, 23.47°N 120.87°E, 2862 m ASL) in Central Taiwan was constructed in 2006 and is the only high-altitude background station in the western Pacific region for studying the influence of continental outflow. In this study, extensive optical properties of aerosols, including the aerosol light scattering coefficient (σ_s) and light absorption coefficient (σ_a), were collected from 2013 to 2014. The intensive optical properties, including mass scattering efficiency (α_s), mass absorption efficiency (α_a), single scattering albedo (ω), scattering Ångstrӧm exponent (Å), and backscattering fraction (b), were determined and investigated, and the distinct seasonal cycle was observed. The value of α_s began to increase in January and reached a maximum in April; the mean in spring was 5.89 m² g⁻¹ with a standard deviation (SD) of 4.54 m² g⁻¹ and a 4.48 m² g⁻¹ interquartile range (IQR: 2.95–7.43 m² g⁻¹). The trend was similar in α_a, with a maximum in March and a monthly mean of 0.84 m² g⁻¹. The peak values of ω (Mean = 0.92, SD = 0.03, IQR: 0.90–0.93) and Å (Mean = 2.22, SD = 0.61, IQR: 2.12–2.47) occurred in autumn. These annual patterns of optical properties were associated with different long-range transport patterns of air pollutants such as biomass burning (BB) aerosol in spring and potential anthropogenic emissions in autumn. The optical measurements performed at LABS during spring in 2013 were compared with those simultaneously performed at the Doi Ang Kang Meteorology Station, Chiang Mai Province, Thailand (DAK, 19.93°N, 99.05°E, 1536 m a.s.l.), which is located in the Southeast Asia BB source region. Furthermore, the relationships among α_s, α_a, and b were used to characterize the potential aerosol types transported to LABS during different seasons, and the data were inspected according to the HYSPLIT 5-day backward trajectories, which differentiate between different regions of air mass origin.