Optical characteristics of southeast Asia's regional aerosols and their sources

The dominant optical characteristics of Southeast Asia (SEA)'s regional aerosols were determined from the cluster analysis of the 26 AERONET aerosol inversion products, including aerosol light scattering/absorption indicators and aerosol size/shape parameters retrieved from 2003 to 2007. The data sets were acquired from four stations: Bac Giang in Vietnam and Mukdahan, Pimai, and Silpakorn University in Thailand. The cluster analysis showed agreement among the aerosol optical characteristics, land cover/uses, season as the surrogate of the prevailing winds, and observations from the literature. The results of this study showed that during the northeast prevailing winds from mid-September to December, the high aerosol exposure events were most frequently observed over the upwind station and less often over the downwind stations. This aerosol exhibited a single scattering albedo (SSA) of approximately 0.95 (440 nm), a relatively low refractive index, and a larger fine-mode size, suggesting it had the characteristics of urban/industrial aerosols reported in the literature. These aerosol sources were upwind from Bac Giang, probably in eastern China. From January to April, the aerosol exhibited a lower SSA of approximately 0.90, a higher refractive index, and a smaller fine-mode size, suggesting biomass burning smoke reported in the literature. The SEA urban aerosol exhibited a mean SSA of approximately 0.90 (440 nm) or lower, and the coarse-mode aerosol, possibly road dust or soil dust, played a role from October to January when seasonal winds are strongest. The results from a canonical discriminant function analysis suggest that the dominant SEA aerosol clusters tended to be separated by a canonical function positively correlated with SSA, the fine-mode asymmetry factor, and the overall fine-mode size and negatively correlated with the refractive index.