We investigated the variability of in-situ and columnar aerosol optical properties (AOPs) at two regional background sites in the Asian continental outflow from 2012 to 2014. The monthly variability of the AOPs at each site is described in terms of its relationship with regional-scale atmospheric circulation patterns and the resulting changes in air mass sources. The median aerosol scattering and absorption coefficient values for sub-10 lam particles (sigma(10 mu m)(sp) and sigma(10 mu m)(ap)) were larger at the polluted marine site, Gosan (GSN; sigma(10 mu m)(sp): 59.94 mm(-1), sigma(10 mu m)(ap): 4.73 Mm(-1)) than at the high-altitude mountain site, Lulin (LLN; sigma(10 mu m)(sp):17.07 mm(-1), sigma(10 mu m)(ap): 1.72 Mm(-1)). Elevated sigma(10 mu m)(sp) and sigma(10 mu m)(ap) at GSN in May and June can be explained by the accumulation of locally emitted aerosols along with long-range transported aerosols during conditions of stagnant synoptic patterns, enhanced particle formation and subsequent growth. The significant peaks of sigma(10 mu m)(sp) and sigma(10 mu m)(ap), with median values of 50.76 Mm(-1) and 5.92 Mm(-1), at LLN during March and April are attributable to biomass burning aerosols transported from northern Indochina and southern China The LLN site was mostly influenced by clean free troposphere air and maritime air masses during the other months, showing medians of 14.65 +/- 19.81 Mm(-1) and 1.46 +/- 1.86 mm(-1) for sigma(10 mu m)(sp) and sigma(10 mu m)(ap) respectively. Lower median sub-micron to sub-10 mu m ratios for the aerosol scattering and absorption coefficients at GSN (0.60 and 0.81, respectively) than at LLN (0.81 and 0.91, respectively) are indicative of the larger proportion of coarse aerosols, such as sea salt and dust, at GSN. The single-scattering albedo for sub-10 lam particles showed similar median values at both sites (GSN: 0.93 +/- 0.02, LLN: 0.91 +/- 0.03).
