Columnar aerosol optical and radiative properties according to season and air mass transport pattern over East Asia

The column-integrated optical and radiative properties of aerosols in the downwind area of East Asia were investigated based on sun/sky radiometer measurements performed from February 2004 to June 2005 at Gwangju (35.23A degrees N, 126.84A degrees E) and Anmyeon (36.54A degrees N, 126.33A degrees E), Korea. The observed aerosol data were analyzed for differences among three seasons: spring (March-May), summer (June-August), and autumn/winter (September-February). The data were also categorized into five types depending on the air mass origin in arriving in the measurement sites: (a) from a northerly direction in spring (S-N), (b) from a westerly direction in spring (S-W), (c) cases with a low ngstrom exponent (<0.8) in spring (dust), (d) from a northerly direction in autumn/winter (AW(N)), and (e) from a westerly direction during other seasons (AW(W)). The highest ngstrom exponents (alpha) at Gwangju and Anmyeon were 1.43 A +/- 0.30 and 1.49 A +/- 0.20, respectively, observed in summer. The lowest column-mean single-scattering albedo (omega) at 440 nm observed at Gwangju and Anmyeon were 0.89 A +/- 0.02 and 0.88 A +/- 0.02, respectively, during a period marked by the advection of dust from the Asian continent. The highest omega values at Gwangju and Anmyeon were 0.95 A +/- 0.02 and 0.96 A +/- 0.02, respectively, observed in summer. Variations in the aerosol radiative-forcing efficiency (beta) were related to the conditions of the air mass origin. The forcing efficiency in summer was -131.7 and -125.6 W m(-2) at the surface in Gwangju and Anmyeon, respectively. These values are lower than those under the atmospheric conditions of spring and autumn/winter. The highest forcing efficiencies in autumn/winter were -214.3 and -255.9 W m(-2) at the surface in Gwangju and Anmyeon, respectively, when the air mass was transported from westerly directions.