Seasonal variations of aerosol optical properties, vertical distribution and associated radiative effects in the Yangtze Delta region of China

Four years of columnar aerosol optical properties and a one-year vertical profiles of aerosol particle extinction coefficient at 527 nm are analyzed at Taihu in the central Yangtze River Delta region in eastern China. Seasonal variations of aerosol optical properties, vertical distribution, and influence on shortwave radiation and heating rates were investigated. Multiyear variations of aerosol optical depths (AOD), Angstrom exponents, single scattering albedo (SSA) and asymmetry factor (ASY) are analyzed, together with the vertical profile of aerosol extinction. AOD is largest in summer and smallest in winter. SSAs exhibit weak seasonal variation with the smallest values occurring during winter and the largest during summer. The vast majority of aerosol particles are below 2 km, and about 62%, 67%, 67% and 83% are confined to below 1 km in spring, summer, autumn and winter, respectively. Five-day back trajectory analyses show that the some aerosols aloft are traced back to northern/northwestern China, as far as Mongolia and Siberia, in spring, autumn and winter. The presence of dust aerosols were identified based on the linear depolarization measurements together with other information (i.e., back trajectory, precipitation, aerosol index). Dust strongly impacts the vertical particle distribution in spring and autumn, with much smaller effects in winter. The annual mean aerosol direct shortwave radiative forcing (efficiency) at the bottom, top and within the atmosphere are -34.8 +/- 9.1 (-54.4 +/- 5.3), -8.2 +/- 4.8 (-13.1 +/- 1.5) and 26.7 +/- 9.4 (41.3 +/- 4.6) W/m(2) (Wm(-2) tau(-1)), respectively. The mean reduction in direct and diffuse radiation reaching surface amount to 109.2 +/- 49.4 and 66.8 +/- 33.3 W/m(2), respectively. Aerosols significantly alter the vertical profile of solar heating, with great implications for atmospheric stability and dynamics within the lower troposphere.