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

Description

Four years of columnar aerosol optical properties and a one-year vertical profile 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. Multi-year variations of aerosol optical depths (AOD), Ångstrom 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 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 was 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 is -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/m2 (Wm-2τ-1), respectively. The mean reduction in direct and diffuse radiation reaching surface amounts to 109.2±49.4 and 66.8±33.3 W/m2, respectively. Aerosols significantly alter the vertical profile of solar heating, with great implications for atmospheric stability and dynamics within the lower troposphere.