LAUSR

Abstract. Aerosol pollution in eastern China is an unfortunate consequence of the region's rapid economic and industrial growth. Here, sun photometer measurements from seven sites in the Yangtze River Delta (YRD) from 2011 to 2015 were used to characterize the climatology of aerosol microphysical and optical properties, calculate direct aerosol radiative forcing (DARF) and classify the aerosols based on size and absorption. Bimodal size distributions were found throughout the year, but larger volumes and effective radii of fine-mode particles occurred in June and September due to hygroscopic growth and/or cloud processing. Increases in the fine-mode particles in June and September caused AOD 440 nm >  1.00 at most sites, and annual mean AOD 440 nm values of 0.71–0.76 were found at the urban sites and 0.68 at the rural site. Unlike northern China, the AOD 440 nm was lower in July and August ( ∼  0.40–0.60) than in January and February (0.71–0.89) due to particle dispersion associated with subtropical anticyclones in summer. Low volumes and large bandwidths of both fine-mode and coarse-mode aerosol size distributions occurred in July and August because of biomass burning. Single-scattering albedos at 440 nm (SSA 440 nm) from 0.91 to 0.94 indicated particles with relatively strong to moderate absorption. Strongly absorbing particles from biomass burning with a significant SSA wavelength dependence were found in July and August at most sites, while coarse particles in March to May were mineral dust. Absorbing aerosols were distributed more or less homogeneously throughout the region with absorption aerosol optical depths at 440 nm ∼  0.04–0.06, but inter-site differences in the absorption Angstrom exponent indicate a degree of spatial heterogeneity in particle composition. The annual mean DARF was − 93  ±  44 to − 79  ±  39 W m −2 at the Earth's surface and ∼ − 40 W m −2 at the top of the atmosphere (for the solar zenith angle range of 50 to 80 ∘ ) under cloud-free conditions. The fine mode composed a major contribution of the absorbing particles in the classification scheme based on SSA, fine-mode fraction and extinction Angstrom exponent. This study contributes to our understanding of aerosols and regional climate/air quality, and the results will be useful for validating satellite retrievals and for improving climate models and remote sensing algorithms.