The optical, physical properties and direct radiative forcing of urban columnar aerosols in Yangtze River Delta, China

The aerosol optical and physical properties as well as its direct radiative forcing (DRF) in urban area of Nanjing (urNJ) are investigated, based on the measurements of Cimel sun-photometer combined with a radiation transfer model. We find that the annual mean 550 nm aerosol optical depth (AOD) of the total aerosols is about 0.65, dominated by scattering aerosols (about 94 %), resulting in a mean single scattering albedo (SSA) of 0.93 at 550 nm and refractive index of 1.44 + 0.0084i at 440 nm during the sampling period. The scattering aerosol has larger size than the absorbing aerosol, with Angstrom exponents (AE) of 1.19 at 440/870 nm, 0.13 smaller than the latter one. The coarse mode fraction for the scattering aerosol (18.03 %) is much smaller than the absorbing aerosol's (43.91 %). Thus, the fine mode aerosols presents more scattering (SSA = 0.95) while the coarse aerosol is more absorption (SSA = 0.82). Analysis implies that there are about 15 % and 27.5 % occurrences of dust and black carbon dominated mixing aerosols, respectively, during the sampling period. All the optical properties follow a simple unimodal pattern. Aerosols in urNJ have a two-mode lognormal pattern in volume size distribution, peaking at the radius of 0.148 and 2.94 µm, and the AOD positively depends on them. Although the fine mode aerosol has a much smaller sizes than the coarse one, they have the same level of the volume concentrations (about 0.12 µm 3 /cm 3 ) due to much higher fraction of the fine aerosol. Estimations present that the mean aerosol DRFs at the top of atmosphere (TOA) are −10.69, −16.45, p5.76 W/m 2 , respectively, for the total, scattering and absorbing aerosols in clear sky. At the surface, the DRFs are 1.1–2.5 times stronger than those at TOA, and the fine aerosol DRFs in these three type of aerosols account for 83.7 %, 91.7 % and 67.2 %, respectively, to their totals. Normally, aerosol DRFs is not very sensitive (no more than 5 %) to its profiles in clear sky condition (extreme cases excepted), although both aerosol scattering and absorption could become weaker to some extent if more aerosols were in lower layers. Both the aerosol properties and DRFs have substantial seasonality.