Description: An integrated aerosol-analytical-system was deployed in Nanjing, a megacity in the Yangtze-River-Delta, to measure size-resolved aerosol mixing states, effective densities, CCN activities, and chemical composition in August 2013. It was found that aerosols were predominantly internally mixed. The average effective densities were 1.38±0.09, 1.48±0.08 and 1.53±0.07 g cm -3 for 50, 80 and 120nm particles, respectively. Although black carbon (BC) represented only 0.3%, 1.6%, 3.3% of the particle mass, on average it was present in 7%, 38% and 47% of the total particle number concentration at 50, 80 and 120 nm, respectively, indicating that BC particles may contribute significantly to the total atmospheric aerosol population. Externally mixed BC was only occasionally observed with an effective density of 0.67-0.97 g cm -3 . Aerosols sampled generally exhibited a relatively high CCN activity and hygroscopicity ( κ = 0.35±0.13). Both newly formed particles and freshly emitted BC particles were observed to age rapidly from photochemical processes, with a significant enhancement in the particle CCN activity and increase in the effective density. Aerosols influenced by four different air masses presented similar CCN activation, indicating that CCN activation would be primarily dependent on the particle size rather than the particle origin (and hence original composition). Our results suggest that under highly active photochemical conditions as encountered in this study, particles from both local sources and regional transport can be rapidly converted into efficient CCN by photochemical aging, thereby making important contributions to the atmospheric CCN budget and exerting profound implications on aerosol indirect climate forcing.