Measurements of cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were performed simultaneously at an urban site in Shanghai from 6 to 9 November 2010. The variations of CCN number concentration (N CCN ) and aerosol activity (activated aerosol fraction, N CCN /N CN ) were examined during a fog–haze co-occurring event. Anthropogenic pollutants emitted from vehicles and unfavorable meteorological conditions such as low planetary boundary layer (PBL) height exerted a great influence on PM 2.5 and black carbon (BC) loadings. N CCN at 0.2% supersaturation (SS) mostly fell in the range of 994 to 6268 cm −3 , and the corresponding N CCN /N CN varied between 0.09 and 0.57. N CCN and N CCN /N CN usually were usually higher in the hazy case due to increased aerosol concentration in the accumulation mode (100–500 nm), and lower in the foggy–hazy and clear cases. The BC mass concentration posed a strong positive effect on N CCN in the foggy–hazy and hazy cases, whereas it poorly correlated with N CCN in the clear case. N CCN /N CN was weakly related with BC in both foggy–hazy and hazy cases. By using a simplified particle hygroscopicity (κ), the calculated critical dry size (CDS) of activated aerosol did not exceed 130 nm at 0.2% SS in spite of diverse aerosol chemical compositions. The predicted N CCN at 0.2% SS was very successful compared with the observed N CCN in clear case ( R 2 =0.96) and foggy–hazy/hazy cases ( R 2 =0.91). In addition, their corresponding ratios of predicted to observed N CCN were on average 0.95 and 0.92, respectively. More organic matter is possibly responsible for this closure difference between foggy–hazy/hazy and clear cases. These results reveal that the particulate pollutant burden exerts a significant impact on N CCN , especially N CCN /N CN promotes effectively during the polluted periods.