Variations of cloud condensation nuclei (CCN) and aerosol activity during fog-haze episode: a case study from Shanghai

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 PM2.5 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 (kappa), 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.