Measurements of surface cloud condensation nuclei and aerosol activity in downtown Shanghai

Cloud condensation nuclei (CCN) and aerosols were measured continuously at an urban site of Shanghai in the period of September 2010–August 2011. Over the entire campaign, annual averages of CCNs at 0.2–1.0% supersaturation (SS) roughly ranged from 4000 cm−3 to 8000 cm−3. CCNs (0.2% SS) varied in a remarkable pattern on monthly or seasonal time scales, showing higher in spring and winter and lower in summer and autumn. High CCN events occurred frequently in spring (21 out of 30 days) and winter (19 out of 30 days) rather than other seasons. The diurnal variation of CCNs popularly showed a pronounced bi-modal pattern in spring, summer and winter while a uni-modal pattern in autumn. The apparent activation diameter was below 100 nm at 0.8% SS regardless of their chemical composition, whereas few particles in diameter smaller than 30 nm were activated. The activation ratio (CCN/CN) at 0.8% SS varied within 0.1–0.9 with an average of 0.47. The CCN activity of CN particles indicated a strong dependence on first aerosol size, which can explain its fundamental changes, and second aerosol chemical composition. High CCN levels were usually observed in the period of particulate pollution (e.g. haze), during which CCN increased up 2.0 times in hazy days higher than clear days, revealing a significant influence of aerosols on CCN in the atmospheric planet boundary.