Observation of aerosol activation in urban Beijing

Aerosol hygroscopicity and cloud condensation nuclei(CCN) activity were characterized in urban Beijing from 11 November 2014 to 1 January 2015 during and after the Asia-Pacific Economic Cooperation (APEC) conference. The observed periods also experienced heating season in North China.High aerosol number concentration (NCN) and CCN number concentration(NCCN) were observed in Beijing. The NCN during heating period was 30% higher than that measured before the heating season. The NCCN varied dramatically in the range of 100-17560 cm-3. The NCCN were 2348cm-3、2820 cm-3、3848 cm-3、4593 cm-3、5377cm-3 at supersaturations of 0.077, 0.09, 0.18, 0.36 and 0.80%, respectively. The hygroscopicity parameter of CCN-active particles (κCCN, derived from the CCN efficiency spectra)increased from 0.10 to 0.55 when particle size diameter(Dp) changed from 70 to 150 nm. The campaign averageκCCN with a value of 0.33 was in good agreement with the results measured in other continental locations. TheκCCN in the period of the APEC was 0.27, which was 25% lower than that derived under other conditions. The less hygroscopicity is mainly due to the particles mostly composed of less soluble organics, but with lower soluble inorganics. Although some references addressed that the observed CCN number concentration could be efficiently predicted using a κ-Köhler model with variable CN size distribution, our closure test showed that consistent chemical composition assumption led to 15% underestimation of NCCN compared to the measured CCN.