Influences of aerosol physiochemical properties and new particle formation on CCN activity from observation at a suburban site of China

With the aim of understanding the impact of aerosol particle size and chemical composition on CCN activity, the size-resolved cloud condensation nuclei (CCN) number concentration (N_CCN), particle number size distribution (PSD) (10–600 nm), and bulk chemical composition of particles with a diameter < 1.0 μm (PM₁) were measured simultaneously at Xinzhou, a suburban site in northern China, from 22 July to 26 August 2014. The N_CCN was measured at five different supersaturations (SS) ranging from 0.075%–0.76%. Diurnal variations in the aerosol number concentration (N_CN), N_CCN, the bulk aerosol activation ratio (AR), the hygroscopicity parameter (κ_chem), and the ratio of 44 mass to charge ration (m/z 44) to total organic signal in the component spectrum (f₄₄), and the PSD were examined integrally to study the influence of particle size and chemical composition on CCN activation. We found that particle size was more related to the CCN activation ratios in the morning, whereas in the afternoon (~ 1400 LST), κ_chem and f₄₄ were more closely associated with the bulk AR. Assuming the internal mixing of aerosol particles, N_CCN was estimated using the bulk chemical composition and real-time PSD. We found that the predicted CCN number concentrations were underestimated by 20–30% at SS < 0.2% probably due to the measurement uncertainties. Estimates were more accurate at higher SS levels, suggesting that the hygroscopicity parameter based on bulk chemical composition information can provide a good estimate of CCN number concentrations. We studied the impacts of new particle formation (NPF) events on size-resolved CCN activity at the “growth” stage and “leveling-off” stage during a typical NPF event by comparing with the case during non-NPF event. It has been found that CCN activation was restrained at the “growth” stage during which larger particle diameters were needed to reach an activation diameter(D_a), and the bulk AR decreased as well. However, during the “leveling-off” stage, a lower D_a was observed and CCN activation was greatly enhanced.