Significantly Enhanced Aerosol CCN Activity and Number Concentrations by Nucleation-Initiated Haze Events: A Case Study in Urban Beijing

The evolution of haze, involving multiple processes such as nucleation, coagulation, and condensation, may exert complex effects on aerosols' cloud condensation nuclei (CCN) activity and number concentration (N_CCN). Based on field campaigns carried out in the winters of 2014 and 2016 in Beijing, we show that N_CCN was significantly enhanced by the evolution of haze, substantially driven by the nucleation process (or new particle formation). The enhancement factor of N_CCN by such nucleation-initiated haze episodes, E_N_CCN, defined as the ratio of N_CCN after haze events to N_CCN prior to haze events, ranged from 2.2 to 6.5 at a supersaturation (S) = 0.76% and from 4.2 to 17.3 at S = 0.23%, the magnitude of which partially depends on the severity of the haze event. The enhancements are much greater than those previously observed and those from model simulations of contribution from new particle formation. This suggests that CCN sources from new particle formation may be underestimated, needing reevaluation in polluted environments where the subsequent growth of newly formed particles can last 2–3 days, yielding more CCN-sized particles. We further quantified that the changes in particle size and composition during the nucleation-initiated evolution of haze are responsible for > 80% and 12–20%, respectively, of the enhancement in CCN activity. The changes in particle composition had a limited impact because most of the ambient particles were already hydrophilic, with hygroscopic parameters of 0.2–0.65.