Cloud Condensation Nuclei and the Contribution of New Particle Formation in the Yangtze River Delta of Eastern China

Cloud condensation nuclei (CCN) serve as a critical link between aerosols and clouds, with new particle formation (NPF) recognized as a major source of CCN. However, the quantitative contribution of NPF to CCN remains highly variable and insufficiently constrained by in situ observations. To address this gap, 2‐year synchronized observations of CCN, particle number size distribution (PNSD), and chemical composition were conducted in an urban area of eastern China. The average CCN concentration (N CCN ) was 2,220 ± 1,050, 3,510 ± 1,500, 4,050 ± 1,640, and 4,400 ± 1,740 cm −3 at 0.2%, 0.4%, 0.6%, and 0.8% supersaturations, respectively. CCN closure analysis indicated that predicting N CCN from PNSD and bulk aerosol hygroscopicity yielded a certain degree of uncertainty, particularly regarding the hygroscopicity of organic‐rich and likely NPF‐origin particles. The concurrent anthropogenic emissions obscure the apparent NPF contribution to CCN. By distinguishing CCN into primary and secondary components and focusing on secondary CCN, NPF‐related increases were estimated at 54%–71% across supersaturations of 0.2%–0.8%, with stronger NPF contribution during spring and summer. Results from the complementary survival probability (SP) approach further confirmed enhanced CCN formation due to higher formation rates and SP in warm seasons. As air quality management continues to reduce PM 2.5 mass concentrations, the organic fraction and NPF frequency are likely to increase. These trends underscore the growing importance of NPF in CCN production and the necessity for coordinated, long‐term observations of CCN and NPF to improve constraints on aerosol–cloud interactions and climate effects.