Seasonal variation in aerosol chemistry drives new particle formation and CCN activity in a coastal city, China: insights from year-long online measurements in Fuzhou

Abstract. New particle formation (NPF) is an important source of cloud condensation nuclei (CCN), which affects the global climate. Continuous observations in the coastal city of Fuzhou, conducted from June 2021 to May 2022, aimed to study NPF events and their impact on CCN. A total of 46 NPF events were identified, with a frequency of 12.7 %. The average formation rate (FR) and growth rate (GR) of particles were 3.94 ± 8.26 cm-3s-1 and 5.20 ± 1.78 nm h−1. The NPF events showed evident seasonal variation: spring (27.17 %), fall (9.89 %), winter (8.89 %), and summer (4.35 %). Spring NPF events were characterized by high FR (5.56 cm-3s-1) and suppressed growth processes, while summer, under the dominance of marine winds, exhibited the lowest FR yet the highest GR among all seasons (peak value 11.68 nm h−1). The influence of NPF on the chemical composition of PM2.5 and CCN also showed seasonal differences. In summer, NPF generated substantial amounts of sulfate and nitrate, resulting in stronger particle hygroscopicity (>0.6). In fall and winter, higher concentrations of black carbon (BC) and primary organic carbon (POC) led to weaker κinorg (≤0.55). XGBoost-SHAP attribution further quantified that FR is dominated by physical processes (nucleation mode 76.2 %, CS 13.8 %), with a sharp NH3 threshold at 4 µg m−3 and a narrow temperature range (20–25 °C). For particle growth, temperature shows a positive linear effect above 20 °C, and RH>60 % consistently suppresses particle concentrations. The enhancement effect of NPF on CCN was most significant in summer (E_NCCN=1.64), accompanied by CCN growth. In spring, the high condensation sink (CS) suppressed growth, leading to an insignificant CCN enhancement effect. In fall and winter, NPF-induced CCN enhancement mainly occurred 3–5 h after the event, with increases ranging from 13 % to 65 %, particularly notable at high supersaturation levels (0.8 %–1.0 % SS).