Meteorology shapes urban particle nucleation, while pollution-related factors modulate growth

Urban new particle formation (NPF) is shaped by meteorology and pollution, yet their stage-specific roles across particle sizes remain difficult to quantify. Here we combine size-resolved particle number size distributions from polluted Beijing and cleaner coastal Qingdao with XGBoost modeling and SHAP attribution to resolve environmental contributions across 10–500 nm particles and three modes. The models reproduced observed particle number concentrations with high skill (mean test R ² = 0.95 in Beijing and 0.88 in Qingdao), enabling size- and mode-specific attribution of model predictions. Meteorological variables, especially relative humidity and solar radiation, accounted for the largest share of nucleation-mode variability (~56–58%), and their contributions intensified during NPF events, reaching ~70% in Qingdao. By contrast, pollution-related variables gained importance in growth-related size modes, with city-specific differences. These results show that meteorological conditions were most strongly associated with observable nucleation-mode bursts, whereas pollution-related conditions gained importance during subsequent growth, providing observational benchmarks for urban aerosol formation and size-resolved aerosol-model sensitivities.