Synchronization of Source and Sink by Boundary Layer Evolution: A Key to New Particle Formation Under Varying Ozone Pollution

Abstract. Atmospheric new particle formation (NPF) is a vital source of aerosol and cloud condensation nuclei, regulated by complex meteorological and chemical factors. Utilizing ground aerosol particle size distributions and vertical observations, this study employs a generalized additive model (GAM) and SHapley Additive exPlanations (SHAP) to quantitatively assess the marginal contributions of factors driving NPF under the NPF scenario and three non-NPF scenarios. We found that NPF depends on the synchronization of enhanced source strength and weakened sink intensity, a process controlled by planetary boundary layer evolution. Under the NPF scenario, the breakup of the inversion layer and entrainment of cleaner air aloft promote vertical mixing. This rapidly reduces the condensation sink (CS) while transporting ozone (O₃) to the surface, allowing precursor formation to coincide with a clean background, thus creating a favorable nucleation window. In contrast, nucleation is inhibited in non-NPF scenarios through distinct mechanisms: insufficient oxidation capacity (Non-O₃ scenario), source–sink desynchronization caused by stable stratification suppressing vertical exchange (Low-O₃ scenario), or rapid scavenging by high background particles (High-O₃ scenario). Correlation analysis and SHAP method corroborate the source–sink competition mechanism. Under the NPF scenario, nucleation (Nuc) mode significantly correlates with SO₂, with high temperature and sufficient precursors contributing positively to its predicted value. However, predicted Nuc values are dominated by background particles under the Low-O₃ scenario and negatively influenced by T and SO₂ under the Non-O₃ scenario, while under the High-O₃ scenario, pre-existing aerosols effectively offset precursor contributions.