Molecular Characterization of Nighttime New Particle Formation in Coastal Urban Environment: Season-Dependent Roles of Sulfuric Acid and Oxygenated Organic Molecules.

New particle formation (NPF) substantially contributes to atmospheric aerosols, which influences human health and climate. Nighttime NPF has been observed with intensities comparable to those of daytime events in both pristine and polluted environments, yet its molecular mechanisms and driving precursors remain largely unknown. Here, we present a comprehensive analysis of nighttime NPF in Dalian, a coastal city in China, based on approximately 1.5 years of measurements using state-of-the-art instruments. Our results reveal seasonally distinct driving precursors. In the spring and winter, nocturnal sulfuric acid (H2SO4) formation primarily drives NPF, with elevated H2SO4 and its clusters closely linked to particle formation, while the H2SO4 dimer formation proceeds near the collision limit. In contrast, during the summer and warm autumn, oxygenated organic molecules (OOMs) abundantly emerge, whereas H2SO4 and its dimers are scarce. OOM dimers show positive correlations with nascent particles, with a mean Pearson correlation coefficient of 0.55. While organic-driven NPF has been reported only in biogenic environments, our observations provide molecular-level evidence suggesting that under anthropogenic influence, OOM dimerization may play an important role in initiating nighttime NPF in coastal urban environments during summer and warm autumn. These findings offer comprehensive mechanistic insights into nighttime NPF and underscore the importance of accounting for seasonal shifts in NPF mechanisms when modeling atmospheric aerosols.