Day-night cloud aqueous chemistry diverges organic molecules pathways and health implications

The atmospheric aqueous phase represents a critical chemical reactor, where day-night alternation establishes distinct physicochemical regimes that transform the composition and impacts of organic aerosol. This study investigates how these contrasting regimes in cloud water govern the molecular characteristics of water-soluble organic matter (WSOM) and its subsequent health implications. Analysis of cloud water samples from Mt. Damaojian, southeastern China, revealed that nocturnal samples exhibited higher liquid water content, greater acidity, and a pronounced shift in chemical dominance from WSOM to secondary inorganic ions compared with daytime. Molecular analysis by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in both ESI ± modes identified CHO and CHON compounds dominated across both periods, but with a significant nocturnal enrichment of CHON species. The chemical signature of daytime WSOM exhibited a higher OSc and a proportion of Lipids reflecting dominant photo-oxidation processes. In contrast, nighttime samples showed higher N/C and a greater abundance of Protein1 (low oxidized protein), consistent with nitration and condensation reactions under dark conditions. Additionally, in vitro exposure experiments indicate that cloud-processed WSOM significantly inhibit cell viability, induce apoptosis, and trigger inflammation. Nighttime samples exhibited stronger cytotoxicity, likely due to their higher levels of nitrogen-containing and aromatic compounds. By integrating organic chemistry and toxicology, this work provides novel insights into the day-night dichotomy in cloud-processed WSOM composition and its corresponding biological impacts.