Abstract
In-cloud brown carbon (BrC) can change cloud lifetimes and radiative effects. However, little is known regarding its molecular-level composition. Herein, the molecular characteristics of water-soluble organic matter (WSOM) in cloud water were analyzed by Fourier transform ion cyclotron resonance mass spectrometry coupled with electrospray ionization (ESI) in negative mode. CHON formulas were dominant in WSOM and exhibited the highest level of aromaticity. Combined with the water-soluble BrC (WS-BrC) light-absorption coefficient (Abs365), the principal component analysis indicated that WS-BrC light absorption was mainly affected by unsaturation, aromaticity, highly oxygenated compounds, and N-containing formulas. Thirteen important potential WS-BrC were identified based on a random forest regression model, which could explain 61.7% of the variation in Abs365. Among them, C7H6N2O5, C8H8N2O5, C9H10N2O5, C10H12N2O5, C8H10O5S, and C8H9NO7S were identified as WS-BrC for the first time in clouds, and these may belong to dinitrophenols or nitrotyrosine, organosulfates, and their derivatives. These findings would improve our understanding of cloud WS-BrC composition.
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