Molecular composition of Beijing PM2.5 Brown carbon associated with fluorescence revealed by gas chromatography time-of-flight mass spectrometry and parallel factor analysis

Abstract

Brown carbon (BrC), the important component of carbonaceous aerosol, attracted much attention for its complex chemical composition which affect its light absorption, fluorescent properties, and atmospheric reactions. In this study, we revealed for the first time the molecular composition of non-polar/low-polar organic matters contributing to fluorescence of Beijing PM2.5 from 2016 to 2018. Firstly, excitation-emission matrix (EEM) spectra of the 122 PM2.5 samples were analyzed and supported by gas chromatography-time of flight mass spectrometry (GC-ToF-MS) and partial least squares regression model, 400 BrC molecules associated with fluorescence were screened out among which 44 were the newly ones. Secondly, six chromophores (C1–C6) were identified by applying parallel factor analysis (PARAFAC) model, which represented categories of compounds consistent with the ToF results. Then, the specific molecule composition of each chromophore was achieved by combining GC-ToF-MS and EEM-PARAFAC. For example, indeno[1,2,3-cd]pyrene is a new fluorescent molecule which contributed high in C3 (high oxygenated humic-like chromophore) and C5 (fluorescein chromophore). The results can, on one side, enrich the database of fluorescent molecular composition, which turns out to be similar as that of light-absorption; on the other side and more importantly, provide chemical characteristics of BrC fluorescent chromophores on the level of molecule.