Excitation-emission matrix fluorescence, molecular characterization and compound-specific stable carbon isotopic composition of dissolved organic matter in cloud water over Mt. Tai

Abstract Summertime bulk and size-segregated cloud water samples were collected in 2014 at the summit of Mount Tai (Mt. Tai, 1534 m a.s.l.) in the North China Plain to investigate the concentrations, size distributions and stable carbon isotopic compositions (δ 13 C) of dicarboxylic acids, oxoacids and α-dicarbonyls as well as fluorescence properties of dissolved organic carbon (DOC). Positive correlations were observed for Peaks A (terrestrial humic-like), M (marine humic-like) and T (protein-like) with diacids and related compounds (r 2  ≥ 0.43), except for Peak M and α-dicarbonyls, illustrating that fluorescent components and dicarboxylic acids were derived from common biological origins and biomass burning. Oxalic acid (C 2 , 2080 μg L −1 ) was the dominant diacid in this study, followed by succinic (C 4 , 640 μg L −1 ) and glyoxylic (ωC 2 , 448 μg L −1 ) acids. Compared to organic precursors, the relatively negative δ 13 C values of C 2 (−19.1‰), glyoxylic acid (ωC 2 , –16.6‰), pyruvic acid (Pyr, −21.5‰) and methylglyoxal (MeGly, −19.6‰) supported that C 2 , Pyr, ωC 2 and MeGly were formed via the oxidation of 13 C-enriched organic precursors in aqueous phase. Furthermore, the larger size (22 μm) of cloud droplet was associated with more abundant organic compounds in general, which might be caused by scavenging ability and condensation of aerosol particles on the larger surface of large droplets. Our study suggested that high loadings of diacids and related compounds in cloud water over Mt. Tai were involved with emissions from anthropogenic and biological sources, followed by photochemical formation during the long-range atmospheric transport.