Distribution and 13C Signature of Dicarboxylic Acids and Related Compounds in Fine Aerosols Near Underground Coal Fires in North China: Implications for Fossil Origin of Azelaic Acid

To understand the characteristics of chemical components and dicarboxylic acids, oxocarboxylic acids and a-dicarbonyls in atmospheric aerosols at underground coal fire environment, we collected fine aerosols (PM2.5) from Wuhai, Inner Mongolia, North China, where the underground coal fire often occurs. PM2.5 samples were analyzed for the measurements of carbonaceous components, stable carbon isotopic composition (delta C-13) of total carbon, and molecular distribution and compound-specific d 13C of diacids, oxoacids and alpha-dicarbonyls. Oxalic acid (C-2) was the most abundant species, accounting for 43.2%-63.1% (avg. 50.8%) of total diacids, followed by phthalic acid (Ph) (13.7%). Azelaic acid (C-9) was the third most (7.4%) abundant, followed by succinic (C-4) and malonic (C-3) acids. Significant correlations of C-2 diacid with glyoxylic (omega C-2) and 3-oxopropanoic (omega C-3) acids and C-3 diacid and their delta C-13 implied that diacids and related compounds were mainly derived from the underground coal fire emissions and subsequent in situ secondary formation. Based on molecular distribution, seasonal pattern and linear relations of C-9 diacid with C-2, C-3,omega C-2 and omega C-3 as well as with Ph acid and total diacids, together with the comparability and similarity of its delta C-13 (-27.0 +/- 0.9%) with that of Ph acid and organic matter in coals, respectively, we found that the C-9 diacid has been mainly derived from its precursors such as unsaturated fatty acids emitted from underground coal fires and their subsequent in situ photochemical oxidation. However, further research is needed to confirm such an importance of coal combustion for azelaic acid loading in atmospheric aerosols. Plain Language Summary Fine aerosols in the atmosphere cause high uncertainty in radiative forcing of the Earth's climate system and have become one of the serious environmental risk factors, causing more than 6 million premature deaths annually. The atmospheric environment over the underground coal fire field is different from that in urban area, because the emission of inorganic gaseous species, volatile organic compounds and particulates that consist oxygen-containing organic species and polycyclic aromatic hydrocarbons and their subsequent photochemical oxidation processes are relatively larger at the coal fire field. Our study on fine aerosols from underground coal fire field at Wuhai, North China showed a rather large abundance of dicarboxylic acids associated with a specific partitioning from the coal fires compared to observations from other environments including an unusual abundance of azelaic acid.