Molecular characterization of PM2.5 in Xi'an, Northwest China to reveal seasonal variation in sources of the organic fraction

Organic aerosols (OA) are an important component of fine particulate matter (PM2.5), constituting a substantial portion of its total mass. Seasonal ambient PM2.5 samples were collected in Xi’an from April 2018 to March 2019. In order to achieve a molecular-level understanding of the OA fraction, the samples were analyzed using an iodide chemical ionization mass spectrometer combined with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS). The sum of compounds identified by FIGAERO-CIMS represented 29% organic matter in PM2.5 on an annual average, including compounds containing CHO, CHON, CHOS, and CHONS. In the winter, the concentration of identified compounds notably surpasses levels observed in the other seasons. It was especially evident during a severe pollution episode in January 2019. The relative contribution of compounds containing only CHO, i.e., no sulfur or nitrogen, increases during summer which is likely due to enhanced photochemical oxidation. It was also indicated by a slightly elevated oxidation state being observed in summer compared to other seasons. The CHON compounds are most likely mainly consisting of nitro-aromatics, e.g., phenols-like compounds, where biomass burning and secondary formation are the dominant sources. Both biogenic and anthropogenic volatile organic compounds are contributing to sulfate-containing organic compounds in urban Xi’an.