Characterization and Seasonal Variation of PM2.5 Composition in Xi’an, Northwest China: Oxygenated and Nitrogenous Organic Aerosol

Oxygenated (CHO) and nitrogenous (CHON) organic aerosols (OA) are important components of fine particulate matter (PM_2.5) in urban environments. To achieve a molecular-level understanding of the seasonal variation of the OA fraction, ambient PM_2.5 samples collected from April 2018 to March 2019 in Xi’an, Northwest China, were analyzed using an iodide Chemical Ionization Mass Spectrometer combined with a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS). The set of compounds identified by FIGAERO–CIMS was estimated to represent 28.6% of the organic matter in PM_2.5. Evaporation temperatures measured by FIGAERO–CIMS indicated that semivolatile organic compounds (SVOCs) were dominant among the identified analytes. Concentrations of CHO (6.01 ± 4.24 μg m^–3) and CHON (3.17 ± 2.34 μg m^–3) increased in winter, especially during a severe haze episode in January 2019. The CHO compounds comprised up to 75.3 ± 3.2% of the total detected compounds. The average carbon oxidation state ($\overline{O{S}_C}$) was slightly elevated in the summer samples. The CHON compounds were mainly nitro-aromatics and their abundance increased substantially in winter, which was attributed to extensive biomass burning demonstrated by high levels of levoglucosan. Biomass-burning related sources accounted for 61.0 ± 19.6% and 68.3 ± 21.9% of the total CHON concentration in autumn and winter, respectively, while secondary formation was the dominant source of CHON species in spring (70.1 ± 11.6%) and summer (79.8 ± 7.0%). These results emphasize the importance of secondary formation and biomass burning as sources of OA components and reveal a clear need to control biomass burning used for heating in Xi’an and its surroundings.