[A-AS11] Atmospheric Chemistry

[AAS11-17] Characterization of urban secondary organic aerosol and its source apportionment by the offline use of an aerosol mass spectrometer

*Baboer Yisikandeer¹, Virginia Tadei², Bhagawati Kunwar³, Qianzhe Sun¹, Nishizawa Michiko³, Daichi Asakawa⁴, Satoshi Takahama², Ann Marie Dillner⁵, Ruichen Zhou³, Michihiro Mochida^1,3 (1.Graduate School of Environmental Studies, Nagoya University, 2.Laboratory of Atmospheric Processes and their Impacts (LAPI), ENAC/IIE, Ecole polytechnique fédérale de Lausanne (EPFL), 3.Institute for Space-Earth Environmental Research, Nagoya University, 4.Osaka City Research Center of Environmental Science, 5.Air Quality Research Center, University of California, Davis)

Abstract: Secondary organic aerosol (SOA) is formed through gas-phase and aqueous-phase reactions in the atmosphere, and is an important component of atmospheric aerosols. SOA exhibits different characteristics from that of primary organic aerosol (OA). Various online-mass-spectrometry-based studies have classified SOA using source factorization method and tracer analysis and investigated its chemical composition. Offline analysis using an aerosol mass spectrometer (AMS) is capable of fractionating OAs, and provides additional information about their chemical composition, which is valuable especially for characterizing a highly oxygenated fraction. In this study, we conducted a one-month intensive aerosol sampling in Nagoya City from February 22 to March 23 in 2024. PM_2.5 samples were collected on filters and then used for AMS, FT-IR, and water-soluble carbon analysis. For the analysis using an AMS, filter samples were extracted into three different fractions: highly polar water-soluble organic matter (HP-WSOM), humic-like substances (HULIS), and water-insoluble organic matter (WISOM). They were measured using the AMS separately in the form of aerosol particles, which were generated by the nebulization of the extract solutions. The resulting mass spectra were used to quantify fractional and bulk OAs and characterize the chemical composition of the OAs. Positive matrix factorization (PMF) analysis was then performed for the offline AMS spectra to identify the sources of the OAs. The PMF analysis resulted in a 5-factor solution with two oxygenated OA (OOA) factors and a 6-factor solution with three OOA factors. These OOA factors are considered to be related to secondary formation. Three high-concentration periods for OA were found; they were largely contributed by more-oxidized OOA factor (MO-OOA) with a very high O/C ratio (1.28) and less-oxidized OOA factor (LO-OOA) with a lower O/C ratio (0.66). Backward airmass trajectory analysis for the studied period indicates the influence of the outflow from East Asian continent. Meteorological data including relative humidity (RH) from the trajectory analysis were used to further investigate the contribution of aqueous-phase SOA (aqSOA) through correlation analysis. One LO-OOA factor from the 6-factor solution showed high positive correlation with RH values under different criteria, implying its association with aqSOA formation.