First field deployment of VIA-NO₃-CIMS for molecular characterization and source apportionment of particle-phase oxygenated organic molecules in eastern China

Abstract. Secondary organic aerosol (SOA) is a major component of fine particulate matter, yet its molecular-level characterization remains challenging. Here, we systematically evaluated the performance of a Volatilization Inlet for Aerosols coupled with nitrate chemical ionization mass spectrometry (VIA-NO₃-CIMS), including particle transmission, volatilization behavior, quantitative response, and operational stability. Following instrument characterization, the first field deployment of VIA-NO₃-CIMS was conducted at the SORPES station in Nanjing, eastern China. A high-confidence dataset containing 1,695 particle-phase oxygenated organic molecules (OOMs) was established. The summed OOM concentration correlated strongly with AMS-derived organic aerosol concentrations (r = 0.88), demonstrating the quantitative capability of VIA–NO₃–CIMS. Compared with gas-phase species, particle-phase OOMs exhibited larger carbon numbers, higher unsaturation, and lower volatility, with low-volatility compounds dominating the particle phase. Positive matrix factorization resolved seven SOA factors, including three biogenic-dominated, two anthropogenic-dominated, one regional background, and one pollution-episode factor. These factors contributed approximately 36.5 %, 35.4 %, 15.6 %, and 12.1 % of the total particle-phase OOM signal, respectively. Their molecular characteristics and temporal behaviors indicate important influences from daytime photochemistry, nighttime NO₃ oxidation, regional transport, and episodic pollution accumulation. Factor contributions evolved systematically with PM_2.5, with freshly formed SOA dominating under clean conditions and the pollution-episode factor becoming increasingly important as pollution levels increased, eventually dominating under the most polluted conditions. These results demonstrate that VIA-NO₃-CIMS enables quantitative molecular-level characterization and source apportionment of particle-phase SOA, providing new insights into the composition and evolution of urban organic aerosols.