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Source apportionment results of particulate matter with an aerodynamic diameter smaller than 2.5 µm (PM2.5) from two multivariate receptor models (positive matrix factorization, PMF; solver for mixture problem, SMP) were compared to assess the comparability of the results in view of model performance, source identification and source quantification. Two receptor models applied the same filter samples of PM2.5 collected between October 2012 and September 2013 in Seoul, Korea, not only reproduced well the measured PM2.5 mass concentrations, especially the SMP model (R2 = 0.99, bias = − 0.02 μg m−3), but also identified the four major PM2.5 contributors: biomass burning (PMF: 12.0%, SMP: 13.2%), vehicles (PMF: 18.9%, SMP: 10.2%), road dust (PMF: 5.0%, SMP: 11.2%) and secondary aerosol sources (PMF: 51%, SMP: 42.5%). The large sulfate fraction in the source profile is thought to be the high vehicle fraction in PMF. The major species constituting the secondary aerosols are OC (PMF: 11.7%, SMP: 16.2%), sulfate (PMF: 21.2%, SMP: 15.3%), nitrate (PMF: 29.4%, SMP: 44.1%), and ammonium (PMF: 16.7%, SMP: 16.9%). The difference in the contribution of nitrate and sulfate between the two models underscores that multiple receptor models may need to be applied to the same dataset to better understand their sources in polluted areas, particularly reducing secondary aerosols. [ABSTRACT FROM AUTHOR]

Despite the effort to control SO 2 emission, sulfate is still one of the major inorganic components of PM 2.5 in urban area. Moreover, there is still a lack of understanding of the sulfate formation mechanism via SO 2 oxidation under various ambient conditions. In this study, we focus on sulfate formation during a haze pollution episode in the spring/summertime of 2016 in Seoul Metropolitan Area (SMA). During the pollution episode, PM 2.5 mass concentration exceeded over 60 μg m−3, and sulfate accounted for about 25% of the total PM 2.5 mass concentration. A sharp increase of sulfur oxidation ratio (SOR) values along with aerosol liquid water content (AWC) under humid conditions could be ascribed to an apparent contribution of aqueous-phase oxidation of SO 2 of sulfate formation during the pollution period. Comparisons of SOR values with four representative oxidants for the aqueous-phase oxidation (i.e., NO 2 , H 2 O 2 , O 3 , and TMIs) indicated that TMIs concentration, especially for Mn (II), showed the best positive correlation. Furthermore, for calculating the sulfate production rate, the contribution of TMIs concentration was found to be dominant within the pH range in SMA (2.1–3.0), which was determined by the chemical composition and derived AWC. These results imply that not only the SO 2 emission but also other chemical components (e.g., TMI and nitrate) would play a critical combined role in sulfate formation under urban haze condition. [Display omitted] • Aqueous-phase formation of sulfate occurred importantly during the haze in SMA. • The oxidation by TMIs is the most significant for sulfate formation. • Within the pH ranges in SMA (2.1–3.0), TMIs are the most contributing oxidants. [ABSTRACT FROM AUTHOR]