Seasonal Composition of PM2.5 and Secondary Inorganic Aerosols at a Rural Site near Livestock Facilities in Ulsan, South Korea

Seasonal observations of particulate composition are needed to characterize rural air quality near livestock facilities without assuming that nearby emissions determine ambient aerosol chemistry. This study examined PM2.5 and eight water-soluble inorganic ions in weekly 24 h filter samples collected in Duseo-myeon, Ulsan, South Korea, from June 2023 to November 2025. Seasonal concentrations and nitrate mass fractions were evaluated alongside particulate ammonium-to-sulfate ratios and a stoichiometric neutralization index. PM2.5 was highest in spring, whereas nitrate was highest in winter and lowest in summer, with seasonal means of 3.00 and 0.26 µg m−3, respectively. Spring and winter both frequently exhibited elevated ammonium-to-sulfate ratios, but nitrate exceeded sulfate by mass much more often in winter. The ratio intervals summarize observed composition and do not define thermodynamic thresholds or precursor-sensitivity regimes. The study provides a multi-year composition baseline for this rural site and shows that the seasonal PM2.5 maximum and the strongest nitrate contribution did not coincide. Weekly sampling, incomplete gas-particle time matching, and the absence of concurrent thermodynamic constraints limit attribution of the observed nitrate abundance to specific sources or formation processes.