PM_2.5 nitrate formation during winter haze episodes: insights from Seoul, South Korea

This study investigates the formation and accumulation mechanisms of particulate nitrate (NO₃^-) during haze events in early winter 2021 in Seoul, South Korea, based on intensive ground-based observations from the Satellite Integrated Joint Monitoring of Air Quality (SIJAQ) campaign. Hourly measurements of particulate matter (PM), major PM_2.5 components and precursor gases revealed a strong correlation between PM_2.5 mass and NO₃^-, particularly during high-pollution episodes, with synoptic meteorological conditions exerting a dominant influence on NO₃^- variability. Under anticyclonic conditions, nocturnal gas-to-particle conversion and limited vertical mixing facilitated NO₃^- accumulation, followed by daytime volatilization at elevated temperatures and enhanced aerosol acidity. In contrast, cold frontal passages promoted rapid and sustained NO₃^- enhancement through regional transport and vertical entrainment, with elevated concentrations persisting during the subsequent stagnation of the air mass. Thermodynamic analysis indicated that ammonium nitrate formation was favored in both regimes, with ambient precursor levels frequently exceeding equilibrium thresholds. NO₃^- enrichment coincided with the increase in droplet-mode particles providing volume for secondary formation. Planetary boundary layer height (PBLH) variations further modulated surface concentrations by influencing vertical mixing and dilution. These findings highlight the complex interplay among local chemistry, aerosol microphysics and synoptic meteorology in driving wintertime haze formation in East Asian megacities, with implications for forecasting and mitigation strategies.