Nitrate Formation Mechanisms Causing High Concentration of PM2.5 in a Residential City with Low Anthropogenic Emissions during Cold Season

During the cold season in South Korea, NO3‒ concentrations are known to significantly increase, often causing PM2.5 to exceed air quality standards. This study investigated the formation mechanisms of NO3‒ in a suburban area with low anthropogenic emissions. The average PM2.5 was 25.3 μg m‒3, with NO3‒ identified as the largest contributor. Ammonium-rich conditions prevailed throughout the study period, coupled with low atmospheric temperature facilitating the transfer of gaseous HNO3 into the particulate phase. This result indicates that the formation of HNO3 played a crucial role in determining particulate NO3‒ concentration. Nocturnal increases in NO3‒ were observed alongside increasing ozone (O3) and relative humidity (RH), emphasizing the significance of heterogeneous reactions involving N2O5. NO3‒ concentrations at the study site were notably higher than in Seoul, the upwind metropolitan area, during a high concentration episode. This difference could potentially attributed to lower local NO concentrations, which enhanced the reaction between O3 and NO2, to produce NO3 radicals. High concentrations of Cl‒ and dust were also identified as contributors to the elevated NO3‒ concentrations.