Characterization of Secondary Aerosol Formation via HONO and HNO3 Reactions and Source Apportionment in Daejeon and Iksan, Republic of Korea

This study investigates the atmospheric formation and sinks of HONO and HNO3 and their contribution to secondary PM2.5 formation in Daejeon (urban) and Iksan (suburban), South Korea. Continuous observations revealed distinct concentration patterns: Iksan exhibited elevated ammonia and nitrate levels associated with agricultural activities and biomass burning, while Daejeon showed higher NOx concentrations driven by traffic and industrial sources. Positive Matrix Factorization (PMF) analysis indicated that secondary formation was the dominant contributor to PM2.5 at both sites, with biomass burning exerting an additional influence in Iksan. Among observed precursors, HNO3 showed the highest conversion to aerosol nitrate, highlighting aerosol-phase reactions as its primary sink, followed by dry deposition. Seasonal analysis demonstrated that HONO loss was largely controlled by photolysis in summer. Externally transported aerosols contributed more than locally formed particles at both sites, emphasizing the role of regional background pollution. These findings provide a scientific basis for region-specific air quality strategies that combine local precursor control with the management of long-range transport.