Influence of atmospheric ammonia on secondary inorganic aerosol formation in PM2.5 during spring 2024 in the Hongseong area, Republic of Korea

Nitrate and organic components have been considered key contributors to high PM 2.5 concentrations in Korea. This study examines the chemical evolution of aerosol species influenced by atmospheric ammonia (NH 3 ) in Hongseong county, Chungcheongnam-do, Republic of Korea, during late spring 2024 (May 20 – June 18), a period characterized by intensive agricultural activity. Using an aerosol chemical speciation monitor and ammonia analyzer, an ammonium-rich atmospheric state (average NH 3 concentration: 26.7 ± 12.4 ppb) was observed, with notable contributions from ammonium (NH 4 + ), nitrate (NO 3 - ), and sulfate (SO 4 2- ), especially under high-humidity conditions. The relative contributions of nitrate to PM 2.5 for low (0–15 μg/m 3 ), medium (16–30 μg/m 3 ), and high (over 30 μg/m 3 ) concentration intervals were 13.8%, 23.4%, and 29.6%, respectively. Elevated relative humidity (RH), averaging 63.1% (low), 82.3% (medium), and 91.7% (high), played a significant role in nitrate formation for those concentration intervals. Elevated nighttime ammonium conversion ratios (NHR) highlighted the importance of abundant atmospheric NH 3 under high RH conditions, facilitating effective heterogeneous uptake and subsequent particulate ammonium formation. Source tracking using the conditional bivariate probability function (CBPF) model identified agricultural fields, power plants, and industrial complexes as major sources of precursor emissions. These results emphasize the need to control emissions of NH 3 , SO 2 , and NOx in rural areas to mitigate PM 2.5 pollution and improve air quality. • Ammonia is a key precursor for the formation of secondary inorganic aerosols. • Relationship of RH with NOR reflects heterogeneous uptake of NO 2 or hydrolysis of N 2 O 5 on wet aerosols. • NOR and NHR demonstrated a strong positive correlation with relative humidity. • Reducing NO x concentrations is essential for controlling high levels of PM 2.5 . • Regions with complex precursor gas emission sources and high RH amplify the formation of secondary inorganic aerosols.