Spatial and Seasonal Variation of Nitric Acid (HNO3) and Understanding the Formation of NH4NO3 in the HNO3–NH3 system in Korea

Abstract Air quality in South Korea is influenced by long-range transported air pollutants from China and domestically emitted pollutants. Among the chemical components of PM 2.5 , nitrate (NO 3 − ) is generally more dominant than sulfate (SO 4 2− ), which makes it necessary to investigate nitrate formation mechanisms in the domestic atmosphere. Although nitric acid (HNO 3 ) and ammonia (NH 3 ) are key precursors of particulate nitrate, their concentrations are not currently provided by the National Institute of Environmental Research (NIER) of Korea. HNO 3 data are extremely limited due to the lack of real-time measurement technology. Field measurements were conducted in Gwangju across all four seasons (spring, summer, fall, and winter) in 2021; in Seoul during summer, fall, and winter from 2021 to 2022; and at Ansan and Baengnyeong Island, observations were conducted in different seasons during the period 2022–2025. HNO 3 was measured using a semi-real time monitoring system developed by the Hankuk University of Foreign Studies (HUFS), while NH 3 and major PM 2.5 components were analyzed simultaneously. Results revealed that ammonium-rich conditions prevailed at all sites. A strong correlation (R 2 > 0.9) between NO 3 ⁻ and excess ammonium confirmed that NH 4 NO 3 formation is primarily governed by HNO 3 and NH 3 . Seasonal characteristics of formation and dissociation were assessed by comparing the theoretical equilibrium constant (K p ) with the observed reaction constant (K m ) derived from measured HNO 3 and NH 3 concentrations. NH 4 NO 3 showed distinct temperature-dependent behavior: dissociation in summer and formation in winter, with both regimes observed during spring and fall. Additional analysis of the molar ratio (R) and SNA (sulfate-nitrate-ammonium) composition provided insight into precursor limitations. In Seoul, Ansan, and Gwangju, HNO 3 limitation was dominant regardless of season, reflecting the influence of anthropogenic emissions and pollutant transport. In contrast, Baengnyeong Island, a background site, generally exhibited HNO 3 limitation but showed NH 3 limitation in winter, highlighting the enhanced role of NH 3 under low-NO x emission conditions. Graphical Abstract