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

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 (NO₃⁻) is generally more dominant than sulfate (SO₄²⁻), which makes it necessary to investigate nitrate formation mechanisms in the domestic atmosphere. Although nitric acid (HNO₃) and ammonia (NH₃) are key precursors of particulate nitrate, their concentrations are not currently provided by the National Institute of Environmental Research (NIER) of Korea. HNO₃ 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. HNO₃ was measured using a semi-real time monitoring system developed by the Hankuk University of Foreign Studies (HUFS), while NH₃ and major PM_2.5 components were analyzed simultaneously. Results revealed that ammonium-rich conditions prevailed at all sites. A strong correlation (R² > 0.9) between NO₃⁻ and excess ammonium confirmed that NH₄NO₃ formation is primarily governed by HNO₃ and NH₃. 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 HNO₃ and NH₃ concentrations. NH₄NO₃ 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, HNO₃ limitation was dominant regardless of season, reflecting the influence of anthropogenic emissions and pollutant transport. In contrast, Baengnyeong Island, a background site, generally exhibited HNO₃ limitation but showed NH₃ limitation in winter, highlighting the enhanced role of NH₃ under low-NO_x emission conditions.