Seasonal patterns and diagnostic values of δ ² H, δ ¹⁸ O, d-excess, and Δ ¹⁷ O in precipitation over Seoul, South Korea (2016–2020)

Abstract. Precipitation stable isotopes are critical tracers for understanding climate variabilities and the hydrological cycles, as they enable the tracing of moisture sources, air mass mixing, and evaporation-condensation mechanisms. In mid-latitude regions such as South Korea, which are influenced by tropical and extratropical circulation, highly resolved and long-term isotope records remain scarce. Here, we analyze stable isotopes in precipitation collected bi-weekly in Seoul, South Korea, from 2016 to 2020. The oxygen isotope ratios (δ18O) ranged widely from 1.15 to –18.21 ‰, deuterium (δ2H) ratios varied from 3.3 to –132.0 ‰, and the 17O-excess ranged from 69 to –28 ‰. All three primary isotopes exhibited a coherent sinusoidal seasonal cycle, with the most depleted values in winter, gradual enrichment through spring, and sharp depletion during the summer monsoon, reflecting the combined influence of temperature and the amount effect. The deuterium excess (d‑excess) was highest during cold, dry months and lowest in humid, rainy months, reflecting shifts in relative humidity and kinetic fractionation. Meanwhile, 17O-excess (Δ17O) exhibited a similar season trend with a smaller amplitude, suggesting that, beyond its known dependence on relative humidity and kinetic fractionation, it is also modulated by large‑scale transport and water vapor mixing. The local meteoric water line closely matches the global line but winter samples show a higher intercept and a slightly steeper δ17O–δ18O slope, suggesting enhanced kinetic fractionation under continental air masses. A consistently negative δ18O–Δ17O relationship was observed except in winter when it weakened. This integrated analysis of δ18O, d‑excess, and Δ17O provides a comprehensive picture of source humidity, transport dynamics, and seasonal precipitation processes in a mid‑latitude East Asia, and offers a valuable reference for refining isotope‑enabled climate models over East Asia.