<p>Tropospheric ozone (O₃) formation in rural environments is controlled by interactions among nitrogen oxides (NOx), volatile organic compounds (VOCs), radical chemistry, and meteorology. This study investigated photochemical O₃ formation and oxidant variability at a rural agricultural site in Buan, South Korea, using three years (2023–2025) of continuous observations. Because direct VOC and radical measurements were unavailable, an observation-based proxy framework constrained by the oxidant indicator Ox (O₃ + NO₂) was applied using routinely measured NO, NO₂, O₃, temperature, and relative humidity. Ox was used to represent oxidant levels while reducing the influence of rapid NO titration. During the study period, annual mean NO₂ declined, whereas O₃ and Ox remained within relatively narrow ranges. O₃ and Ox showed clear seasonal cycles, with elevated levels from spring to early summer. Proxy-derived OH, VOC oxidation, HO₂, RO₂, and p(O₃) also exhibited pronounced warm-season maxima, indicating enhanced daytime photochemical activity under favorable conditions. Annual statistics of integrated daytime proxy p(O₃) and observed daytime oxidant enhancement (ΔOx) showed only modest interannual variability, with a slight decrease from 2023 to 2025. Regime filtering further identified a high-photochemistry regime under warm, relatively dry, and low-NO conditions, in which both integrated proxy p(O₃) and observed daytime ΔOx were substantially higher than under a contrasting low-photochemistry regime. These results indicate that local daytime oxidant enhancement at the Buan site was most clearly expressed under conditions favorable for radical production and photochemical processing. Although the proxy-derived variables do not represent absolute radical concentrations or ozone production rates, the framework provides a practical tool for diagnosing relative photochemical behavior in rural environments where direct VOC and radical measurements are unavailable.</p> Graphical Abstract <p></p>