Multisite Observations of the Oxygen Isotope Anomaly in Surface Ozone Across China Indicate an Imprint of Stratosphere-Troposphere Exchange

The mass-independent oxygen-17 anomaly in ozone (Δ17O–O3) provides a unique constraint on atmospheric transport and oxidation processes, yet its spatial variability over continental regions remains poorly characterized. Here, we present the first multisite observations of surface Δ17O–O3 reported to date across China, revealing pronounced spatial heterogeneity (22.6–29.4 ‰) and a latitude-related pattern. This pattern cannot be fully accounted for by known temperature- and pressure-dependent tropospheric isotope effects alone. Its covariation with tropopause height, potential vorticity, humidity, and diagnosed stratospheric-intrusion conditions indicates a measurable imprint of stratosphere-troposphere exchange on surface Δ17O–O3. Using an isotope mixing framework, we derive an apparent stratospheric-like contribution of up to 26 ± 18% to surface ozone, which should be interpreted as a semiquantitative upper-bound estimate. Sensitivity analysis further shows that spatial variations in Δ17O–O3 can influence inferred nitrate formation pathways, with recalculations of previous nitrate studies indicating biases of up to 18%. These results suggest that spatially resolved Δ17O–O3 observations can help constrain stratospheric influence on surface ozone and reduce biases in Δ17O-based interpretations of atmospheric oxidation pathways.