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.