Sensitivity of ground-level ozone to surface elevation in CMIP6: Role of stratosphere-troposphere exchange

Ground-level ozone, a critical air pollutant affecting human health and ecosystems, exhibits significant correlation with surface elevation. Using CAMS reanalysis and CMIP6 historical simulations, this study quantifies the surface elevation-dependent sensitivity of ground-level ozone across the Northern Hemisphere (NH). Reanalysis results reveal that ground-level ozone in the NH mid- and high-latitudes is sensitive to surface elevation, with ozone increasing at rates of 1.85 and 6.06 ppbv/km, respectively. Particularly, high-elevation regions such as the Tibetan Plateau and Greenland exhibit significant elevation dependence with rates of 5.21 (r = 0.81) and 4.57 ppbv/km (r = 0.78), respectively, likely due to more efficient stratosphere-troposphere exchange (STE). CMIP6 models generally capture this surface elevational sensitivity, though with different ozone increase rates of 2.5 and 2.7 ppbv/km in NH mid- and high-latitude for the multi-model mean. It is found that ‌STE-driven ozone explains 38 % and 37 % of the variability in NH mid- and high-latitude, respectively, where surface elevation acts as a ‌“STE enhancer” by compressing the vertical distance for stratospheric intrusion. Notable inter-model disparities of the contribution from STE emerge, with IPSL-CM5A2-INCA exhibiting minimal elevational sensitivity (0.01 ppbv/km). Excluding this outlier elevates STE's contribution to 46 %. These findings highlight ‌surface elevation-specific ozone response rates‌ critical for air quality management in mountainous and plateau regions and underscore the need to refine STE parameterizations in climate-chemistry models.