Ozone pollution in China affected by stratospheric quasi-biennial oscillation

Abstract. In recent years, near-surface ozone (O₃) level has been rising fast in China, with increasing damages to human health and ecosystems. In this study, the impact of stratospheric quasi-biennial oscillation (QBO) on interannual variations in summertime tropospheric O₃ over China is investigated based on GEOS-Chem model simulations and satellite retrievals. QBO has a significant positive correlation with near-surface O₃ concentrations over central China (92.5°–112.5° E, 26°–38° N) when the sea surface temperature (SST) over the eastern tropical Pacific is warmer than normal, with a correlation coefficient of 0.53, but QBO has no significant effect on O₃ under the cold SST anomaly. Compared to the easterly phase of QBO, the near-surface O₃ concentrations have an increase of up to 3 ppb (5 % relative to the average) over central China during its westerly phase under the warm SST anomaly. O₃ also increases above the surface and up to the upper troposphere, with a maximum increase of 2–3 ppb (3–5 %) in 850–500 hPa over central China comparing westerly phase to easterly phase. Process-based analysis and sensitivity simulations suggest that the O₃ increase over central China is mainly attributed to the anomalous downward transport of O₃ during the westerly phase of QBO when a warm SST anomaly occurs in the eastern tropical Pacific, while the local chemical reactions and horizontal transport processes partly offset the O₃ increase. This work suggests a potentially important role of QBO and the related vertical transport process in affecting near-surface O₃ air quality, with an indication for O₃ pollution prediction and prevention.