Study on the characteristics of urban background ozone pollution based on long-term observations from mountain sites in China during 2015–2022

In recent years, Chinese cities have faced persistent O₃ pollution challenges. O₃ concentrations at urban mountain sites, serving as proxies for urban background O₃ levels, can help address data gaps in O₃ research in China. This study, for the first time, analyzed O₃ data from ten urban mountain sites during 2015–2022 using statistical methods like the Mann-Kendall test, Theil-Sen slope estimation, and Pearson correlation analysis to reveal characteristics and trends in urban background O₃ and proposed a metric for assessing mountain sites as urban background stations. Results indicate that O₃ at mountain sites were generally higher than at plain sites, regardless of mean, 10th percentile, or 90th percentile the average maximum daily 8-h average (MDA8) O₃. Although mountain sites exhibited similar annual and diurnal variations as plain sites, their amplitudes were significantly lower. Significant upward trends (P < 0.05) in MDA8 O₃ were observed mainly at plain sites, whereas trends at mountain sites were insignificant (P > 0.05) during 2015–2022. The increase in urban ground-level O₃ was largely due to changes in VOCs and NO_x emission structures, which reduce nighttime titration effects while enhancing daytime O₃ production. The insignificant trends observed at mountain sites suggest that these mountain sites may be in NO_x-limited or transitional regimes rather than VOCs-limited regimes, which is also associated with their lower NO_x concentrations compared to plain sites. These findings suggest that if China continues to prioritize the reduction of NO_x emissions, urban ground-level O₃ could further increase and approach the O₃ at mountain sites. Finally, this study proposes that the correlation in nighttime O₃ concentrations between mountain and plain stations can serve as a valuable indicator for evaluating the suitability of mountain stations as regional background stations in urban areas. It recommends mountain sites in cities such as Changji, Nanping for urban background O₃ monitoring stations.