Long-term variation of O3 in the Yangtze River Delta and its influencing factors from a regional perspective

To tackle regional O-3 pollution, understanding long-term trends and drivers of background O-3 is crucial. This comprehensive study analyzed O-3 data from the Lin'an regional atmospheric background station (LAN) in the Yangtze River Delta (YRD), utilizing the Ensemble Empirical Mode Decomposition (EEMD) method to discern trends and the Random Forest (RF) model to evaluate anthropogenic and meteorological impacts, while also employing the satellite-based HCHO/NO2 column ratio (FNR) as a proxy to assess O-3 production sensitivity. EEMD effectively filtered out high-frequency signals, revealing a significant decreasing trend of -0.7 ppb/decade (p < 0.001) in O-3 mixing ratios from 2005 to 2022. In contrast, direct analysis of the raw O-3 data showed an insignificant increase of 0.4 +/- 0.9 ppb/decade (p = 0.67), highlighting the importance of data preprocessing methods in assessing long-term trends. Meteorological normalization using the RF model, anthropogenic emissions drove a slight O-3 rise (1.7 ppb/decade), partly offset by meteorology. Using the FNR, a noticeable shift in O-3 photochemical production sensitivity from VOCs-limited to transitional regime was observed since 2014, potentially attributed to decreased NOx emissions. Effective regional O-3 pollution mitigation in the YRD demands a comprehensive strategy, focusing on concurrent reductions in NOx and VOCs emissions, with emphasis on efficient VOCs control measures.