Long-Term Variation of O3 In the Yangtze River Delta and its Influencing Factors from a Regional Perspective

Abstract

To tackle regional O3 pollution, understanding long-term trends and drivers of background O3 is crucial. This comprehensive study analyzed O3 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 O3 production sensitivity . EEMD analysis showed a significant decreasing trend of -0.7 ppb/decade (p < 0.001) in O3 mixing ratios from 2005 to 2022, whereas the direct trend from raw O3 data showed an insignificant increase of 0.4 ppb/decade (p = 0.67). Meteorological normalizated using the RF model , anthropogenic emissions drove a slight O3 rise (1.7 ppb/decade), partly offset by meteorology. Using the FNR, a noticeable shift in O3 photochemical production sensitivity from VOCs-limited to NOx-limited regime was observed since 2014, possibly due to reduced NOx emissions but increased VOC emissions. Effective regional O3 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.

Keywords: regional background ozone, long-term variations, Ensemble Empirical Mode Decomposition, Random Forest model, photochemical O3 production sensitivity