Long-term trends of ozone in the Yangtze River Delta, China: Spatiotemporal impacts of meteorological factors, local, and non-local emissions.

The Yangtze River Delta (YRD) region has witnessed a consistent decrease in NO, CO, and PM from 2016 to 2023. However, ozone has exhibited fluctuating patterns. Quantifying ozone contributions from emissions, both within and outside the YRD, is essential for understanding city-cluster-scale ozone pollution (CCSOP). To address these concerns, a comprehensive approach combining Kolmogorov-Zurbenko filtering, Empirical Orthogonal Function, Absolute Principal Component Score, and Multiple Linear Regression methods (KZ-EOF-APCs-MLR) was employed to quantify the impacts of meteorological factors, local and non-local emission contributions of ozone (LECO and NECO). Emission changes were identified as the predominant factor shaping annual fluctuations in ambient ozone. Notably, during the previous and middle stages of the COVID-19 pandemic (from 2017 to 2021), emissions reductions led to a marked decrease in YRD ozone levels (-7.01 µg/m), with a pronounced rebound post-pandemic (2022 to 2023) (+8.04 µg/m). Seasonally, the emission-induced ozone exhibited fluctuating upward trend during autumn and winter, suggesting a transition of ozone pollution towards colder seasons. Spatially, high LECO concentrated in the eastern YRD (EYRD) across spring, autumn, and winter, becoming prominent in the central YRD (CYRD) during summer. During CCSOP, the CYRD exhibited the highest LECO and exceedance frequency (20.82 µg/m and 45.27 %). LECO explained a large portion of ozone variability during CCSOP, particularly in the EYRD, while NECO showed less explanatory power but consistently high contributions (148.05 ± 15.52 µg/m). These findings offer valuable insights for a deeper understanding of the evolving patterns of ozone pollution and the issue of CCSOP in the YRD.