Ozone Pollution in the Western Yangtze River Delta During the 2020 and 2021 Warm Seasons: Roles of Meteorology and Air Mass Transport.

Surface ozone (O), a key hurdle in air quality improvement in China, often displays regional pollution characteristics. This study investigated the influence of meteorological conditions and air mass transport on O and non-methane hydrocarbons (NMHCs) concentrations in Nanjing, located in the western Yangtze River Delta (YRD) region of China during April-September of 2020 and 2021 based on online observations of O and its precursors and meteorological conditions, backward-trajectory analysis, and an observation-based box model (OBM). O concentrations rose with temperature, albeit non-linearly. Southeastern trajectories constituted the most dominant air mass transport pathway (29.3%) and were associated with the highest O concentrations. The concentration-weighted trajectory analyses of O and NMHCs during four O pollution episodes suggested that urban/industrial areas in central and eastern YRD were potential source regions. The OBM results indicated that while O sensitivity was dominated by the NMHCs-limited regime, the relative contributions of three O sensitivity regimes varied across air mass trajectory clusters. The southeastern air masses with long-range transport showed the highest frequency of the transition and NO-limited O sensitivity regimes. These findings underscore the crucial role of regional air mass transport not only in determining O and NMHCs concentrations but also in shaping O formation sensitivity, highlighting the necessity of implementing regionally coordinated control strategies for effective O and NMHCs pollution mitigation.