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

Surface ozone (O₃), 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 O₃ 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 O₃ and its precursors and meteorological conditions, backward-trajectory analysis, and an observation-based box model (OBM). O₃ 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 O₃ concentrations. The concentration-weighted trajectory analyses of O₃ and NMHCs during four O₃ pollution episodes suggested that urban/industrial areas in central and eastern YRD were potential source regions. The OBM results indicated that while O₃ sensitivity was dominated by the NMHCs-limited regime, the relative contributions of three O₃ 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_x-limited O₃ sensitivity regimes. These findings underscore the crucial role of regional air mass transport not only in determining O₃ and NMHCs concentrations but also in shaping O₃ formation sensitivity, highlighting the necessity of implementing regionally coordinated control strategies for effective O₃ and NMHCs pollution mitigation.