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 NOx-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.