Causes of a continuous summertime O₃ pollution event in Jinan, a central city in the North China Plain

In the summer of 2017, measurements of ozone (O₃) and its precursors were carried out at an urban site in Jinan, a central city in the North China Plain (NCP). A continuous O₃ pollution event was captured during 4–11 August, with the maximum hourly O₃ mixing ratio reaching 154.1 ppbv. Model simulation indicated that local photochemical formation and regional transport contributed 14.0±2.3 and 18.7±4.0 ppbv h⁻¹, respectively, to the increase in O₃ during 09:00–15:00 LT (local time) in this event. For local O₃ formation, the calculated OH reactivities of volatile organic compounds (VOCs) and carbon monoxide (CO) were comparable between O₃ episodes and non-episodes (p>0.05), so was the OH reactivity of nitrogen oxides (NO_x). However, the ratio of OH reactivity of VOCs and CO to that of NO_x increased from 2.0±0.4 s⁻¹ s¹ during non-episodes to 3.7±0.7 s⁻¹ s¹ during O₃ episodes, which resulted in the change in the O₃ formation mechanism from the VOC-limited regime before the O₃ pollution event to the transitional regime during the event. Correspondingly, the simulated local O₃ production rate during the event (maximum: 21.3 ppbv h⁻¹) was markedly higher than that before the event (p<0.05) (maximum: 16.9 ppbv h⁻¹). Given that gasoline and diesel exhaust made large contributions to the abundance of O₃ precursors and the O₃ production rate, constraint on vehicular emissions is the most effective strategy to control O₃ pollution in Jinan. The NCP has been confirmed as a source region of tropospheric O₃, where the shift in regimes controlling O₃ formation like the case presented in this study can be expected across the entire region, due to the substantial reductions of NO_x emissions in recent years.