Causes of a continuous summertime O<sub>3</sub> pollution event in Ji’nan, a central city in the North China Plain

Abstract. In summer 2017, measurements of ozone (O3) and its precursors were carried out at an urban site in Ji’nan, a central city in the North China Plain (NCP). A continuous O3 pollution event was captured during August 4–11, with the maximum hourly O3 reaching 154.1 ppbv. The O3 pollution was associated with the uniform pressure field over the NCP, and was also aggravated by the low pressure trough on August 9–10. Model simulation indicated that local photochemical formation and regional transport contributed 14.0±2.3 and 18.7±4.0 ppbv/hr to O3 increase during 09:00–15:00 local time (LT), respectively. In the evening, vertical transport dominated the O3 sources with the contribution of 54.5±9.6 ppbv/hr. For local O3 formation, the calculated OH reactivity of volatile organic compounds (VOCs) and carbon monoxide (CO) was comparable between O3 episodes and non-episodes (p>0.05), so was the OH reactivity of nitrogen oxides (NOx). However, the ratio of OH reactivity of VOCs and CO to that of NOx increased from 2.0±0.4 s−1/s−1 during non-episodes to 3.7±0.7 s−1/s−1 during O3 episodes, which resulted in the change of O3 formation mechanism from VOC-limited regime (non-episodes) to transition area (O3 episodes) where O3 production rates were among the highest. Correspondingly, the simulated local O3 production rate during O3 episodes (maximum: 21.3 ppbv/hr) was remarkably higher than that during non-episodes (p<0.05) (maximum: 16.9 ppbv/hr). Given that gasoline and diesel exhaust made great contributions to O3 precursors and O3 production rate, constraint on vehicular emissions is the most effective strategy to control O3 pollution in Ji’nan. Specifically, 10 % reduction of diesel and gasoline exhaust led to a decrease of the simulated O3 production rate of 0.58 and 0.47 ppbv/hr, respectively. Both the observation and model simulation imply that the NCP is a source region of tropospheric O3, and may export photochemical pollution to the surrounding areas.