Causes of a continuous summertime O-3 pollution event in Jinan, a central city in the North China Plain

Abstract

In the summer of 2017, measurements of ozone (O-3) and its precursors were carried out at an urban site in Jinan, a central city in the North China Plain (NCP). A continuous O-3 pollution event was captured during 4-11 August, with the maximum hourly O-3 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 O-3 during 09:00-15:00 LT (local time) in this event. For local O-3 formation, the calculated OH reactivities of volatile organic compounds (VOCs) and carbon monoxide (CO) were comparable between O-3 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 O-3 episodes, which resulted in the change in the O-3 formation mechanism from the VOC-limited regime before the O-3 pollution event to the transitional regime during the event. Correspondingly, the simulated local O-3 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 O-3 precursors and the O-3 production rate, constraint on vehicular emissions is the most effective strategy to control O-3 pollution in Jinan. The NCP has been confirmed as a source region of tropospheric O-3, where the shift in regimes controlling O-3 formation like the case presented in this study can be expected across the entire region, due to the substantial reductions of NOx emissions in recent years.