Bypassing the NOx titration trap in ozone pollution control in Beijing

Abstract Ozone concentrations have increased sharply since the implementation of the Clean Air Action plan in China, but the reasons for the increase are still unclear. Here, we analyzed the annual ozone trends at different altitudes from 2013 to 2018 based on ozonesonde observations in the southern suburb of Beijing. Large differences in ozone concentrations were observed among different years and were most significant within 2 km above the ground. Ozone in the lower boundary layer exhibited a strong increase before 2016 and a weak decrease after 2016, while ozone in the upper boundary layer exhibited a weak increase before 2016 and a strong decrease after 2016. The main factor responsible for these ozone trends was ozone precursors, which induced the transition of ozone production from being sensitive to VOC to being sensitive to NOx. This study indicated that a large amount of NOx reduction would be conducive to ozone pollution control in China.