Variations in summertime ozone in Nanjing between 2015 and 2020: roles of meteorology, radical chain length and ozone production efficiency

Changes in ozone (O₃) can be evaluated to inform policy effectiveness and develop reasonable emissions reduction measures. This study investigated the causes of summertime maximum daily 8-h average (MDA8) O₃ variation between 2015 and 2020 in Nanjing, China, a megacity in the Yangtze River Delta (YRD) region, from the perspective of meteorological conditions and anthropogenic emissions of O₃ precursors (VOCs and No_x). Compared with 2015, the observed MDA8 O₃ decreased by 19.1 µg/m³ in August 2020. The indirect and indirect impacts of meteorological conditions contributed 44% of the decline, with temperature, relative humidity, and wind playing important roles in the O₃ variation. The O₃ drop by 10.7 µg/m³ (56% of the total decrease) may have been due to the decreases in anthropogenic emissions of VOCs and NO_x by 7.8% and 11.7%, respectively. The longer hydroxyl (OH) radical chain length and higher ozone production efficiency (OPE) indicated that the reduction of anthropogenic emissions accelerated the RO_x (RO_x = OH + HO₂ + RO₂) and NO_x cycles in O₃ production, making O₃ more sensitive to NO_x. This corresponded to the O₃ formation shifting from a VOC-limited regime in 2015 to a transition regime in 2020 and O₃ decrease with anthropogenic emission reduction. Hence, the joint control of O₃ precursor emissions can effectively mitigate O₃ pollution in Nanjing.