The importance of NOx control for peak ozone mitigation based on a sensitivity study using CMAQ-HDDM-3D model during a typical episode over the Yangtze River Delta region, China

In recent years, ground-level ozone (O₃) has been one of the main pollutants hindering air quality compliance in China's large city-clusters including the Yangtze River Delta (YRD) region. In this work, we utilized the process analysis (PA) and the higher-order decoupled direct method (HDDM-3D) tools embedded in the Community Multiscale Air Quality model (CMAQ) to characterize O3 formation and sensitivities to precursors during a typical O₃ pollution episode over the YRD region in July 2018. Results indicate that gas-phase chemistry contributed dominantly to the ground-level O₃ although a significant proportion was chemically produced at the middle and upper boundary layer before reaching the surface via diffusion process. Further analysis of the chemical pathways of O₃ and O_x formation provided deep insights into the sensitivities of O₃ to its precursors that were consistent with the HDDM results. The first-order sensitivities of O₃ to anthropogenic volatile organic compounds (AVOC) were mainly positive but small, and temporal variations were negligible compared with those to NO_x. During the peak O₃ time in the afternoon, the first- and second-order sensitivities of O₃ to NO_x were significantly positive and negative, respectively, suggesting a convex response of O₃ to NO_x over most areas including Shanghai, Hangzhou, Nanjing and Hefei. These findings further highlighted an accelerated decrease in ground-level O₃ in the afternoon corresponding to continuous decrease of NO_x emissions in the afternoon. Therefore, over the YRD region including its metropolises, NO_x emission reductions will be more important in reducing the afternoon peak O₃ concentration compared with the effect of VOC emission control alone.