## 3 Results and Discussion
### Effect of 13 th FYP on Air Quality during Heavy Pollution Episodes in Haizhu in 2020
(2) To adopt a proper reduction ratio of  
Figure 9: Maps of the simulated O\({}_{3}\) mean daily 8 h maximum concentration distribution over Haizhu for 2020 scenarios on 6 August (a: 2020A; b: 2020B; c: 2020C; d: 2020D; HZ: Haizhu; GZ: Guangzhou). VOCs/NO\({}_{x}\) and flatten the O\({}_{3}\) increase to the largest extent during the transient from VOC-limited region to NO\({}_{x}\)-limited region. But this pathway requires a great deal of effort to determine a favourable reduction ratio of VOCs/NO\({}_{x}\) and cut the emission of VOCs from various sectors. The findings in the present study highlight the complexity and nonlinear chemistry of O\({}_{3}\) formation and call for further investigations. For example, the reduction ratio of VOCs/NO\({}_{x}\) for anthropogenic sources was generally suggested to be 1:2 in PRD [10]. However, considering the change of precursor emissions and meteorological conditions, the ratio of VOCs/NO\({}_{x}\) might need to be further studied. Besides, the NO\({}_{2}\) is overpredicted in the present study implying that the VOC-limited nature in Haizhu district might be overpredicted, although there is very low chance that Haizhu district is a NOx-limited region in 2015 because the NOx is still at an elevated level and this region (PRD) has been repeatedly diagnosed as a VOC-limited region (e.g., [PERSON] et al., 2014 [91]). Moreover, the formation of O\({}_{3}\) depends on the local mixture of NO\({}_{x}\) and VOCs. Therefore, the location/sector of the emission of NO\({}_{x}\) and VOC, apart from the overall reduction of NO\({}_{x}\) and VOC in an area, might have an impact on the changes of O\({}_{3}\). Although we have made some effort on such topic by evaluating the effect of only controlling traffic emission on air quality (scenario 2020D), it would be of interest to adopt more sophisticated tools, e.g., a tagging technique ([PERSON], 2013 [92]), to evaluate the effect of various combinations of emission reduction in sectors in future study. The simulated O\({}_{3}\)-8h distribution on 6 August is shown in Figure 9 to demonstrate the difference of regional O\({}_{3}\) pollution in Haizhu district. Ozone pollution would be relieved in scenario 2020C (Figure 9c) compared with that in other scenarios (Figure 9a,b,d). It is noticeable that ozone pollution in the southwest area (red area, suburban and rural area) in 2020C (Figure 9c) could be smaller than those in other scenarios (Figure 9a,b,d), which may be because of the suitable ratio of precursors in that area of scenario 2020C, although the pollution in Haizhu (urban area) seemed to have no significant difference. The difference of regional O\({}_{3}\) pollution shows that in the southwest area (suburban and rural area), the policy in scenario 2020C could decrease the O\({}_{3}\) significantly (less red area), and in other regions, e.g., in Haizhu (urban area), the effect is less noticeable, which indicated that the control of ozone pollution should be taken according to local conditions, i.e., measures in urban areas should be different from those in rural areas. We note that transboundary transport of O\({}_{3}\) and its precursors (CO, VOCs, and NOx\({}_{x}\)) due to mesoscale dynamics complicates O\({}_{3}\) pollution control and requires strong and efficient cooperation among the adjacent regions.