Diurnal emission variation of ozone precursors: Impacts on ozone formation during Sep. 2019

With the issue of ozone (O₃) pollution having increasingly gained visibility and prominence in China, the Chinese government explored various policies to mitigate O₃ pollution. In some provinces and cities, diurnal regulations of O₃ precursor were implemented, such as shifting O₃ precursor emission processes to nighttime and offering preferential refueling at night. However, the effectiveness of these policies remains unverified, and their impact on the O₃ generation process requires further elucidation. In this study, we utilized a regional climate and air quality model (WRF-Chem, v4.5) to test three scenarios aimed at exploring the impact of diurnal industry emission variation of O₃ precursors on O₃ formation. Significant O₃ variations were observed mainly in urban areas. Shifting volatile organic compounds (VOCs) to nighttime have slight decreased daytime O₃ levels while moving nitrogen oxides (NOx) to nighttime elevates O₃ levels. Simultaneously moving both to nighttime showed combined effects. Process analysis indicates that the diurnal variation in O₃ was mainly attributed to chemical process and vertical mixing in urban areas, while advection becomes more important in non-urban areas, contributing to the changes in O₃ and O₃ precursors levels through regional transportation. Further photochemical analysis reveals that the O₃ photochemical production in urban areas was affected by reduced daytime O₃ precursors emissions. Specifically, decreasing VOCs lowered the daytime O₃ production by reducing the ROx radicals (ROx = HO + HO˙₂ + RO˙₂), whereas decreasing NOx promoted the daytime O₃ production by weakening ROx radical loss. Our results demonstrate that diurnal regulation of O₃ precursors will disrupt the ROx radical and O₃ formation in local areas, resulting in a change in O₃ concentration and atmospheric oxidation capacity, which should be considered in formulating new relevant policies.