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

With the issue of ozone (O) pollution having increasingly gained visibility and prominence in China, the Chinese government explored various policies to mitigate O pollution. In some provinces and cities, diurnal regulations of O precursor were implemented, such as shifting O precursor emission processes to nighttime and offering preferential refueling at night. However, the effectiveness of these policies remains unverified, and their impact on the O 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 O precursors on O formation. Significant O variations were observed mainly in urban areas. Shifting volatile organic compounds (VOCs) to nighttime have slight decreased daytime O levels while moving nitrogen oxides (NOx) to nighttime elevates O levels. Simultaneously moving both to nighttime showed combined effects. Process analysis indicates that the diurnal variation in O 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 O and O precursors levels through regional transportation. Further photochemical analysis reveals that the O photochemical production in urban areas was affected by reduced daytime O precursors emissions. Specifically, decreasing VOCs lowered the daytime O production by reducing the ROx radicals (ROx = HO + HO˙ + RO˙), whereas decreasing NOx promoted the daytime O production by weakening ROx radical loss. Our results demonstrate that diurnal regulation of O precursors will disrupt the ROx radical and O formation in local areas, resulting in a change in O concentration and atmospheric oxidation capacity, which should be considered in formulating new relevant policies.