Identify the key emission sources for mitigating ozone pollution: A case study of urban area in the Yangtze River Delta region, China.

Ozone (O) has become the most critical air pollutant in the Yangtze River Delta (YRD) region of China. Research on the O formation mechanism and its precursor sources (including nitrogen oxides (NO) and volatile organic compounds (VOCs)) could provide a theoretical basis for mitigating O pollution in this region. In this study, simultaneous field experiments were conducted for air pollutants in a typical urban area (Suzhou) in the YRD region in 2022. The capacity of in-situ O formation, O-NO-VOCs sensitivities and sources of O precursors were analyzed. The results showed that in-situ formation contributed 20.8 % of the O concentration in the warm season (April to October) of the Suzhou urban area. Compared with the warm season average, the concentrations of various O precursors increased on pollution days. The O-NO-VOCs sensitivity was the VOCs-limited regime based on the average concentrations during the warm season. O formation was most sensitive to anthropogenic VOCs, of which oxygenated VOCs, alkenes and aromatics were the key species. There was a VOCs-limited regime in spring and autumn, while a transitional regime in summer due to the changes in NO concentrations. This study considered NO emission from VOCs sources and calculated the contribution of various sources to O formation. The results of VOCs source apportionment showed that diesel engine exhaust and fossil fuel combustion had a dominant proportion, but O formation presented significant negative sensitivities to the above two sources because of their high NO emissions. There were significant sensitivities of O formation to gasoline vehicle exhaust and VOCs evaporative emissions (gasoline evaporation and solvent usage). The contribution of VOCs evaporative emissions during the O pollution episode was significantly higher than the average; therefore, controlling VOCs evaporative emissions during the O pollution episode is critical. These results provide feasible strategies to mitigate O pollution.