Variation of biogenic VOC contribution to ozone formation with reduced anthropogenic precursor emissions: Coupling...

As an essential component of urban natural sources, isoprene has strong interactions and synergies with anthropogenic precursors (volatile organic compounds and nitrogen oxides) of ozone (O₃), influencing O₃ formation in urban areas. However, the variability of these effects under different anthropogenic emission scenarios has not been fully understood. This study, utilizing observational data from Dezhou (a medium-sized city in the center of North China Plain) from May to September in both 2019 and 2020, and incorporating four future scenarios based on Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5), unravels the mechanisms of O₃ formation and the impact of isoprene on O₃ production using an observation-based box model. Our observation results showed that O₃ concentrations were lower in 2020 compared to 2019. The box model analysis suggests a shift in O₃ formation from a VOC-limited regime in 2019 to a transition regime in 2020, primarily due to a significant reduction in anthropogenic emissions. Isoprene photochemistry contributed to 7% and 11% of the daytime average net O₃ production rates in 2019 and 2020, respectively. The increase can be attributed to a notable rise in RO₂ radicals, from 14% in 2019 to 19% in 2020. Under the future scenario with the lowest projected anthropogenic emissions (SSP1-2.6), isoprene-derived RO₂ radicals (generated through isoprene oxidation) are expected to account for 36% of the total RO₂ radicals. We also use a quasi-EKMA diagram to illustrate the contribution of isoprene to O₃ concentrations, highlighting the nonlinear amplification or reduction of its contribution depending on changes in anthropogenic VOCs and NO_x concentrations. This nonlinear relationship is influenced by NO_x concentrations, with the impact of isoprene being reduced at lower NO_x levels. To effectively mitigate long-term O₃ pollution, especially given the growing contribution of biogenic precursors, it is crucial to focus on reducing NO_x emissions.