Explaining the Earlier Spring Ozone Pollution in a typical industrial city in China: Multipath Peroxy Radical Enhancement by Alkenes and OVOC.

Springtime ozone (O3) pollution has shown an earlier annual onset in recent years, yet its chemical drivers remain less understood compared to summer. Using an observation-based model, this study investigates the chemical drivers of this earlier spring O3 onset by comparing two contrasting spring episodes, emphasizing the role of specific volatile organic compounds (VOCs). More severe O3 pollution occurred in spring 2024 than 2023, with higher concentrations and earlier onset. The increased pollution level of O3 is primarily driven by elevated VOCs level and meteorological conditions, with meteorological factors showing a stronger influence. OVOC could accelerate the formation of peroxy radicals through photolysis and acyl peroxy radicals (RCO3·) reactions, while active alkenes through reactions with hydroxyl radical or Criegee Intermediates decomposition. The consistently higher peroxy radical formation rates in spring 2024 are key to the worsened O3 pollution. Source analysis, excluding meteorological influences, identified gasoline vehicles and industrial processes as the dominant contributors to VOCs and O3 in spring 2024. These sources emitted substantial amounts of active alkene and OVOC, respectively, enhancing photochemical activity and leading to higher O3 formation potential in 2024 (32.70% and 36.90%). Controlling the two sources could significantly reduce the peroxy radicals and O3 formation through RCO3·-related pathway. In summary, the elevated emissions of gasoline vehicles and industrial processes are identified as key drivers of the intensified photochemical activity in the 2024 episode. These findings quantitatively resolve the previously underrecognized roles of alkenes and OVOC in promoting peroxy radical formation under spring conditions, extending insights from summer studies and highlighting the need for season specific control measures.