Pathway-specific responses of isoprene-derived secondary organic aerosol formation to anthropogenic emission reductions in a megacity in eastern China

Abstract. Isoprene-derived secondary organic aerosol (iSOA) represents a major biogenic source of atmospheric OA, with its formation strongly influenced by anthropogenic emissions. However, long-term iSOA measurements in polluted urban regions remain scarce, limiting the understanding of anthropogenic influences on iSOA formation. In this study, field observations of iSOA were conducted in Shanghai, China during summers and winters of 2015, 2019, and 2021, aiming to assess the iSOA response to emission reductions over this period. The particulate iSOA tracers, formed via reactive uptake of isoprene-derived epoxides, were measured by gas chromatography-mass spectrometry and high-resolution liquid chromatography-mass spectrometry. After accounting for the measurement uncertainties, both total iSOA and organosulfates (OSs) tracers (including 2-methyltetrol sulfates and 2-methylglyceric acid sulfate) decreased annually, while summertime polyol tracers like 2-methyltetrols (2-MTs) and 2-methylglyceric acid (2-MG) varied insignificantly despite strong NOx reductions. Declining aerosol reactivity toward isoprene-derived epoxides and reduced atmospheric oxidizing capacity drove the decrease in OSs but could not explain the trend of summertime polyols. Simulations with the Community Multiscale Air Quality model in 2015 and 2019 captured the decrease in total iSOA and OSs, confirming a driving role of chemical processes. However, the model failed to replicate relatively stable polyol levels in summer, suggesting additional factors (e.g., potential unaccounted sources of 2-MTs and methacrolein, the precursor to 2-MG) may buffer their variations. These findings highlight pathway-specific iSOA responses to emission reductions in a megacity and the importance of targeted anthropogenic emission reductions for mitigating biogenic SOA formation through regulating atmospheric oxidizing capacity and aerosol reactivity.