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 and its formation is profoundly influenced by anthropogenic emissions. However, long-term iSOA measurements in polluted urban regions remain limited, hindering 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 epoxydiol (IEPOX), a hydroperoxy radical-dominated pathway, and hydroxymethylmethyl-α-lactone (HMML) and/or methacrylic acid epoxide (MAE), a NO_x-dominated pathway, were measured by mass spectrometry. Both total and IEPOX-derived iSOA decreased markedly from 2015 to 2021, while summertime HMML/MAE-SOA did not vary significantly despite strong NOx reductions. Declining aerosol reactivity toward IEPOX/HMML/MAE and reduced atmospheric oxidizing capacity drove the decrease in IEPOX-SOA but could not explain the trend of summertime HMML/MAE-SOA. Simulations of iSOA with the Community Multiscale Air Quality model in 2015 and 2019 captured the decreasing IEPOX-SOA trend and confirmed a driving role of chemical processes. However, the model failed to replicate relatively stable HMML/MAE-SOA levels in summer, suggesting additional factors (e.g., the potential unaccounted sources of methacrolein, the precursor to HMML/MAE) may buffer HMML/MAE-SOA variations. These findings demonstrate the pathway-specific iSOA responses to emission reductions in a megacity and emphasize the importance of regulating atmospheric oxidizing capacity and aerosol reactivity to mitigate biogenic SOA formation in urban environments.