Drivers of biogenic secondary organic aerosol from the past to the future

Abstract. Biogenic secondary organic aerosol (SOA) makes up a substantial fraction of atmospheric fine particulate mass, with important implications for climate and human health. However, its chemical formation processes remain poorly understood and are often oversimplified in 3D atmospheric models. Recent studies have found that the peroxy radical (RO₂) isomerization and the RO₂ accretion reactions (RO₂ + RO₂) can lead to SOA formation. We expand the RO₂ chemical mechanism in the Community Earth System Model version 2 to include these two additional pathways for biogenic SOA formed through the OH oxidation of volatile organic compounds (VOCs). Using this mechanism, we quantify the contribution of each RO₂ pathway to biogenic SOA formation and examine how these contributions evolve from the pre-industrial (PI) to present-day (PD) and future (F). We find that in PD conditions, RO₂ isomerization accounts for 44–46 % of monoterpene SOA, while the contribution from RO₂ + RO₂ pathways is minor. From PI to F, the RO₂ fate varies in response to atmospheric NO_x levels and climate, but the contribution from RO₂ isomerization is consistently high for monoterpenes, underscoring the importance of representing this pathway in SOA parameterizations. In addition, total biogenic SOA formed through OH oxidation decreases by 41 % from PI to PD and increases by 113 % from PD to F, driven primarily by changes in biogenic VOC emissions. Our results highlight the need to better constrain RO₂ pathways for SOA formation through laboratory studies and represent this RO₂ chemistry in SOA modeling.