Exploring the Factors Controlling the Long‐Term Trend (1988–2019) of Surface Organic Aerosols in the Continental United States by Simulations.

Observed surface organic aerosols (OA) concentrations slightly increased in the western US (WUS) but significantly decreased in the eastern US (EUS) in summer, and continuously decreased in winter over the US region. To understand the driving factors for the long‐term surface OA trend, we apply a revised version of the Community Atmosphere Model version 6 with comprehensive tropospheric and stratospheric chemistry representation, considering the heterogeneous formation of isoprene‐epoxydiol‐derived secondary organic aerosols (SOAIE) and fast photolysis rate of monoterpene‐derived secondary organic aerosols (MTSOA) to diagnose the OA evolution in 1988–2019. Compared to older versions, the revised model better reproduces the climatology, seasonal cycle, and long‐term trend of surface OA as evaluated against the Interagency Monitoring of Protected Visual Environments measurements. We find the decrease in EUS summertime OA is likely attributed to the interplay between SOAIE and MTSOA. With anthropogenic emissions reduction, primary organic aerosols (POA) declined, SOAIE decreased along with sulfate, while MTSOA increased along with biogenic emissions driven by a warming climate. POA from wildfires with a significant trend of 2.9% yr−1 and considerable interannual variation of 62.8% drive the statistically insignificant but increasing WUS summertime OA, while anthropogenic POA dominates the decreasing wintertime OA in the US. Through sensitivity experiments, we find MTSOA show linear responses to the increasing monoterpenes emissions and negligible responses to NOx emissions reduction due to the mutual offsets between MTSOA components from different oxidation pathways. This study reveals the increasingly important role of MTSOA in summertime OA under a warming climate. Plain Language Summary: As the major components of fine particles, organic aerosols (OA) increased in the western United States and decreased in the eastern United States in the summer, and kept decreasing in the winter in the past decades. The driving factors for the long‐term trend of OA and their components remain unclear and are investigated by conducting a series of long‐term simulations. We find the isoprene‐epoxydiol‐derived secondary organic aerosols decrease with sulfate emission controls, which is partly offset by the increasing monoterpene‐derived secondary organic aerosols (MTSOA) under global warming and the statistically insignificant increase of primary organic aerosols driven by wildfires in summer. In winter, anthropogenic emissions dominate the declining surface OA. We also find MTSOA are more sensitive to increasing biogenic emissions than anthropogenic emissions reduction. Our results reveal the important role of MTSOA in total summertime OA under a warming climate. Key Points: Monoterpene‐derived and isoprene‐epoxydiol‐derived secondary organic aerosols (SOA) contribute to the descending surface summertime organic aerosols (OA) in the eastern USPrimary organic aerosols (POA) from wildfires drive the statistically insignificant but increasing trend of surface summertime OA in the western USMonoterpene‐derived SOA linearly respond to increasing monoterpenes emissions but negligibly respond to NOx emissions reduction [ABSTRACT FROM AUTHOR]