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. Organic aerosol (OA) has been considered as one of themost important uncertainties in climate modeling due to the complexity inpresenting its chemical production and depletion mechanisms. To betterunderstand the capability of climate models and probe into the associateduncertainties in simulating OA, we evaluate the Community Earth System Modelversion 2.1 (CESM2.1) configured with the Community Atmosphere Model version6 (CAM6) with comprehensive tropospheric and stratospheric chemistryrepresentation (CAM6-Chem) through a long-term simulation (1988–2019) withobservations collected from multiple datasets in the United States. We findthat CESM generally reproduces the interannual variation and seasonal cycleof OA mass concentration at surface layer with a correlation of 0.40 compared to ground observations and systematically overestimates (69 %)in summer and underestimates (−19 %) in winter. Through a series ofsensitivity simulations, we reveal that modeling bias is primarily relatedto the dominant fraction of monoterpene-formed secondary organic aerosol(SOA), and a strong positive correlation of 0.67 is found betweenmonoterpene emission and modeling bias in the eastern US during summer. In termsof vertical profile, the model prominently underestimates OA and monoterpeneconcentrations by 37 %–99 % and 82 %–99 %, respectively, in the upper air(&gt; 500 m) as validated against aircraft observations. Our studysuggests that the current volatility basis set (VBS) scheme applied in CESMmight be parameterized with monoterpene SOA yields that are too high, whichsubsequently results in strong SOA production near the emission source area. Wealso find that the model has difficulty in reproducing the decreasing trendof surface OA in the southeastern US probably because of employing pure gas VBS torepresent isoprene SOA which is in reality mainly formed through multiphasechemistry; thus, the influence of aerosol acidity and sulfate particle changeon isoprene SOA formation has not been fully considered in the model. Thisstudy reveals the urgent need to improve the SOA modeling in climate models.

Analysis of secondary organic aerosol simulation bias in the Community Earth System Model (CESM2.1)