A New Simplified Parameterization of Secondary Organic Aerosol in the Community Earth System Model Version 2 (CESM2; CAM6.3)

Abstract. The Community Earth System Model (CESM) community has been providing versatile modeling options, with simple to complex chemistry and aerosol schemes in a single model, in order to support the broad scientific community with various research interests. While different model configurations are available in CESM and these can be used for different fields of Earth system science, simulation results that are consistent across configurations are still desirable. Here we develop a new simple secondary organic aerosol (SOA) scheme in the Community Atmosphere Model version 6.3 (CAM6.3), the atmospheric component of the CESM. The main purpose of this simplified SOA scheme is to reduce the differences in aerosol concentrations and radiative fluxes between CAM and CAM with detailed chemistry (CAM-chem) while maintaining the computational efficiency of CAM. CAM simulation results with the current and the new SOA schemes are compared to the CAM-chem results as a reference. More consistent SOA concentrations are obtained globally when using the new SOA scheme, for both temporal and spatial variabilities. Furthermore, the overestimation of other carbonaceous aerosols (black carbon and primary organic aerosol) in CAM is greatly reduced, which results in improved global atmospheric burden and concentrations at the high latitudes of the Northern Hemisphere compared to the full chemistry version (CAM-chem). As a result, the high bias of radiative flux in the Arctic region is significantly reduced for both nudged and free-running simulations. We find that the current SOA scheme in CAM can still be used for radiative forcing calculation as the high biases exist both in pre-industrial and present conditions, but studies focusing on the instantaneous radiative effects would benefit from using the new SOA scheme. The new SOA scheme also has technical advantages including the use of identical SOA precursor emissions as CAM-chem from the online biogenic emissions, instead of pre-calculated emissions that may introduce differences. Future parameter updates on the CAM-chem SOA scheme can be easily translated to the new CAM SOA scheme as it is derived from the CAM-chem SOA scheme.