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The Community Earth System Model (CESM1) CAM4-chem has been used to performthe Chemistry Climate Model Initiative (CCMI) reference and sensitivitysimulations. In this model, the Community Atmospheric Model version 4 (CAM4)is fully coupled to tropospheric and stratospheric chemistry. Details andspecifics of each configuration, including new developments and improvementsare described. CESM1 CAM4-chem is a low-top model that reaches up toapproximately 40 km and uses a horizontal resolution of 1.9°latitude and 2.5° longitude. For the specified dynamics experiments,the model is nudged to Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis. We summarize the performance of the threereference simulations suggested by CCMI, with a focus on the last 15 years ofthe simulation when most observations are available. Comparisons withselected data sets are employed to demonstrate the general performance of themodel. We highlight new data sets that are suited for multi-model evaluationstudies. Most important improvements of the model are the treatment ofstratospheric aerosols and the corresponding adjustments for radiation andoptics, the updated chemistry scheme including improved polar chemistry andstratospheric dynamics and improved dry deposition rates. These updates leadto a very good representation of tropospheric ozone within 20 % of valuesfrom available observations for most regions. In particular, the trend andmagnitude of surface ozone is much improved compared to earlier versions ofthe model. Furthermore, stratospheric column ozone of the Southern Hemispherein winter and spring is reasonably well represented. All experiments stillunderestimate CO most significantly in Northern Hemisphere spring and show asignificant underestimation of hydrocarbons based on surface observations

Representation of the Community Earth System Model (CESM1) CAM4-chem within the Chemistry-Climate Model Initiative (CCMI)