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. We document and evaluate the aerosol schemes as implemented in the physical and Earth system models, the Global Coupled 3.1 configuration of the Hadley Centre Global Environment Model version 3 (HadGEM3-GC3.1)and the United Kingdom Earth System Model (UKESM1), which are contributing to the sixth Coupled Model Intercomparison Project (CMIP6).The simulation of aerosols in the present-day period of the historical ensemble of these models is evaluatedagainst a range of observations.Updates to the aerosol microphysics scheme are documented as well as differences in theaerosol representation between the physical and Earth system configurations.The additional Earth system interactions included in UKESM1 lead to differences in the emissions ofnatural aerosol sources such as dimethyl sulfide, mineral dust and organic aerosol and subsequent evolutionof these species in the model.UKESM1 also includes a stratospheric–tropospheric chemistry scheme which is fully coupled to the aerosol scheme,while GC3.1 employs a simplified aerosol chemistry mechanism driven by prescribed monthly climatologies ofthe relevant oxidants.Overall, the simulated speciated aerosol mass concentrations compare reasonably well with observations.Both models capture the negative trend in sulfate aerosol concentrations over Europe and the eastern UnitedStates of America (US)although the models tend to underestimate sulfate concentrations in both regions.Interactive emissions of biogenic volatile organic compounds in UKESM1 leadto an improved agreement of organic aerosol over the US.Simulated dust burdens are similar in both models despite a 2-fold difference in dust emissions.Aerosol optical depth is biased low in dust source and outflow regions but performs well inother regions compared to a number of satellite and ground-based retrievals of aerosol optical depth.Simulated aerosol number concentrations are generally within a factor of 2 of the observations, withboth models tending to overestimate number concentrations over remote ocean regions, apart fromat high latitudes, and underestimate over Northern Hemisphere continents.Finally, a new primary marine organic aerosol source is implemented in UKESM1 for the first time.The impact of this new aerosol source is evaluated. Over the pristine Southern Ocean, it is found to improve the seasonalcycle of organic aerosol mass and cloud droplet number concentrations relative to GC3.1although underestimations in cloud droplet number concentrations remain.This paper provides a useful characterisation of the aerosol climatology in both modelsand will facilitate understanding in the numerous aerosol–climate interaction studies that willbe conducted as part of CMIP6 and beyond.

Description and evaluation of aerosol in UKESM1 and HadGEM3-GC3.1 CMIP6 historical simulations