Aerosol-cloud interactions (ACI) effects are expected to change dramatically as we enter a post-fossil world, characterized by strong reductions in anthropogenic aerosol emissions but with increasingly larger impacts from natural aerosols. ACI in this post-fossil state may considerably differ from preindustrial conditions, owing to shifts in climate and changes in sources region characteristics. CleanCloud (Clouds and climate transitioning to post-fossil aerosol regime) is a Horizon Europe Cluster 5 project, comprised of 20 European research institutes with expertise in land- and space-based remote sensing, laboratory and in-situ atmospheric aerosol and cloud chamber experiments, and kilometer and large-scale aerosol, cloud, and climate modelling. CleanCloud, aims to quantify and understand regional and temporal effects of ACI-related processes, and how they will evolve in the transition to the post-fossil regime.