Reduced effective radiative forcing from cloud-aerosol interactions (ERF<sub>aci</sub>) with improved treatment of early aerosol growth in an Earth System Model

Abstract. Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric green- house gas concentrations. The strength of this negative aerosol forcing is, however, highly uncertain – especially the part originating from cloud-aerosol interactions. An important part of this uncertainty originates from our lack of knowledge about the pre-industrial aerosols and how many of these would have acted as cloud condensation nuclei (CCN). In order to simulate CCN concentrations in models, we must adequately model secondary aerosols, including new particle formation (NPF) and early growth, which contributes with a large part of atmospheric CCN. In this study, we investigate the effective radiative forcing (ERF) from cloud–aerosol interactions (ERFaci ) with an improved treatment of early particle growth, presented in Blichner et al. (2020). We compare the improved scheme to the default scheme, OsloAero, both part of the atmospheric component of the Norwegian Earth System Model v2 (NorESM2). The improved scheme, OsloAeroSec, includes a sectional scheme that treats the growth of the particles from 5–39.6 nm which thereafter inputs the particles to the smallest mode in the pre-existing, modal aerosol scheme. The default scheme parameterizes the growth of particles from nucleation and up to the smallest mode, a process that can take several hours. The explicit treatment of the early growth in OsloAeroSec on the other hand, captures the changes in atmospheric condition during this growth time both in terms of air mass mixing, transport and condensation and coagulation.We find that the ERF aci with the sectional scheme is −1.16 Wm−2 , which is 0.13 Wm−2 weaker compared to the default scheme. This reduction originates from OsloAeroSec producing more particles than the default scheme in pristine, low-aerosol- concentration areas and less NPF particles in high-aerosol areas. We find, perhaps surprisingly, that NPF inhibits cloud droplet activation in polluted/high-aerosol-concentration regions because the NPF particles increase the condensation sink and reduces the growth of the larger particles which may otherwise activate. This means that in these high-aerosol regions, the model with lowest NPF – OsloAeroSec – will have highest cloud droplet activation and thus more reflective clouds. In pristine/low aerosol regions however, NPF enhances cloud droplet activation, because the NPF particles themselves tend to activate.Lastly, we find that sulphate emissions in the present day simulations increase the hygroscopicity of the secondary aerosols compared to the pre-industrial simulations. This makes NPF particles more relevant for cloud droplet activation in the present day than the pre-industrial atmosphere, because the increased hygroscopicity means they can activate at smaller sizes.