The effect of organic nucleation on the indirect radiative forcing with a semi-explicit chemical mechanism for highly oxygenated organic molecules (HOMs)

Abstract. Highly oxygenated organic molecules (HOMs) can significantly contribute to new particle formation (NPF). HOMs-derived NPF in preindustrial (PI) environments provides the baseline for calculating radiative forcing, yet global model studies examining this are lacking. Here, we use a global climate model with a semi-explicit HOMs chemistry and the associated nucleation scheme to systematically quantify the effect of HOMs-derived NPF on CCN formation and effective radiative forcing due to aerosol–cloud interactions (ERF_aci). The improved model shows better agreement with measured cloud condensation nuclei (CCN) numbers. Aerosols generated from organic NPF nearly double the globally averaged CCN burden in PI (39 %) compared to PD (18 %) experiments. This weakens the ERF_aci by 0.4 W m^-2, corresponding to a 16 % reduction, with most of this reduction occurring in tropical regions where pure organic nucleation rates shows larger value in PI atmosphere. Unlike the findings of Gordon et al. (2016), the reduction is mainly driven by a greater enhancement of the sub-20 nm growth rate (GR) in the PI atmosphere compared to PD instead of the ~1 nm nucleation rate (j_1.7nm). The greater enhancement of GR is due to higher HOM concentrations in the PI atmosphere, while the greater j_1.7nm in PD environment results from higher sulfuric acid concentrations, leading to higher heteromolecular nucleation rates involving sulfuric acid and organics. The reduction underscores the critical role of biogenic NPF in CCN formation, particularly in the PI climate when cloud droplet concentrations and albedo are more sensitive to aerosol changes.