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). HOM-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 HOM chemistry and the associated nucleation scheme to systematically quantify the effect of HOM-derived NPF on cloud condensation nuclei (CCN) formation and effective radiative forcing due to aerosol–cloud interactions (ERFaci). The model shows better agreement with measured CCN numbers after including organic NPF mechanisms. Aerosols generated from organic NPF nearly double the globally averaged CCN burden in PI (39 %) compared to present-day (PD) (18 %) experiments. This weakens the ERFaci by 0.4 W m−2, corresponding to a 16 % reduction, with most of this reduction occurring in tropical regions where the pure organic nucleation rate shows a larger value in the PI atmosphere. The reduction is mainly driven by a greater enhancement of the sub-20 nm growth rate (GR) in the PI atmosphere compared to PD, in contrast to the findings of Gordon et al. (2016) that the ∼1 nm nucleation rate (j1.7 nm) drives the reduction. The greater enhancement of GR is due to higher HOM concentrations in the PI atmosphere, while the greater j1.7 nm in the PD environment results from higher sulfuric acid concentrations, leading to higher heteromolecular nucleation rates involving sulfuric acid and organics. The significant 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.