Causes and importance of new particle formation in the present-day and preindustrial atmospheres

New particle formation has been estimated to produce around half of cloud-forming particlesin the present-day atmosphere, via gas-to-particle conversion. Here we assess the importance of newparticle formation (NPF) for both the present-day and the preindustrial atmospheres. We use a globalaerosol model with parametrizations of NPF from previously published CLOUD chamber experimentsinvolving sulfuric acid, ammonia, organic molecules, and ions. We find that NPF produces around 67% ofcloud condensation nuclei at 0.2% supersaturation (CCN0.2%) at the level of low clouds in the preindustrialatmosphere (estimated uncertainty range 45–84%)and 54% in the present day (estimated uncertaintyrange 38–66%). Concerning causes, we find that the importance of biogenic volatile organic compounds(BVOCs) in NPF and CCN formation is greater than previously thought. Removing BVOCs and hence allsecondary organic aerosol from our model reduces low-cloud-level CCN concentrations at 0.2%supersaturation by 26% in the present-day atmosphere and 41% in the preindustrial. Around three quartersof this reduction is due to the tiny fraction of the oxidation products of BVOCs that have sufficiently lowvolatility to be involved in NPF and early growth. Furthermore, we estimate that 40% of preindustrialCCN0.2% are formed via ion-induced NPF, compared with 27% in the present day, although we caution thatthe ion-induced fraction of NPF involving BVOCs is poorly measured at present. Our model suggests that theeffect of changes in cosmic ray intensity on CCN is small and unlikely to be comparable to the effect of largevariations in natural primary aerosol emissions