The Effects of New Particle Formation by Products Associated with the Oxidation of Alpha-Pinene

Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period (PI). Here, a model is developed with a set of comprehensive nucleation schemes of diacyl peroxides formed during the oxidation of alpha-pinene. The model is able to reproduce the profile of organic aerosol number concentrations measured in the Amazon, although some profiles in comparison with Atom observations have deficiencies. The model simulation is in reasonable agreement with the annual average concentration as well as the seasonal and spatial pattern of organic carbon (OC) concentrations in the US, while it fails to predict the seasonal pattern of OC in Europe due to the lack of sufficient primary organic aerosol emissions in the winter. After including organic nucleation, the bias of the annual average surface particle number concentration is decreased at 60% of the available observation sites and the temporal correlation coefficients are increased at 67% of the sites.With the current model, organic nucleation enhances the direct radiative forcing (DRF) by 4.6% and reduces the warm cloud indirect radiative forcing (IRF) by 13.5%. The addition of climate and land use change enhances the DRF (-0.52 W m^-2) by 3.0% due to the size distribution and number concentration change of SOA and sulfate, while the IRF (-1.36 W m^-2) is reduced by 8.3%. Overall, we find a 15% reduced radiative forcing due to the increase in anthropogenic aerosol when including organic nucleation together with climate and land use change.