Global Modeling of Organic-related New Particle Formation and its Contribution to Global Aerosol Number Concentrations using CAM6-Chem

New particle formation involving organic compounds has been identified as an important process affecting aerosol particle number concentrations in the global atmosphere. Here a global chemistry-climate model has been developed to include explicit chemical reactions of highly oxygenated molecules (HOMs) and accretion products based on monoterpene-derived peroxy radical (RO2) unimolecular autoxidation and self- and cross-reactions with other RO2 species. The improved model incorporates a comprehensive biogenic organic nucleation scheme including heteromolecular nucleation of sulfuric acid and organics, neutral pure organic nucleation, and ion-induced pure organic nucleation. These organic-related mechanisms are combined with an inorganic nucleation scheme derived from published chamber experimental data from the CLOUD project. Additionally, the organic condensational growth rate for newly formed particles (sub-20nm) is taken into account. The updated model captures the occurrence frequency of new particle formation events (normalized mean bias, NMB changes from -96% to -15%) and shows reasonable agreement with measured rates of nucleation (NMB changes from -97% to -64%) and growth (NMB changes from -54% to 39%) globally except in China (NMB of nucleation rate > -100%). The model successfully reproduces surface-level aerosol number concentrations over oceans and vertical profiles over the Amazon Basin. Globally, we find that organics contribute to 45% of the annual average vertically integrated nucleation rate and 25% of the vertical mean growth rate. The inclusion of organic-related processes leads to a 39% increase in the global annual mean aerosol concentration and a 33% increase in cloud condensation nuclei at 0.5% supersaturation compared to a simulation with only inorganic nucleation. Our work also indicates that organic initial growth is more important for particle number than organic nucleation on global average.