Global impacts of organic aerosol phase state

Organic aerosol (OA) in the atmosphere can exist in liquid, semi-solid, or solid states, influenced by molecular properties and environmental conditions. However, regional and global models typically assume OA to be only in a liquid phase. Recent studies underscore that OA can present in a semi-solid or even solid state in low-temperature, dry environments. Under such conditions, increased viscosity can impede heterogeneous reaction rates by reducing diffusion. We develop a novel phase state scheme within the GEOS-Chem global model, enabling real-time simulation of OA phase states from various sources under diverse environmental conditions. Subsequently, we investigate the effects of OA phase states on heterogeneous chemical processes, including gas-particle partitioning, reactive uptake, and ice particle nucleation. Finally, our simulated OA concentrations are evaluated against global vertical profiles from aircraft observations. Our simulations indicate that on a global scale, viscosity is higher in polar regions compared to tropical regions and increases with altitude, with little to no liquid phase present above 500 hPa. Additionally, anthropogenic secondary OA (SOA) exhibits greater viscosity than biogenic SOA, hydrophobic primary OA (POA), and hydrophilic POA. The increased viscosity leads to slower gas-phase partitioning and uptake processes, thereby reducing near-source concentrations and increasing concentrations in remote areas. Considering solid-phase OA as heterogeneous ice nuclei enhances OA removal. Overall, our simulations demonstrate that incorporating phase state effect results in a reduction of OA concentrations, particularly for the more viscous SOA.