Global Impacts of Simulating Organic Aerosol Phase State

Organic aerosol (OA) in the atmosphere exists in liquid, semisolid, or solid states, controlled by molecular properties and environmental conditions. However, regional and global models typically assume that OA exists exclusively in the liquid phase. We develop a source-specific OA phase-state model that enables key heterogeneous processes, including gas-particle partitioning, reactive uptake, and ice particle nucleation, to be explicitly phase-state-dependent. Our simulations show that globally more than 70% of surface OA exists in a semisolid state, consistent with observations (r = 0.9, NMB = -15%). OA viscosity is higher over continental midlatitude regions and increases with altitude, with little to no liquid phase above 2.5 km due to generally lower temperatures and relative humidity, compared to the surface. The phase state substantially slows gas-particle partitioning and uptake while enhancing OA removal at higher altitudes, where solid-state OA can act as ice-nucleating particles. Consequently, global annual mean surface OA concentrations decrease by 28%, while concentrations at 7 km increase by 8%, leading to a 36% reduction in the vertical OA concentration gradient, with a modest decrease in the global OA burden (2.64 Tg to 2.32 Tg). Incorporating the OA phase state improves agreement with observed vertical OA profiles from both near-source (EMERGE-AS: r from 0.92 to 0.93 and NMB from 136% to 75%) and remote (ATom: r from 0.3 to 0.6 and NMB from 122% to 96%) aircraft campaigns.