Global impacts of simulating organic aerosol phase state

Organic aerosol (OA) in the atmosphere exists in liquid, semi-solid, or solid states, controlled by molecular properties and environmental conditions. However, regional and global models typically assume that OA exists exclusively in a 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 global simulations show that surface OA predominantly exists in a semi-solid state, consistent with observations (r = 0.9, NMB = –14%). OA viscosity is higher over continental mid-latitude regions and increases with altitude, with little to no liquid phase above 2.5 km due to low temperature and relative humidity. Considering 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 2.34 Tg). Incorporating the OA phase state improves agreement with observed vertical OA profiles from both near-source (EMERGE-AS) and remote (ATom) aircraft campaigns.