Abstract: Ultrafine aerosols can significantly influence Earth’s climate if they are able to grow to sizes large enough to interact with the incoming solar radiation and nucleate cloud droplets. In clear air, aerosol growth occurs via gas-to-particle conversion of condensable trace gases, including sulfuric acid, nitric acid, hydrochloric acid, ammonia, and myriad oxidation products of many different volatile organic compounds. Nearly all aerosol models developed to date to simulate the aerosol growth assume instantaneous equilibrium between semivolatile gases and submicron-sized particles. This assumption effectively favors the growth of the largest pre-existing particles in accordance with Raoult’s law. In this talk, I will use a combination of laboratory, field, and modeling studies to show that the equilibrium assumption may not always hold, especially when aerosol growth occurs from condensation of supersaturated vapors and/or when the pre-existing aerosols are in a solid or semisolid phase state. In such cases, gas-particle partitioning must be treated as a dynamic process and size-resolved bulk diffusion limitation must be considered inside semisolid particles. I will outline a computationally efficient dynamic gas-particle partitioning treatment in the MOSAIC aerosol model to address this issue and discuss implications on the growth of ultrafine aerosols from semivolatile vapors.