The Community Multi-scale Air Quality model (CMAQ) aerosol component utilizes a modal representation, where the size distribution is represented as a sum of three lognormal modes. Though the aerosol treatment in CMAQ is quite advanced compared to other operational air quality models, various members of the CMAQ community have commented that improvements to the aerosol module should be made in order to bring it to the state of the science found in research-grade air quality models. Among the shortcomings noted are that (1) a trimodal representation is insufficient to resolve the aerosol size and composition distribution; (2) interactions of the gas phase with coarse particles, such as sea-salt, are neglected; and (3) the assumption of instantaneous equilibrium between gas-phase species and fine-mode aerosol is inaccurate. In the present study, we seek to understand the effects of the latter two assumptions on predicted fine-particle mass and chemical composition in regional-scale air quality models.