The optical properties of an internally mixed aerosol with a lognormal size distribution can be approximated in terms of analytic functions of the wet surface mode radius with coefficients that can be related to the wet refractive index. The wet radius is calculated from the dry radius and relative humidity using either the Köhler theory or the MOSAIC thermodynamic model. The hydration state of the aerosol in the hysteresis region between the crystallization and deliquescence relative humidities is diagnosed by comparing the aerosol water from the previous time step with the current water content of the hydrated aerosol. The wet refractive index is estimated from the volume fractions and refractive indices of all components of the aerosol, including water, using volume mixing for soluble components and an effective medium approximation for the insoluble components. The parameterization is evaluated by comparing with Mie solutions for ammonium sulfate, black carbon, and a 50:50 mixture for a wide range in size distributions and relative humidity. Errors are usually less than 20% and are less then 30% for all conditions except when absolute values are small. The parameterization is suitable for any aerosol model that uses lognormal size distributions composed of internal mixtures of multiple aerosol components.