We use a size- and composition-resolved externally mixed aerosol microphysical model and a delta-Eddington exponential-sum-fit radiation algorithm to examine the interactions between aerosol particles and cloud drops, and their influence on solar radiation. Both the aerosol model and the radiation code are designed to explicitly handle external and internal aerosol particle and cloud drop mixtures. Using observations from the Monterey Area Ship Track (MAST) Experiment, we model changes in aerosol and cloud drop size distributions for a clean marine cloud and ship track and a continentally influenced marine cloud and ship track. Linking these results to the radiation algorithm with a Mie-scattering subroutine, we investigate the corresponding changes in cloud albedo, cloud absorption, and transmission. The differences in 0.3--3.0 μm albedo and transmission between the clouds and ship tracks as a result of the changes in drop size distribution and composition are found to be substantial, and the composition of the cloud drops is found to be important particularly in the continentally influenced case. Both the clouds and ship tracks enhance atmospheric absorption with respect to a clear sky, with a cloud forcing ratio ranging from 1.15 to 1.37, where the clear sky is defined to be cloud- and aerosol-free. Sensitivity studies are performed with respect to the updarft velocity, the updraft area fraction, dilution of the ship emissions, and the composition of supermicron continental particles. The radiation results are also compared with Meteorological Research Flight (MRF) C-130 in situ aircraft measurements and with parameterizations of the Twomey effect. ¿ 2001 American Geophysical Union