LES of a cloudy marine boundary layer (MBL) observed during the Atlantic Stratocumulus Transition Experiment were performed to study the influence of entrainment of free tropospheric cloud condensation nuclei (CCN) on cloud microphysics, dynamics, and radiative properties. The initial CCN concentration is 100 cm^-3 in one simulation, while in the second simulation it varies from 100 cm^-3 below the cloud top to a peak of 1200 cm^-3 at the inversion. In the case with the elevated pollution layer, penetrating cumulus are less effective at supplying surface moisture to the cloud which results in a much lower liquid water path (LWP) relative to the clean case. Thus, although entrained CCN enhance the droplet concentration, the net effect on the cloud albedo is small. For this case study, the simulations show how elevated polluted layers can change cloud radiative forcing in ways that would not be easily predicted by large-scale models.