Aircraft measurements are presented of the interaction of aerosols and stratocumulus cloud within the effluent of ships powered by partial combustion of low-grade diesel fuel. Two case studies are shown where aerosol is modified by cloud processing over times of 1--2 hours, or <1 hour accumulatively spent within cloud. One case shows a good example of the Twomey effect, whereby the cloud droplet effective radius reduced and the extinction coefficient increased quite dramatically for a given liquid water content. The other cloudy case shows relatively little cloud perturbation due to the ship aerosol. In both cases a Hoppel dip appears in the dehydrated aerosol spectrum between 0.1 and 0.2 μm diameter and a new mode grows out to about 0.45μm. Scattering coefficients of particles between 0.1--0.7 μm show that the modified spectra have greater efficiency at scattering solar radiation. A modeling study of one of these cases indicates that aqueous-phase sulfur chemistry within cloud can explain the aerosol features. By comparison with the concomitant processing of the background aerosol, it is shown that the Hoppel dip and hence critical size for droplet activation lies at larger particle sizes in polluted clouds. A third case study is presented where the plume evolves in a cloud-free boundary layer. Here no modal growth of the aerosol was seen and the aerosol was diluted primarily by mixing with background air. The aerosol was highly deliquesced, which suggests that haze particle chemistry was probably insignificant in modifying the aerosol in any of the cases. ¿ 2001 American Geophysical Union