New airborne measurements provide support for the hypothesis that layers of high concentrations of Aitken nuclei near the tops of marine clouds are due to photochemical nucleation. They also reveal a significant correlation between cloud condensation nucleus (CCN) concentrations in the boundary layer and mean cloud droplet concentration in stratus clouds topping a marine boundary layer. Nonsea salt sulfate mass and the concentration of CCN active at 1% supersaturation are also significantly correlated. These results provide quantitative support for some facets of the DMS-cloud-climate hypothesis. ¿American Geophysical Union 1991