Measurements of aerosol size distributions (0.005<r<20 μm), cloud droplet spectra, SO2, O3, CN, and other supporting quantities were made in the cloud-topped and clear marine boundary layer (MBL) from an airship operating within about 50 km of the Oregon coast. Comparison of size distribution of interstitial aerosol within the cloud with the size distribution below the cloud clearly indicates that the processing of the aerosol through (nonprecipitating) stratus can lead to increased mass of the subset of particles which had served as cloud condensation nuclei (CCN). This increase in mass in the CCN results in a distinct ''cloud residue'' mode in the size distribution measured below the cloud. In all cases the aerosol mass in the cloud residue mode greatly exceeded the mass in the interstitial mode, even though the number concentration of interstitial particles sometimes exceeded the CCN concentration. Evidence of new particle formation in clear air was also found on numerous occasions. Analyses of the data indicate that the growth of newly formed particle sinto the observed size range is consistent with gas phase oxidation of SO2 to sulfate and subsequent condensation on the aerosol. However, the exact nucleation process, whether by homogeneous nucleation, ion-assisted nucleation, or heterogeneous nucleation on precursor embryos, is still an open question. ¿ American Geophysical Union 1994