Accurate measurements of the size distribution of atmospheric aerosols in the size (radius) range from 0.006 to 2.5 μm made from a ship within 300 km of the east coast of the United States are presented. As air advects from land to sea, there is a rather rapid decay of particles smaller than 0.05 μm during the first day or so. After this initial decay of small particles the size distribution was often found to remain remarkably stable for hours. Significant changes in the size distribution were often associated with changes in air masses and meteorological conditions. Some of the changes can be explained qualitatively. Others, such as a pronounced double peak which occurred occassionally in the fine-particle range, are more difficult to understand. Possible physical mechanisms which might produce a double-peaked distribution are discussed, including the increase in the mass of cloud condensation nuclei as a result of the chemical formation of new material from gases absorbed in cloud droplets during a cloud formation-evaporation cycle. Using the measured size distributions, volume extinction coefficients were calculated for the wavelength range 0.3--10 μm. The calculations compare favorably to the measured scattering coefficients at 0.55--μm wavelength. Particles in the radius range 0.1--0.3 μm made the largest contribution to the total extinction at optical wavelengths on nearly all occasions.