Measurements are presented of the size distribution and volatility of particles in the cloudy atmospheric boundary layer of the North Atlantic Ocean. The results suggest that particle nucleation of acid sulfates occurs but is neither spatially nor temporally homogeneous. Such nucleation appears to be inversely correlated with preexisting particle surface area. The few measurements obtained in clouds show a different acid sulfate distribution from those in clear air. ¿ American Geophysical Union 1993