Recent experiments demonstrate that cloud processes in the free troposphere, and at times in the marine boundary layer (MBL), can result in the production of new aerosol particles linked to sulfuric acid production that is photochemically driven. Here we provide observational evidence from the tropics and mid-latitudes indicating that this nucleation is often in accord with predictions of classical binary nucleation theory when it is applied to the high humidity environment near evaporating clouds edges located above the MBL. Coupled with suitable models for the relevant chemical species and conditions, these observations suggest a means for modeling natural particle production in these regions. ¿ 1999 American Geophysical Union