The southern hemisphere marine Aerosol Characterization Experiment (ACE 1) was the first of a series of experiments that will quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. The goals of this series of process studies are to reduce the overall uncertainty in the calculation of climate forcing by aerosols and to understand the multiphase atmospheric chemical system sufficiently to be able to provide a prognostic analysis of future radiative forcing and climate response. ACE 1, which was conducted from November 15 to December 14, 1995, over the southwest Pacific Ocean, south of Australia, quantified the chemical, physical, radiative, and cloud nucleating properties and furthered our understanding of the processes controlling the aerosol properties in this minimally polluted marine atmosphere. The experiment involved the efforts of scientists from 45 research institutes in 11 countries. ¿ 1998 American Geophysical Union