During the First Aerosol Characterization Experiment (ACE 1) a number of meteorological and atmospheric composition measurements were made to identify anthropogenically impacted conditions and to infer information about air parcels during transport to the sampling point. Radon was used as an indicator of land contact supported by back trajectory calculations; additional information on anthropogenic sources was obtained from the concentrations of condensation nuclei and elemental carbon. A limited extension of the air mass categorization utilizes altitude profiles of potential temperature and water content derived from balloon soundings at the fixed sites. The four surface platforms were found to sample a very similar range of conditions, from very clean marine air masses to those strongly influenced by anthropogenic sources. Although this was expected because the study area is essentially encompassed by the one meteorological system, the precise origins of air masses were usually different for each platform, even, for example, when the Discoverer was within a hundred kilometers of Cape Grim. There was good agreement between the different air mass indicators under conditions when they could be compared. Radon was present when the back trajectory indicated passage over land at low altitude. Vertical profiles of potential temperature and water concentration were generally consistent with the origin of back trajectories. Excursions in the concentrations of condensation nuclei and elemental carbon occurred during periods of elevated radon concentrations when the path from land was short. However, these coincident periods of elevated concentration were rare when there was a long enough time from land for the different processing of each species to take effect. ¿ 1998 American Geophysical Union