Radon, chemical, and condensation nuclei measurements were used to characterize air in which marine fogs formed off the coast of Nova Scotia in August 1975. Tracing of air mass trajectories from weather maps showed that either continental or maritime air could reside at the fog sites, with some fog events bridging the boundary between these two types of air. Radon and chemical data were used to assess the relative continental contributions to the foggy air parcels as well as to define boundaries between parcels. Chemical results aslo showed that the fog waters contained varying levels of continental and marine components, probably derived from the orignial aerosol content of the air. Meteorological limitations on air mass histories supported evidence for K enrichment in the sea-surface-derived aerosol component. Similarly, evidence of SO4= being enriched in those aerosols, as well as being added to the fog water from continental sources, was found. A device for acquisition of fog water samples and the measurement of their ionic conductivities in real time is described. It enabled monitoring changes in the salt load of the air within fogs and thereby marked air mass transitions in some of them. Besides magnitudes, the rates of change of the monitored variables appear to be practical real-time indicators.