Studies on impacts of air pollutants on vegetation have focused primarily on individual pollutants: ozone, sulfur dioxide and nitrogen dioxide. The impacts of pollutant combinations have not been extensively studied and there has been no concerted effort to ensure that experimental regimes for combined pollutant exposures are representative of ambient pollutant concentration, frequency, duration and time intervals between events. Most studies concerning the impact of pollutant combinations on vegetation have used concentrations of 0.05 ppm and greater. Therefore, co-occurrence was defined as the simultaneous occurrence of hourly averaged concentrations of 0.05 ppm or greater for pollutant pairs (SO2/NO2, O3/SO2, or O3/NO2). Air quality information from three data bases (EPA-SAROAD, EPRI-SURE and TVA) was analyzed to determine the frequency of co-occurrence for pollutant pairs. Ambient air quality data representing a diverse range of monitoring sites (e.g. rural, remote, city center, urban near urban, etc.) were used in the analysis. Results showed that at most sites (1) the co-occurrence of two-pollutant mixtures lasted only a few hours per episode, (2) the time interval between episodes was generally large (weeks, sometimes months) and (3) most studies have used more intense exposure regimes than occurred at most monitored sites. When designing vegetation experiments for assessing pollutant mixture effects, it may be desirable to give greater emphasis to sequential patterns of exposure. (Copyright (c) 1984 Pergamon Press Ltd.)