The increased consumption of 'green' fuels, which are rich in monocyclic aromatic hydrocarbons (MAH), in particular due to vehicles which are not equipped with a catalysator, is likely to lead to an increase of these compounds in the urban atmosphere. It seemed thus necessary to follow up the MAH content changes in a number of Paris areas and to compare this evolution to that of usual pollution indicators for motor vehicles and to that of various levels of fuel consumption. The measurement of seven hydrocarbons (benzene, toluene, ethyl-benzene, meta- and para-xylenes, ortho-xylene, 1,2,4-trimethylbenzene) was carried out after a 24-h sampling of air on activated charcoal with a 700 ml/min flow rate. Gas chromatographic analysis with an internal standard was used to measure minimum contents of the order of 2 mu g . m(-3) for each hydrocarbon. This technique was validated by a first set of measurements. Three measurement sites were selected in Paris: narrow street with medium traffic; intersection with dense traffic; underground car park. For each outside site, daily samplings were performed for the whole duration of the study. For the underground car park, six sets of measurements of one week each were considered. The first results obtained show a comparable evolution of the seven hydrocarbons measured. Toluene concentrations are the highest (daily contents of 140 pg . m(-3) were obtained in the ambient outside air). In underground structures, higher contents were recorded (800 mu g . m(-3) for toluene, and 240 mu g . m(-3) for benzene, for example). These results show the necessity of a quasi-permanent control of the pollutants contained in the ambient air and mainly of benzene, due to its toxicity, and of toluene, due to the large amount present.
