A comparison of benzene, toluene and C-2-benzenes mixing ratios in automotive exhaust and in the suburban atmosphere during the introduction of catalytic converter technology to the Swiss Car Fleet

Time-resolved chemical ionization mass spectrometry (CIMS) has been used to investigate the variations of the mixing ratios of benzene, toluene and th e C-2-benzenes (xylenes and ethyl benzene) in automotive exhaust during tra nsient engine operation. A significant increase of the benzene/toluene rati os from 0.35 to 1.31 (median) was found upon introduction of a catalytic co nverter system. A preliminary emission model was developed from these test stand measurements to simulate benzene/toluene ratios of passenger car flee ts with variable proportions of three-way catalyst vehicles. Although only the emissions of gasoline-driven passenger cars have been considered so far , the predicted increase of the benzene/toluene ratios during the introduct ion period of the three-way catalyst from 1980 to 2000 is in good agreement with the observed increase of the atmospheric benzene/toluene ratio measur ed at a suburban monitoring site (Dubendorf, Switzerland) which is strongly influenced by road traffic emissions. At this site, the atmospheric concen trations of benzene and alkyl benzenes have been detected at hourly interva ls since 1993. A steady decrease of the yearly mean from 3.54 to 2.00 ppb f or toluene and from 2.87 to 1.33 ppb for the sum of C-2-benzenes was found from 1994 to 1998, respectively, when the proportion of three-way catalyst passenger cars increased from 60 to 82%. Nevertheless, the mean benzene con centration was only affected to a small degree (from 1.10 to 0.97 ppb) with in the same period of time. Thus, the observed increase of the atmospheric benzene/toluene-mixing ratios from 0.32 to 0.58 (mean) is in good agreement with the predicted values from the presented emission model. Reduced catal yst conversion efficiency for benzene with respect to alkylated benzenes ca n explain most of the observed increase of the benzene/toluene and benzene/ C-2-benzenes mixing rations. In addition, benzene emissions e.g. from the c lass of light duty vehicles, which are operated more frequently at sub-opti mal combustion conditions, may also contribute to the unexpectedly stable a tmospheric benzene concentration at the investigated suburban monitoring si te. (C) 2000 Elsevier Science Ltd. All rights reserved.