Vehicles with combustion engines are significant emitters of air pollutants and therefore have a major influence on the environment. Road and tunnel studies are important tools in evaluating the traffic emission models and in assessing the achievements of new technologies for emission reduction.
During three tunnel measurement campaigns namely: the Lundby Tunnel in Sweden (March, 2001), the Plabutsch Tunnel in Austria (November, 2001), the Kingsway Tunnel in England (March, 2003) and an additional field experiment conducted along a section of the A656 motorway in Germany (May 2001), emissions from a large variety of vehicles were investigated. The measurements covered the standard pollutants CO2, NOₓ and non methane volatile organic compounds (NMVOC).
The data obtained from the measurements carried out in these European locations were analysed with respect to the composition of the NMVOC mixture and the ratio of individual compounds to benzene, CO2 and NOₓ. The NMVOC profiles observed in the tunnel as well as in motorway measurements are in good agreement with the NMVOC profiles observed in other campaigns. Among the aromatic hydrocarbons, toluene showed the highest mixing ratio, whereas iso-pentane and propene had the highest mixing ratios among the alkanes and alkenes, respectively. During the motorway experiment, vertical gradients were observed for several NMVOC downwind of the motorway, in contrast to the upwind measurement site, where NMVOC were constantly distributed. This indicates that the studied NMVOCs are directly emitted by the traffic. The ratio toluene/benzene, which is an indicator of traffic activity, was found in the performed studies to be: 3.09 in the Lundby Tunnel, 2.44 in the Plabutsch Tunnel and 2.87 in the Kingsway Tunnel. During the A656 motorway experiment, the ratio was constantly 2.5 except for stop and go periods when this could increase up to 3.6. These values are higher than the previously measured ratios at traffic sites. This finding can be explained by an over proportional decrease in the benzene emissions compared with the aromatic compounds since the year 2000, when the new European Directive 98/70/EC limiting the benzene content in gasoline to 1% started to be applied.
Within the framework of extensive emission data validation experiments, the emission factors for the Lundby Tunnel, the Plabutsch Tunnel and the A656 motorway have been determined using the local traffic measurements in combination with new emission models. The models provide emission factors with a temporal resolution of 20 min for the tunnel studies and 3 h for the motorway study. The models took as input parameters the emission factors for CO2 - model 1; total hydrocarbons (THC) - model 2; and benzene, toluene, xylene (BTX) - model 3 from the Handbook Emission Factors for Road Transport (HBEFA) version 2.1. The input parameters were modified by taking into account the slope gradients for the corresponding measurement sites as well as the fleet composition and the driving pattern. By comparing the direct calculated values of emission factors with the model 1 values, a good correlation of data was obtained for the time intervals characterised by fluent and high traffic density. This proves that the emission factors for CO2 in the HBFEA 2.1 are a good estimation for the real traffic situation. In the case of the Lundby and the Kingsway Tunnels, for all analysed compounds except benzene, the emission factors calculated through model 1 were higher than those obtained from model 2 and 3. For the Plabutsch Tunnel, the models 2 values were higher. The data analysis of THC emission factors for the given traffic fleets was used in order to clarify the discrepancy between measured and model calculated emission data.
Two hypotheses were verified:
- the THC emission factor of heavy duty vehicles is not correct; and/or
- the percentage of catalyst vehicles determined from the fleet data analysis does not reflect the reality, i.e. cars that are registered with catalyst in fact have a malfunctioning catalyst and should be counted as non-catalyst vehicles with the corresponding emission factor. The influence on the emission factors of the about 12% malfunctioning catalysts was demonstrated.
From the overall emission factors determined for weekday traffic, in the Plabutsch Tunnel, the emission factors for light duty vehicles (LDV) and heavy duty vehicles (HDV) were calculated by means of a regression analysis. The comparison of the LDV emission factors for benzene and toluene obtained in the Plabutsch Tunnel, to data from a previous study performed in the same tunnel shows a clear decrease of the emission factors in this tunnel. The A656 motorway experiment, showed for benzene, toluene, ethylbenzene and 1,2,4 trimethylbenzene/decane that within the statistical errors the measured values are in good agreement with the modeled ones.
The results of the present study show that the NMVOC split is influenced by the fleet composition and that the most important contribution to the NMVOC emissions results from poorly maintained vehicles and those with malfunctioning catalysts. The results of the present study reflect the current situation and the improvements of the European car fleet with respect to hydrocarbons emissions after the introduction of the new European legislation in 2000.