Experimental Evaluation of Ozone Forming Potentials of Motor Vehicle Emissions

Evaluations of air quality impacts from vehicle emissions are based on the assumptions that all the important reactive species in the exhaust have been identified and quantified and that they are accurately represented in airshed models. To test this, environmental chamber experiments were carried out with exhaust from vehicles fueled by LPG, M100, M85, CNG, diesel, and Phase 2 reformulated gasoline (RFG), with the latter including vehicles representing a range of mileages, types, and pollution levels. Baseline FTP tests and speciation analyses were carried out for all vehicles studied but diesel. The chamber experiments consisted of irradiations of the exhausts themselves, “incremental reactivity” experiments with the exhaust added to two different surrogate VOC NOx mixtures simulating conditions of photochemical smog, and irradiations of synthetic exhaust mixtures based on the results of the exhaust analyses. Computer model simulations of the experiments were also carried out. The results of most of the experiments were consistent with results of experiments using synthetic exhausts derived to represent them, and were generally consistent with model predictions. This indicates that the major constituents contributing to their ozone impacts have probably been identified, and that a current chemical mechanism is reasonably successful in predicting these impacts. The major exception to this was the one experiment with diesel exhaust, where a complete analysis was not conducted. There was some evidence, albeit inconclusive, that the model may be underpredicting the ozone impacts of some of the constituents of exhausts from the two highest mileage RFG-fueled vehicles in some experiments, but the model gave reasonably good simulations of effects of adding these to realistic ambient VOC NOx mixtures, as was the case for all the other exhausts for which complete analyses were conducted.