Smog chamber studies on the influence of diesel exhaust on photosmog formation

In an outdoor smog chamber, volatile organic compounds (VOC)/NOx/air mixtures were irradiated by natural sunlight in the presence and the absence of diesel exhaust. The VOC mixture contained n-butane, ethene and toluene with a fixed mixing ratio. Diesel exhaust was generated by a diesel engine mounted on a motor test bed directly at the chamber facility. Five different diesel fuel formulations were used. Each experiment was carried out under similar initial conditions for VOC and NOx. In the presence of diesel exhaust, the formation of ozone was significantly increased. Simulation of the experiments performed using a chemical box model yielded good agreement between measured and calculated concentrations for all chamber runs. The increase in ozone formation observed on addition of diesel exhaust was mainly caused by the exhaust concentrations of nitrous acid and formaldehyde, which serve as strong radical sources in the initial phase of each exhaust experiment. A sensitivity analysis showed that the photooxidant formation was not dependent on the formulation of the diesel fuel used. The different ozone formation rates observed for the single exhaust experiments were clearly caused by deviations in initial reactant concentrations as well as photolysis conditions.