Rapid identification of high particle number emitting on-road vehicles and its application to a large fleet of diesel buses

Pollutant concentrations measured in the exhaust plume of a vehicle may be related to the pollutant emission factor using the CO₂ concentration as a measure of the dilution factor. We have used this method for the rapid identification of high particle number (PN) emitting on-road vehicles. The method was validated for PN using a medium-duty vehicle and successfully applied to measurements of PN emissions from a large fleet of on-road diesel buses. The ratio of PN concentration to CO₂ concentration, Z, in the exhaust plume was estimated for individual buses. On the average, a bus emitted about 1.5 × 10⁹ particles per mg of CO₂ emitted. A histogram of the number of buses as a function of Z showed, for the first time, that the PN emissions from diesel buses followed a gamma distribution, with most of the values within a narrow range and a few buses exhibiting relatively large values. It was estimated that roughly 10% and 50% of the PN emissions came from just 2% and 25% of the buses, respectively. A regression analysis showed that there was a positive correlation between Z and age of buses, with the slope of the best line being significantly different from zero. The mean Z value for the pre-Euro buses was significantly greater than each of the values for the Euro I and II buses. © 2007 American Chemical Society.