A high-resolution NOx emission factor model for North American motor vehicles

Criteria air contaminant inventories in Canada show that approximately 60% of NOx is produced from the transportation sector alone. NO2, which is present at higher levels in the winter than in the summer, has been identified as the main pollutant responsible for non-traumatic deaths and hospitalization in Canadian cities. Therefore, accurate emission estimates of vehicle-generated NOx are needed, especially at the micro-scale level. The MOBILE emission model, which is widely used in Canada, provides only aggregated emission factors at very low resolution. Many studies have shown that MOBILE emission data are unreliable for many applications—particularly for air-quality modelling. In contrast to the aggregated nature of the MOBILE data, the new disaggregated model, MicroFacNOx, uses time-dependent site-specific vehicle fleet information to calculate emission factors at high spatial and temporal resolution. MicroFacNOx evaluation in a range of traffic fleet and meteorological conditions shows very encouraging results. Sensitivity analyses of the model reveal that its results depend on traffic fleet composition, speed and ambient temperature. MicroFacNOx results are suitable for use in human exposure assessment and by city planners in decision making for growth management. The effect of vehicle-related NOx on ozone in the vicinity of a large urban area is a good example of an unsolved problem that would benefit from the MicroFacNOx modelling approach.