Estimation Of Motor Vehicle Emission Factors From Road Measurements Of Pollutant Concentrations

Motor vehicle emission factors for application in traffic/transport models are often estimated indirectly, through measurements of pollutant concentrations in the close vicinity of a road link. Such an approach provides insight into total vehicle fleet emissions on that link. However, estimation of emission factors from the measured concentration raises a set of complex issues. The concentrations depend not only on the emission factors, but also to a large degree on meteorological factors, as well as on the topography of the area.Using the number/mass balance model, a theoretical relationship was developed earlier that allows for calculation of emission factors from the concentrations measured at a specific point above the road, taking into account wind as the only removal mechanism of the emitted pollutants. Using this model, average emission factors for all vehicles on the road can be estimated and then used to arrive at emission factors for individual types of vehicles, given a known vehicle fleet composition.The work presented here is an extension of the model to include the effect of humidity and temperature, as well as topographical aspects, on the concentrations measured. Starting from the principles of combustion processes, as well as of the dynamics of pollutant interaction and transport, some of the general mechanisms affecting emissions or concentrations were included in the model. The empirical dependencies were derived from the results of extensive monitoring of traffic flow and associated emissions at two city sites. The latter included a freeway link and a major signal controlled urban arterial link. The model and experimental results can be applied to estimate average emission factors from measurements conducted at any road link.