| The primary goals of this research were to clarify the relationships between vehicular emissions and (1) vehicular and socioeconomic characteristics, and (2) ambient pollution concentrations. To this end, the study utilizes data from three inspection and maintenance (I/M) programs, the US Census, and the US Environmental Protection Agency's National Emission Inventory and Air Quality System.; The study finds that vehicle characteristics do affect vehicle emissions. Model-year, fuel economy, mileage, and engine size are the most significant vehicle characteristics, although other characteristics, such as vehicle weight, make, and certain engine attributes are also found to influence emissions. The dirtiest 10% of the vehicular fleet, the high-emitters, are responsible for a disproportionately large percentage of CO, HC, and NO emissions. For example, high-emitters account for more oil-road CO emissions than the cleanest 75% of the vehicle fleet. High emitters are found to be a problem for all makes, model-years, and vehicle types. A strong connection between fuel efficiency and emissions is found in all vehicle makes and for all model-years. The current regulatory disconnect between vehicle fuel economy and emission standards thus needs to be bridged. This can be done by changing the way vehicle emissions are regulated: by regulating emissions in grams/gallon, rather than the current grams/mile standard.; The socioeconomic characteristics of vehicle owners have an important effect on vehicle emissions. Both income and education are the most important socioeconomic variables with regard to emissions, although education proves to be a slightly better explanatory variable than income. Race and the urban character of an area are also found to be important predictors of vehicle emissions and emission test failure.; Lastly, the study finds that the NET on-road car and truck emissions estimates and those derived from the study's Massachusetts I/M data to be fairly consistent with one another. Although a relationship is found between vehicular CO emissions and ambient CO, none was found between NO emissions and ambient NO. The analysis of the relationship between vehicular NO and HC emissions and ambient ozone levels led to confounding results, due to data limitations and the long-distance character of ozone pollution. |
