Effect of county-level income on vehicle age distribution and emissions

In conjunction with a statewide emissions inventory of on-road mobile sources in Tennessee, a county-by-county analysis of vehicle registration data was performed. Several interesting trends were observed in the kinds and ages of vehicles driven in Tennessee counties compared with national statistics and compared with the average personal income of county residents. In particular, median vehicle age correlated strongly with average personal income for each county. Vehicle fleets were oldest in lowest-income counties and newest in the highest-income county; median vehicle age was 10.8 years in the former and only 5.9 years in the latter. This difference in vehicle age results in average mobile-source emissions factors 63% higher for nitrogen oxides, 73% higher for carbon monoxide, and 104% higher for volatile organic compounds in the lowest-income counties than in the highest-income counties, based on the MOBILE6 emissions model run for calendar year 2000. The low-income counties also registered 76% more light-duty trucks per capita than the national average, and these trucks were 5 years older than the national median age. It is concluded that county-level personal income is a good predictor of vehicle age and can be used as a readily obtainable indication of whether local vehicle registration data should be used to improve the accuracy of emissions inventories (instead of national defaults or statewide averages). County-level personal income also can be used as a basis for determining whether more than one vehicle age distribution should be used for modeling mobile-source emissions within a state, a metropolitan area, or an airshed.