Aerosol samples of PM2.5 and PM10 in a period of intensive haze-fog (HF) events were collected to investigate the chemical characteristics of air pollution in Beijing. The air quality in HF episodes was much worse than that in nonhaze-fog (NHF) days. The concentrations of elements and water-soluble (WS) ions (K+, So4(2-), and NO3-) in HF episodes were more than 10 times higher than those in NHF days. Most of the chemical species in PM2.5 and the secondary species (NH4+, So4(2-), and NO3-) in PM10 showed significant difference between HF from westerly direction (HFW) and southerly direction (HFS). The concentrations of secondary species in HFS were much higher than those in HFW, and other chemical species in HFS were lower than those in HFW. The sources of PM2.5 were more from areas on the regional scale due to its tendency for long-range transport, while PM10 was more limited to the local sources. Aerosol particles were more acidic in HFS and more alkaline in HFW. The secondary species were the major chemical components of the aerosol in HF episodes, and their concentrations increased in the order of NHF < HFW < HFS. High concentrations of the secondary aerosol in HF episodes were likely due to the higher sulfur and nitrogen oxidation rate in aqueous-phase reactions. The serious air pollution in HF episodes was strongly correlated with the meteorological conditions and the emissions of pollutants from anthropogenic sources.

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 lowestincome 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.

The MOBILE6 emissions model, currently under development, uses mileage accumulation rates that assume that the average 25-year-old car has been driven more than 337 962 km (210,000 mi), and that the average 25-year-old pickup truck has been driven more than 402 336 km (250,000 mi). Cumulative mileage is used in the model to calculate emission factors that increase with mileage due to air pollution control device “deterioration.” Odometer readings taken in previous studies by the Environmental Protection Agency and additional data indicate that average mileage accumulations may be much less [i.e., 201 168 km (125,000 mi)] than those used in the MOBILE6 model. This has the effect of overestimating emissions from older vehicles. A simple model is presented that accounts for scrappage of older vehicles as a function of cumulative mileage. This model predicts average cumulative mileage that is much closer to actual odometer readings (taken in Nashville, Tennessee) than the default values used in MOBILE6. The use of more accurate cumulative mileage values for older vehicles should provide improvements to the estimation of emissions from the vehicle fleet.