Mobile sources critical review: 1998 NARSTO assessment

Mobile sources of air pollutants encompass a range of vehicle, engine, and fuel combinations. They emit both of the photochemical ozone precursors, hy drocarbons and oxides of nitrogen. The most important source of hydrocarbon s and oxides of nitrogen are light- and heavy-duty on-road vehicles and hea vy-duty off-road vehicles, utilizing spark and compression ignition engines burning gasoline and diesel respectively. Fuel consumption data provide a convenient starting point for assessing current and future emissions. Moder n light-duty, gasoline vehicles when new have very low emissions. The in-us e fleet, due largely to emissions from a small "high emitter" fraction, has significantly larger emissions. Hydrocarbons and carbon monoxide are highe r than reported in current inventories. Other gasoline powered mobile sourc es (motorcycles, recreational vehicles, lawn, garden, and utility equipment , and light aircraft) have high emissions on a per quantity of fuel consume d basis, but their contribution to total emissions is small. Additional unc ertainties in spatial and temporal distribution of emissions exist. Heavy-d uty diesel vehicles are becoming the dominant mobile source of oxides of ni trogen. Oxides of nitrogen emissions may be greater than reported in curren t inventories, but the evidence for this is mixed. Oxides of nitrogen emiss ions on a fuel-consumed basis are much greater from diesel mobile sources t han from gasoline mobile sources. This is largely the result of stringent c ontrol of gasoline vehicle emissions and a lesser (heavy-duty trucks) or no control (construction equipment, locomotives, ships) of heavy-duty mobile sources. The use of alternative fuels, natural gas, propane, alcohols, and oxygenates in motor vehicles is increasing but remains small. Vehicles util izing these fuels can be but are not necessarily cleaner than their gasolin e or diesel counterparts. Historical vehicle kilometers traveled growth rat es of about 2% annually in both the United States and Canada will slow some what to about 1.5%. Mexican growth rates are expected to be greater. Fuel c onsumption growth in recent years of about 1.4% annually is projected to co ntinue as slowing gains in fuel economy from fleet turnover are more than o ffset by growth and the increasing number of Sport Utility Vehicles. This g rowth also will erode the emissions reductions resulting from cleaner new v ehicles and fuels. Uncertainties in these projections are high and affected by economic activity, demographics, and the effectiveness of emissions con trol programs - especially those for reducing in-use emissions. (C) 2000 El sevier Science Ltd. All rights reserved.