Because of continued growth in the number of vehicles and miles traveled, hydrocarbon emissions from conventional gasoline vehicles will begin to increase after 2005, despite continued improvements in emission control systems. But if an aggressive clean fuels program is adopted, overall hydrocarbon emissions will continue to decline dramatically. Some fuels are inherently cleaner than gasoline because they emit less nitrogen oxides or hydrocarbons, and because the hydrocarbons they do emit are less likely to react in the atmosphere to form ozone. Battery-powered cars have the potential for zero tailpipe and evaporative hydrocarbon and nitrogen oxide emissions, though power plant emissions must be accounted for. Today's electric vehicle technology is limited, but promising recent developments may lead to more widespread use in the future.
Propane is a byproduct of petroleum refining and natural gas production.
Propane vehicles emit considerably less ozone-forming hydrocarbons than do vehicles fueled with conventional gasoline. The petroleum industry is studying ways to change refinery procedures to make a cleaner-burning gasoline.