Vehicle nitrogen oxides emissions make a significant contribution to acid deposition. The major gases leading to the formation of acid deposition are sulphur dioxide (SO2) and nitrogen oxides (NOx). Vehicles do not produce much sulphur dioxide as petroleum contains very little sulphur. In 1999, vehicles contributed 1% of the UK sulphur dioxide emissions whilst power stations being the major source of the total UK sulphur dioxide emissions contributed 65%. Nitrogen oxides from vehicles, however, are a major contributor to acid deposition. In 1999, road transport accounted for 44% of nitrogen oxides emissions in the UK.
A motor vehicle produces air pollutants when fuel is burnt to give mechanical power. In a totally efficient combustion process, hydrocarbons in fuel, and oxygen will react to form carbon dioxide and water. However, the combustion process is never perfect; some of the hydrocarbon fuel is only partially burnt forming carbon monoxide and water, whilst some of the hydrocarbons are not combusted at all. These can, and often are, emitted from the exhaust as unburned hydrocarbons. During the combustion process the temperature can reach 2500°C. At these temperatures nitrogen and oxygen from the air in the combustion chamber react to form nitrogen oxides.
Nitrogen oxides pollution from petrol-driven vehicles can be significantly reduced by fitting a catalytic converter to the exhaust system. This is a relatively low cost method of pollution control (around £350) which has little effect on vehicle performance and fuel consumption. All new cars sold in Britain from January 1993 onwards have catalytic converters.