Road transport is a major source of air pollution in the United Kingdom. The motor engine emits many types of pollutants including;
- nitrogen oxides (NOx)
- volatile organic compounds (VOC's)
- carbon monoxide (CO)
- carbon dioxide (CO2)
- particulates (e.g. PM10's)
- sulphur dioxide (SO2)
- ozone (O3)
- hydrocarbons (HC)
Emissions are related to the use of the engine, mainly the fuel type and the temperature of combustion. If the engine is 100% efficient, then the products of combustion will be CO2 and water (H2O).
At low loads engines are inefficient and therefore the products of incomplete combustion dominate, for example CO, VOC's and smoke (particulate matter) in diesels and CO and VOC's in petrol engines. As the temperature of combustion increases, the inefficiency of conversion to CO2 and water increases. However impurities in the fuel such as nitrogen are oxidised to nitrogen dioxide (NO2). Hence with heavier vehicles at a greater speed, NO2 production dominates.