Road transport is a source of air pollutants which can have adverse effects at various scales. The air pollutants which are currently causing greatest concern in terms of local air quality, primarily because of their impacts on human health, are airborne PM, NO2 and ground-level ozone5. Road transport is an important contributor to all three. Emissions of nitrogen oxides (NOx) from road vehicles are also implicated in regional phenomena such as acidification, eutrophication and loss of biodiversity, as well as the formation of secondary PM in the atmosphere. Moreover, road transport is a major source of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).
The significance of road transport as a source of air pollution can be illustrated by reference to sectoral emissions for the EU-27 countries, based on submissions to the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP). Road transport is a significant contributor to NOx emissions (41% in Europe) and is also a major contributor to PM emissions. In urban areas its impact is even greater due to the density of the road network, the volume of traffic, and the close proximity of the population to the emission source.
Legislation and strategies to reduce exhaust emissions from road vehicles have been in place for some time. Calculations have established that emissions of regulated pollutants from road transport have been reducing as controls on vehicles and fuels have tightened. However, in many urban areas the concentrations of NO2 and PM10 (particulate matter with an aerodynamic diameter of less than 10 μm) still frequently exceed health-based limits and are not decreasing.
The importance of NO2 and particulate matter is explained in more detail below.