§ Mr. WigginTo ask the Secretary of State for Environment, Food and Rural Affairs what her estimate is of the contribution of conventionally powered vehicles to(a) summer smog and (b) winter smog. [133280]
§ Mr. BradshawThe most important component of summer smog is ground level ozone. Ozone is not emitted directly by vehicles. It is a secondary pollutant formed from precursor gases. Due to the complexities of170W the chemistry of ozone formation it is not possible to provide a quantitative estimate of the contribution of conventionally powered vehicles to summer smog.
The major chemical precursors for ground level ozone in industrialised nations are oxides of nitrogen and hydrocarbons produced mainly by conventionally powered motor vehicles and industry. Ozone and its precursors can be transported over hundreds of kilometres making ozone a transboundary pollutant. Local emissions of precursors do not generally make a significant contribution to local ozone concentrations in the UK.
The most important components of winter smog are particulate matter (PM10) and nitrogen dioxide (NO2). PM to is a complex mixture of local primary combustion particles, mechanically generated particles and secondary particles formed in the atmosphere from oxides of nitrogen and sulphur dioxide and transported over hundreds of kilometres. 18 per cent. of UK PM10 emissions are from road traffic sources and 42 per cent. of NOx emissions are from road traffic sources. The contribution from road traffic sources to ambient concentrations in cities during winter smog episodes is likely to be higher than implied by national emission totals. Studies suggest that between 25 per cent. and 40 per cent. of PM10 concentrations and 50-70 per cent. of oxides of nitrogen concentrations are likely to be derived from road traffic in urban areas.