Development of an urban inventory for road transport emissions of NO2 and comparison with estimates derived from ambient measurements

A primary nitrogen dioxide (NO2) emissions inventory has been developed for road transport sources in London based on recent exhaust emissions measurements of nitrogen oxides (NOX=NO+NO2) and NO2 from a range of vehicle types and technologies. The emissions inventory estimate of the mean primary volumetric NO2:NOX ratio (NO2 fraction) on road links in London was 10.2%. The emissions inventory highlights important spatial gradients in primary NO2 emissions across London. On major roads such as motorways, where the vehicle speed is high (>80 km h−1), the primary NO2 fraction is estimated to be less than 5.0% by volume. By contrast, on the congested roads of Central London, where emissions from diesel vehicles such as taxis and buses are high because of their high flows in this area of London, the primary NO2 fraction is typically 12.0% by volume. These differences have important implications for the estimation of ambient NO2 concentrations close to roads in London, and highlight why it is likely to be more difficult to meet international limits for ambient NO2 concentrations in Central London compared with other areas of the capital. The emissions inventory estimates of primary NO2 fraction have been compared with estimates derived from 41 roadside measurement sites in London, the latter being based upon an analysis of hourly mean NOX and NO2 concentrations. The results show that the mean primary NO2 fraction of 11.3% derived from ambient measurements agrees well with the mean estimate of 10.2% derived from the emissions inventory. The range of primary NO2 emission fractions derived from the ambient measurements is, however, much wider than that derived from the emissions estimates. Some of the possible reasons for the disagreement are discussed. It is concluded that a larger database of emissions measurements from a range of vehicle types and pollution control technologies is required to develop a more robust emissions inventory for primary NO2 in urban areas.