Estimation of the emission factors of particle number and mass fractions from traffic at a site where mean vehicle speeds vary over short distances

Emission factors for particle number in three size ranges (11–30; 30–100 and >100 nm) as well as for PM2.5, PM2.5−10 and PM10 mass have been estimated separately for heavy and light-duty vehicles in a heavily trafficked street canyon in London where traffic speeds vary considerably over short distances. Emissions of NOx were estimated from published emission factors, and emissions of other pollutants estimated from their ratio to NOx in the roadside concentration after subtraction of the simultaneously measured urban background. The estimated emission factors are compared with other published data. Despite many differences in the design and implementation of the various studies, the results for particulate matter are broadly similar. Estimates of particle number emissions in this study for light-duty vehicles are very close to other published data, whilst those for heavy-duty vehicles are lower than in the more comparable studies. It is suggested that a contributory factor may be the introduction of diesel particle oxidation traps on some of the bus fleet in London. Estimates of emission factors for particle mass (PM2.5 and PM2.5−10) are within the range of other published data, and total mass emissions estimated from the ratio of concentration to NOx are tolerably close to those estimated using emission factors from the National Atmospheric Emissions Inventory (NAEI). However, the method leads to an estimate of carbon monoxide emissions 3–6 times larger than that derived using the NAEI factors.