Generalised extreme value distribution fitted well to PNC data at both TIs. Despite the relatively high traffic volume at 3–way TI, average PNCs at 4–way TI were about twice as high as at 3–way TI, indicating less favourable dispersion conditions. Daily averaged particle number distributions at TIs reflected the effect of fresh emissions with peaks at 5.6, 10 and 56nm. The aims were to: (i) assess the differences in PNCs measured at studied TIs, (ii) identify the best fit probability distribution curves for the PNCs, (iii) determine vertical and horizontal decay profiles of PNCs, (iv) estimate particle number emission factors (PNEFs) under congested and free–flow traffic conditions, and (v) quantify the pedestrian exposure in terms of respiratory deposition dose (RDD) rates at the TIs. We measured size–resolved PNCs in the 5–560 nm range at two different types (4– and 3–way) of TIs in Guildford (Surrey, UK) at fixed sites (~1.5 m above the road level), sequentially at 4 different heights (1, 1.5, 2.5 and 4.7 m), and along the road at five different distances (10, 20, 30, 45 and 60 m). (2013) Mixed fleet 3e950 6.03 ± 0.19 Road in a semi-urban area This study a Mixed fleet 5e560 6.41 ± 3.32 3eway TI This study a Mixed fleet 5e560 7.69 ± 10.9 4eway TI a The details of methodology used to estimate them are available in Section 2.5 and the results are discussed in Section 3.5. (2006) Mixed fleet 7e3000 7.90 Motorway Jones and Harrison (2006) HDV 11e450 6.36 Street canyon Jones and Harrison (2006) LDV 11e450 0.12 Street canyon Rose et al. (2005) Diesel vehicle 18e880 2.04 Stop and start condition in street canyon Hueglin et al.
(2005) Petrol vehicle 18e880 0.22 Stop and start condition in street canyon Morawska et al. (2005) Mixed fleet 10e400 4.70 Rural motorway Morawska et al. Vehicle type Range (nm) PNEF± s (# veh À1 km À1 ) Â 10 14 Environment Corsmeier et al. However, the method leads to an estimate of carbon monoxide emissions 3–6 times larger than that derived using the NAEI factors. 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). 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 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.
Despite many differences in the design and implementation of the various studies, the results for particulate matter are broadly similar. The estimated emission factors are compared with other published data. 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. 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.
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