Size-resolved particle emission factors of vehicular traffic derived from urban eddy covariance measurements

Abstract

Road traffic is one of the major sources of atmospheric particles in urban areas. Modelling the contribution of this source to urban pollution requires reliable estimates of emission factors possibly resolved in size. In this work, size-resolved particle emission factors (EFs) of the mixed vehicle fleet were measured in the urban area of Lecce (south-eastern Italy). The approach used is based on vertical fluxes measured with eddy-covariance, counting of vehicles, and estimation of footprints. Results show that the average EF in number (range 0.009-3 μm) was 2.2*10¹⁴ #/Veh km, being dominated by ultrafine particles (D_p < 0.25 μm) due to exhaust emissions. EF number size distribution decreases with particle size. A reduction of more than four orders of magnitude was observed at D_p ≅ 0.9 μm. EF mass size distribution reaches a maximum around D_p ≅ 0.3 μm then decreases until D_p ≅ 0.9 μm. For larger particles EF in mass increases for the influence of non-exhaust emissions. Average emission factor of PM₁ was 56 mg/Veh Km and that of PM_2.5 was 63 mg/Veh Km. A comparison of measurements taken in 2010 and 2015 in the same area shows a decreasing trend of the average total EF in number of about 56%, likely as a consequence of the increased use of new generation vehicles following more restrictive limits for particle emissions.

Keywords: Eddy-covariance measurements; Footprint analysis; Size-resolved particle fluxes; Urban aerosols; Vehicle emission factors.