Size-resolved particle emission factors of vehicular traffic derived from eddy covariance measurements in two European cities.

Aerosol flux measurements from two different locations, Innsbruck (Austria) and Lecce (Italy), were compared to evaluate road traffic emissions in urban areas. The present study aims to derive emission factors (EFs) of particles for different size ranges from vertical turbulent fluxes obtained by the eddy-covariance method and simultaneous counting of vehicles. The measurements were carried out between 27 July and 21 August 2015 in Innsbruck, and between 10 March and 24 April 2015 in Lecce. Both measuring stations were located on the rooftop of university buildings within their respective city centres, at 38.6 m and 14 m height above ground level, with zero-plane displacement height d0 = 14 m and 7 m, and roughness length z0 = 2 m and 1.3 m in Innsbruck and Lecce, respectively. Source areas for concentrations and fluxes (Fig.1) were estimated for both sites. The instruments used consisted of eddy-covariance stations based on ultrasonic anemometers, CPCs, and OPCs at both sites; in addition, the Innsbruck site included an ELPI. Fluxes were evaluated for the size ranges between 0.009 and 3 µm, for the Lecce site, and between 0.006 and 10 µm, for the Innsbruck site, over 30-mins averages after despiking, detrending, stationarity selection, and correction for high frequency losses. This method allows to have direct measurements of the traffic-related particle pollution strengths (EFs) evaluating the surface areas mainly influencing measurements. The size-resolved EFs are evaluated considering the particle exchange between surface and atmosphere (in terms of number and mass of particles) in both the sites, and compared with previously published results obtained in other urban areas. Road traffic represents a significant contribution to particle concentrations in number and in mass in urban areas. During the campaigns positive upward aerosol fluxes were dominant even though deposition cases were present for large particles and when the traffic rate was low. At the Lecce site, deposition cases during the weekend were about 5% of the measurement time for small particles (Dp < 0.25 µm) and about 10% for larger particles (Conte et al., 2018). At the Innsbruck site, the median emission flux value was of about 1.3*104 #/cm2 s and the median deposition value was about -0.7*103 #/cm2 s (Deventer et al., 2018). The results show that particles with Dp < 0.25 µm, mainly due to exhaust emissions, dominate the traffic emission factors with an average value of 2.2*1014 #/ Veh km for Lecce and of 2.5*1014 #/Veh km for Innsbruck. The comparison shows that the average values of EFs in number are in good agreement in both sites. The EF number size distribution decreases with particle diameter, very steeply for small particles and much slower for particles with Dp > 0.9 µm. Since the EFs are representative of the entire vehicle fleet, and due to the limited number of data points available for the footprint evaluation, the EFs should be considered as "estimates" and treated accordingly. Mixed vehicles fleet EFs evaluated in this work compare well with other works performed in different other urban areas under real operating conditions.