Dispersion modelling of traffic induced ultrafine particles in a street canyon in Antwerp, Belgium and comparison with observations Irina Nikolova a, b, ⁎, Stijn Janssen a , Peter Vos a, b , Karl Vrancken a, b , Vinit Mishra b , Patrick Berghmans a a Flemish Institute for Technological Research VITO, Boeretang 200, 2400 Mol, Belgium b University of Antwerp, Bio-Engineering Department, Groenenborgerlaan 171, Antwerp, Belgium abstract article info Article history: Received 20 July 2011 Received in revised form 28 September 2011 Accepted 28 September 2011 Available online 26 October 2011 Keywords: Ultrafine particles Modelling CFD Street canyon Simultaneous measurements P-TRAK The aim of this study is to investigate the dispersion of ultrafine particles and its spatial distribution in a street canyon and its neighbourhood with the 3D CFD model ENVI-met®. The performance of the model at street scale is evaluated and the importance of the boundary conditions like wind field and traffic emissions on the UFP concentration is demonstrated. To support and validate the modelled results, a short-term measure- ment campaign was conducted in a street canyon in Antwerp, Belgium. The UFP concentration was measured simultaneously with P-TRACK (TSI Model 8525) at four different locations in the canyon. The modelled UFP concentrations compare well with the measured data (correlation coefficient R from 0.44 to 0.93) within the standard deviation of the measurements. Despite the moderate traffic flow in the street canyon, UFP concen- trations in the canyon are in general double of the background concentrations, indicating the high local con- tribution for this particle number concentration. Some of the observed concentration profiles are not resembled by the model simulations. For these specific anomalies, further analysis is performed and plausible explanations are put forward. The role of wind direction and traffic emissions is investigated. The perfor- mance evaluation of ENVI-met® shows that in general the model qualitatively and quantitatively describes the dispersion of UFP in the street canyon study. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Traffic is recognised as the main contributor to the ultrafine particle (UFP) pollution in the ambient air of many cities (Kittelson et al., 2004; Gidhagen et al., 2005). Typical concentrations range between 10 4 and 10 6 particles/cm 3 depending on factors such as driving speed, composi- tion of the fleet and meteorological conditions (Morawska et al., 2008). It has been observed that ultrafine particles (diameter less than 0.1 μm) are associated with adverse effects on human health (Oberdorster and Utell, 2002; Donaldson et al., 2005; Politis et al., 2008; Crüts et al., 2008). Currently there is no limit value to control the number concentra- tion of UFP. On a European level, a euro standard for number concentra- tions “is to be defined as soon as possible and at the latest upon entry into force of Euro 6” (Regulation (EC) No 715/2007). The regulation stipulates that “In order to ensure that emissions of ultrafine particulate pollutants (PM0, 1 μm and below) are controlled (Regulation (EC) No 595/2009) the Commission should also establish specific proce- dures, tests and requirements for type approval, as well as a revised measurement procedure for particulates and a particle number based limit value, and to adopt measures concerning the use of defeat devices, access to vehicle repair and maintenance information and test cycles used to measure emissions”. This is a positive action initiated by the community and a step forward to introduce a standardised UFP emis- sions procedure. Such a step would reduce the uncertainties in the UFP emissions (Kumar et al., 2011). Another key step would be to in- clude UFP measurements in standardised monitoring stations. The main difficulties in comparing the results of various monitoring cam- paigns arise from the different operating principles and size cut-offs of the instruments (Kumar et al., 2010). Including standardised measure- ments of UFP in air quality monitoring networks would provide compa- rable datasets, more detailed analysis and last but not least the so necessary data for validation of dispersion models. Holmes and Morawska (2006) presented an overview of different dispersion models. Their conclusions state that many factors influence the concentration of UFP, notably fluctuations in the wind flow and emissions. Often, for example in cities, the areas show a complex ge- ometry leading to complex air flows and turbulence that can have a great impact on the concentration of the ultrafine particles. To deal with these aspects, the CFD (Computational Fluid Dynamics) methods become of interest. In recent years the use of CFD models in air quality modelling gained a lot of attention. These models are useful for stud- ies requiring high resolution within the urban canopy although they are complex, resource demanding and a thorough validation of a CFD model setup is not straightforward. CFD models allow to simulate the flow characteristics and the dispersion of pollutants at the urban Science of the Total Environment 412 (2011) 336–343 ⁎ Corresponding author at: University of Antwerp, Bio-Engineering Department, Groenenborgerlaan 171, Antwerp, Belgium. Tel.: + 32 14 33 67 59; fax: + 32 14 32 11 85. E-mail addresses: irina.nikolova@vito.be, irinanikolova@mail.bg (I. Nikolova). 0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2011.09.081 Contents lists available at SciVerse ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv