A methodology to urban air quality assessment during large time periods of winter using computational fluid dynamic models Parra M.A. a , Santiago J.L. b, * , Martín F. b , Martilli A. b , Santamaría J.M. a a Laboratorio Integrado de Calidad Ambiental (LICA), Departamento de Química y Edafología, Facultad de Ciencias, Universidad de Navarra, Irunlarrea s/n, 31080 Pamplona, Navarra, Spain b Atmospheric Pollution Unit, Environmental Department, CIEMAT, Av. Complutense 22, 28040 Madrid, Spain article info Article history: Received 18 December 2009 Received in revised form 8 March 2010 Accepted 10 March 2010 Keywords: Ambient wind direction Computational fluid dynamics Experimental measurements Pamplona Urban flow and dispersion abstract The representativeness of point measurements in urban areas is limited due to the strong heterogeneity of the atmospheric flows in cities. To get information on air quality in the gaps between measurement points, and have a 3D field of pollutant concentration, Computational Fluid Dynamic (CFD) models can be used. However, unsteady simulations during time periods of the order of months, often required for regulatory purposes, are not possible for computational reasons. The main objective of this study is to develop a methodology to evaluate the air quality in a real urban area during large time periods by means of steady CFD simulations. One steady simulation for each inlet wind direction was performed and factors like the number of cars inside each street, the length of streets and the wind speed and direction were taken into account to compute the pollutant concentration. This approach is only valid in winter time when the pollutant concentrations are less affected by atmospheric chemistry. A model based on the steady-state Reynolds-Averaged NaviereStokes equations (RANS) and standard k-3 turbulence model was used to simulate a set of 16 different inlet wind directions over a real urban area (downtown Pamplona, Spain). The temporal series of NO x and PM 10 and the spatial differences in pollutant concentration of NO 2 and BTEX obtained were in agreement with experimental data. Inside urban canopy, an important influence of urban boundary layer dynamics on the pollutant concentration patterns was observed. Large concentration differences between different zones of the same square were found. This showed that concentration levels measured by an automatic monitoring station depend on its location in the street or square, and a modelling methodology like this is useful to complement the experimental information. On the other hand, this methodology can also be applied to evaluate abatement strategies by redistributing traffic emissions. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Citizens are exposed to atmospheric pollutants that have a severe impact on health (WHO, 2000). To mitigate this effect, the European Directive on ambient air quality (2008/50/CE) obliges the European Union Countries to assess air quality and establish plans to improve it where the standards are exceeded. Modelling is an important tool both in complementing measurements to assess air quality as well as in evaluating air pollution abatement plans. Motor vehicles are the main pollutant sources in urban areas. Toxic contaminants such as Nitrogen Oxides (NO x ), Volatile Organic Compounds (VOC) and Particulate Matter (PM) are emitted by traffic, and they contribute to the formation of secondary pollutants like ozone through photochemical reactions. The dispersion of those hazardous materials in urban areas is deter- mined by the interactions between meteorological conditions (wind speed and direction, atmospheric stability), and building and street configurations. Since these interactions are very complex, research to gain insight into the processes and features of flow and pollutant dispersion in cities is necessary. To date, a significant amount of investigation has been carried out in two complemen- tary directions:  Experimentally in wind tunnel (Meroney et al., 1996; Kastner- Klein and Plate, 1999) and at field scale (Dobre et al., 2005; Klein et al., 2007) over different urban configurations.  With Computational Fluid Dynamics (CFD) models over different idealized geometries (Sini et al., 1996; Assimakopoulos et al., 2003; Kim and Baik, 2004; Santiago et al., 2007). * Corresponding author. E-mail address: jl.santiago@ciemat.es (J.L. Santiago). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv ARTICLE IN PRESS 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.03.009 Atmospheric Environment xxx (2010) 1e9 Please cite this article in press as: Parra, M.A., et al., A methodology to urban air quality assessment during large time periods of winter using..., Atmospheric Environment (2010), doi:10.1016/j.atmosenv.2010.03.009