A field study of factors influencing the concentrations of a traffic-related pollutant in the vicinity of a complex urban junction A.S. Tomlin a, * , R.J. Smalley a,1 , J.E. Tate b , J.F. Barlow c , S.E. Belcher c , S.J. Arnold c, 2 , A. Dobre c , A. Robins d a Energy and Resources Research Institute (ERRI), University of Leeds, Leeds LS2 9JT, UK b Institute for Transport Studies, University of Leeds, Leeds LS2 9JT, UK c Department of Meteorology, University of Reading, Reading, RG6 6BB, UK d Faculty of Engineering and Physical Sciences, University of Surrey, UK article info Article history: Received 18 February 2009 Received in revised form 28 June 2009 Accepted 29 June 2009 Keywords: Dispersion Intersection Street canyon Traffic pollution Turbulence DAPPLE field campaign abstract The paper describes a field study focused on the dispersion of a traffic-related pollutant within an area close to a busy intersection between two street canyons in Central London. Simultaneous measurements of airflow, traffic flow and carbon monoxide concentrations ([CO]) are used to explore the causes of spatial variability in [CO] over a full range of background wind directions. Depending on the roof-top wind direction, evidence of both flow channelling and recirculation regimes were identified from data collected within the main canyon and the intersection. However, at the intersection, the merging of channelled flows from the canyons increased the flow complexity and turbulence intensity. These features, coupled with the close proximity of nearby queuing traffic in several directions, led to the highest overall time-average measured [CO] occurring at the intersection. Within the main street canyon, the data supported the presence of a helical flow regime for oblique roof-top flows, leading to increased [CO] on the canyon leeward side. Predominant wind directions led to some locations having significantly higher diurnal average [CO] due to being mostly on the canyon leeward side during the study period. For all locations, small changes in the background wind direction could cause large changes in the in-street mean wind angle and local turbulence intensity, implying that dispersion mechanisms would be highly sensitive to small changes in above roof flows. During peak traffic flow periods, concentrations within parallel side streets were approximately four times lower than within the main canyon and intersection which has implications for controlling personal exposure. Overall, the results illustrate that pollutant concentrations can be highly spatially variable over even short distances within complex urban geom- etries, and that synoptic wind patterns, traffic queue location and building topologies all play a role in determining where pollutant hot spots occur. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Urban areas are susceptible to elevated concentrations of pollutants due to the high density of traffic and the dense arrangement of buildings that modifies the airflow and can inhibit the ventilation of pollutants emitted at street level. The accurate prediction of concentrations, necessary for air quality management, requires knowledge of the modified wind flow between buildings which affects dispersion and entrainment mechanisms, as well as the magnitude and location of traffic emissions. Many previous studies of dispersion within urban areas have concentrated on the idealised case of the 2D street canyon (Sini et al., 1996; Pavageau and Schatzmann, 1999; Louka et al., 2000). The skimming flow regime that may occur during near perpendicular winds for canyons with height to width ratios between 0.65 and 1 (Oke, 1987; Leonardi et al., 2003) has received much attention, since pollutants released by vehicles at street level are then transported by a single across-canyon vortex leading to elevated concentrations on the canyon leeward side. The presence of such a cross canyon vortex may also affect the extent of vertical mixing as well as turbulence levels and thus the mean and turbulent flux of pollutants out of the canyon (Baik and Kim, 2002). The dependence of vortex structure on canyon aspect ratio was studied using a Reynolds Averaged Navier Stokes (RANS) model by Baik and Kim (1999). As pointed out in Ahmad et al. (2005), less attention has been paid to airflow patterns established under oblique background * Corresponding author. Tel.: þ44 113 3432500; fax: þ44 113 2467310. E-mail address: a.s.tomlin@leeds.ac.uk (A.S. Tomlin). 1 Present address: Bureau of Meteorology, GPO Box, 1289, Melbourne, Victoria 3001, Australia. 2 Present address: Golder Associates (UK) Ltd, Nottinghamshire, NG12 5BL, UK. Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.06.047 Atmospheric Environment 43 (2009) 5027–5037