Time-series analysis to study the impact of an intersection on dispersion along a street canyon†‡ Jennifer Richmond-Bryant, * a Alfred D. Eisner, b Intaek Hahn, c Christopher R. Fortune, b Zora E. Drake-Richman, b Laurie A. Brixey, b M. Talih, d Russell W. Wiener e and William D. Ellenson b Received 7th April 2009, Accepted 28th September 2009 First published as an Advance Article on the web 6th November 2009 DOI: 10.1039/b907134m This paper presents data analysis from the Brooklyn Traffic Real-Time Ambient Pollutant Penetration and Environmental Dispersion (B-TRAPPED) study to assess the transport of ultrafine particulate matter (PM) across urban intersections. Experiments were performed in a street canyon perpendicular to a highway in Brooklyn, NY, USA. Real-time ultrafine PM samplers were positioned on either side of an intersection at multiple locations along a street to collect time-series number concentration data. Meteorology equipment was positioned within the street canyon and at an upstream background site to measure wind speed and direction. Time-series analysis was performed on the PM data to compute a transport velocity along the direction of the street for the cases where background winds were parallel and perpendicular to the street. The data were analyzed for sampler pairs located (1) on opposite sides of the intersection and (2) on the same block. The time-series analysis demonstrated along-street transport, including across the intersection when background winds were parallel to the street canyon and there was minimal transport and no communication across the intersection when background winds were perpendicular to the street canyon. Low but significant values of the cross-correlation function (CCF) underscore the turbulent nature of plume transport along the street canyon. The low correlations suggest that flow switching around corners or traffic-induced turbulence at the intersection may have aided dilution of the PM plume from the highway. This observation supports similar findings in the literature. Furthermore, the time-series analysis methodology applied in this study is introduced as a technique for studying spatiotemporal variation in the urban microscale environment. Introduction The work presented here is part of the Brooklyn Traffic Real- time Ambient Pollutant Penetration and Environmental Dispersion (B-TRAPPED) study performed in Brooklyn, NY, USA. 1,2 In the B-TRAPPED study, time-resolved particulate matter (PM) measurements were obtained to learn about street canyon transport, infiltration, and indoor air transport in a complex urban environment. The portion of the study pre- sented here focuses on ultrafine PM transport across an inter- section. Although the majority of urban microscale dispersion studies have focused on street canyons assumed to be infinitely long, 3–8 there is a growing body of literature exploring transport across street intersections (e.g., Kaur et al. 9 and Gokhale and Khare 10 ). This information can aid in understanding the physical processes dictating flow across these features and in planning for field studies and interpreting street canyon data from the field in an urban environment with inhomogeneous building structures. More importantly, studies of microscale transport and dispersion in the urban environment can enhance understanding of human exposure to traffic-related pollutants. The majority of data examining flow across intersections has been obtained in wind tunnel or numerical simulations. Kastner- Klein and Rotach performed wind tunnel studies of a detailed urban environment in Nantes, France. 11 This study showed significant differences between air velocities within canyons and at intersections. When the predominant wind direction was perpen- dicular to the street canyon, mean along-street wind was near zero. Mean and turbulent velocities at the intersection were significantly higher. In both cases, maxima in turbulent kinetic energy and shear stress were located just above the buildings’ rooftops. Likewise, in wind tunnel and large eddy simulations of airflow around an array of buildings with staggered position and varying height, Xie et al. found that turbulence kinetic energy was at a maximum at the top and sides of the buildings and that the motion around neighboring buildings caused a downward motion of the airflow between buildings. 12 Ahmad et al. executed wind tunnel studies of simu- lated automobile traffic in street canyons and intersections. 13 Their a National Center for Environmental Assessment, US Environmental Protection Agency, 109 T. W. Alexander Drive, MC B243-01, Research Triangle Park, NC, 27711, USA b Alion Science and Technology, PO Box 12313, Research Triangle Park, NC, 27709, USA c National Center for Environmental Research, US Environmental Protection Agency, Washington, DC, 20460, USA d Urban Public Health Program, Hunter College, City University of New York, 425 East 25th Street, New York, NY, 10010, USA e National Homeland Security Research Center, US Environmental Protection Agency, Research Triangle Park, NC, 27711, USA † Part of a themed issue on a real-time study of airborne particulate dispersion in urban canyons. ‡ Electronic supplementary information (ESI) available: Supplemental Table A1. See DOI: 10.1039/b907134m This journal is ª The Royal Society of Chemistry 2009 J. Environ. Monit., 2009, 11, 2153–2162 | 2153 PAPER www.rsc.org/jem | Journal of Environmental Monitoring