A simplified benchmark of ultrafine particle dispersion in idealized urban street canyons: A wind tunnel study L. Stabile a, * , F. Arpino a , G. Buonanno a, b , A. Russi a , A. Frattolillo a a Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Cassino (FR), Italy b Queensland University of Technology, Brisbane, Australia article info Article history: Received 10 February 2015 Received in revised form 14 May 2015 Accepted 16 May 2015 Available online 10 June 2015 Keywords: Street canyon Ultrafine particles Particle image velocimetry (PIV) Wind tunnel Pollutant dispersion in urban areas CFD benchmark abstract The paper proposes a benchmark for computational fluid-dynamic models of ultrafine particle (particles' diameter lower than 100 nm) dispersion in urban street canyons. In particular, an on-scale symmetric street canyon was designed and settled in a wind tunnel considering a perpendicular wind condition. An experimental campaign was carried out to: i) investigate the fluid-dynamic conditions inside the canyon through a Particle Image Velocimetry (PIV), ii) evaluate the uncertainty budget of the PIV measurements, iii) design proper particle injection and sampling systems in the street canyon not influencing the fluid- dynamic conditions of the canyon itself, iv) provide particle number concentration profiles at different sections of the canyon (at different heights on both the canyon facades and at a background level) through ultrafine particle generation (Submicrometer Aerosol Generator 3940, TSI Inc.) and measuring devices (CPC 3775 and SMPS 3936, TSI Inc.). The study can be considered the very first attempt to set a benchmark for ultrafine particle dispersion in street canyons through wind tunnel systems. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The specialized cancer agency of the World Health Organization, the International Agency for Research on Cancer (IARC), recently classified outdoor air pollution as carcinogenic to humans [1]. In particular, for the first time, Particulate Matter (PM), a major component of outdoor air pollution, was “evaluated separately and classified as carcinogenic to humans” (Group 1, Beelen, Raaschou- Nielsen [2]) since an increasing risk of lung cancer with increasing levels of exposure to PM and air pollution was found whatsoever the PM chemical composition. Moreover, scientific evidences also allowed to recognize the PM as responsible of car- diovascular death due to long-term exposure [3,4]. Among the different particle size ranges, recent epidemiological and toxicological studies [5e9] identified the Ultrafine Particles (UFPs, particles smaller than 100 nm in diameter, as defined by the International Organization for Standardization [10]) as critical in terms of health effects due to their ability in crossing human res- piratory system, depositing in the deepest and most defenseless regions of the lungs [11], and carrying with them a number of toxic compounds [12]. In fact, studies aimed to relate the lung cancer risk to the combustion-generated ultrafine particles recognized a po- tential lung cancer risk related to the UFPs much higher than coarse particles' one [13,14]. 1.1. Air quality in urban areas Urban areas represent the outdoor microenvironments mainly contributing to the daily exposure to particles [15e17] due to proximity to the particle sources, i.e. vehicular traffic [18,19]. In fact, people may be significantly exposed as a function of the different urban/transportation microenvironments where they spend time (e.g. walking on sidewalk, using private or public vehicles, living and working in buildings flanking congested roads [20e27]). Currently, the human exposure to air pollution in urban areas is based on the evaluation of a threshold value determined at a Fixed Sampling Point (FSP) of the examined area and adopted for the entire population living nearby [28]. However, this is a questionable approach since outdoor exposure is influenced by several micro- environments within the urban area. Moreover, these standards are still referred to threshold limit values based on a time-integrated measurement of PM 10 (mass of particulate matter with * Corresponding author. Department of Civil and Mechanical Engineering, Uni- versity of Cassino and Southern Lazio, Via Di Biasio 43, 03043 Cassino (FR), Italy. E-mail address: l.stabile@unicas.it (L. Stabile). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv http://dx.doi.org/10.1016/j.buildenv.2015.05.045 0360-1323/© 2015 Elsevier Ltd. All rights reserved. Building and Environment 93 (2015) 186e198