Spatial distribution assessment of particulate matter in an urban street canyon using biomagnetic leaf monitoring of tree crown deposited particles Jelle Hofman * , Ines Stokkaer, Lies Snauwaert, Roeland Samson PeSTO Research Group, Department of Bio-science Engineering, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerpen, Belgium article info Article history: Received 6 July 2012 Received in revised form 8 September 2012 Accepted 12 September 2012 Keywords: Biomonitoring Urban air pollution SIRM PM London plane (Platanus  acerifolia Willd.) abstract Recently, biomagnetic monitoring of tree leaves has proven to be a good estimator for ambient partic- ulate concentration. This paper investigates the usefulness of biomagnetic leaf monitoring of crown deposited particles to assess the spatial PM distribution inside individual tree crowns and an urban street canyon in Ghent (Belgium). Results demonstrate that biomagnetic monitoring can be used to assess spatial PM variations, even within single tree crowns. SIRM values decrease exponentially with height and azimuthal effects are obtained for wind exposed sides of the street canyon. Edge and canyon trees seem to be exposed differently. As far as we know, this study is the first to present biomagnetic moni- toring results of different trees within a single street canyon. The results not only give valuable insights into the spatial distribution of particulate matter inside tree crowns and a street canyon, but also offer a great potential as validation tool for air quality modelling. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction To date, a continuously increasing urbanization has resulted in health-threatening concentrations of air pollutants, especially in urban areas. Urban outdoor air pollution is estimated to cause 1.3 million deaths worldwide per year (WHO, 2011). Particulate matter (PM) is of particular interest because it affects more people than any other pollutant (WHO, 2011). In three Belgian cities (Antwerp, Brussels and Liège), evidence suggested that 5.5% of the mortality is attributable to PM 10 concentrations higher than the reference level (20 mg/m 3 )(Remy et al., 2011). PM consists of a complex mixture of both liquid and solid particles of organic and inorganic substances suspended in the air, and is divided into different fractions based on its aerodynamic diameter. The coarse fraction represents particles smaller than 10 mm and is health related because of its ability to enter the respiratory tract (Langner, 2007). Within the PM 10 frac- tion, fine particles represent particles smaller than 2.5 mm and ultrafine particles are smaller than 0.1 mm. While the coarse frac- tion largely consists of particles that originate from natural sources, the fine and ultrafine particles originate from anthropogenic sources. The smaller the particles, the further they are transported in our pulmonary alveoli, which will result in more serious health impacts (Donaldson et al., 2001). Health impacts of chronic PM exposure involve cardiovascular and respiratory diseases, as well as lung cancer (Donaldson et al., 2001; Remy et al., 2011). Because of the adverse health effects, the mitigation of partic- ulate air pollution is mainly aimed at source measures, where emission reductions, limitations and targets are pursued to constrain atmospheric concentration levels (e.g. WHO Air Quality Guidelines (WHO, 2006)). Although source regulations are indis- pensable in the mitigation of air pollution, growing interest has heightened the need for exposure measures that influence atmo- spheric pollutant concentrations by stimulating deposition and/or dispersion processes. In this context, former research concentrated on the potential mitigation effect of vegetation (Beckett et al., 1998; Yang et al., 2005; Litschke and Kuttler, 2008). In addition to already known ecosystem services like carbon sequestration, micro-climate regulation, noise reduction, rainwater drainage, psychological and recreational values (Ulrich, 1984; Bolund and Hunhammar, 1999; Chen and Jim, 2008; Jim and Chen, 2009; Li et al., 2010), the miti- gation of air pollution can serve as an important additional ecosystem service of urban green. Because of its high leaf area relative to the ground area it covers, vegetation (especially trees) can influence local atmospheric PM concentrations through both direct and indirect effects. While vegetation will lower ambient particle concentration by stimulating deposition on its surface (direct effect), vegetation will also affect wind speed and direction and therefore the dispersion of PM polluted air (indirect effect) (Langner, 2007). Research indicated that this indirect effect is able * Corresponding author. E-mail addresses: Jelle.Hofman@ua.ac.be (J. Hofman), Roeland.Samson@ua.ac.be (R. Samson). Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2012.09.015 Environmental Pollution 183 (2013) 123e132