Spatio-temporal variations of ozone and nitrogen dioxide concentrations under urban trees and in a nearby open area Federica Fantozzi a,⇑ , Fabrizio Monaci a , Tijana Blanusa b,c , Roberto Bargagli a a Department of Environment, Earth and Physical Sciences, University of Siena, Via P.A. Mattioli, 4, Siena 53100, Italy b Plant Sciences Department, Royal Horticultural Society, Wisley, Woking GU23 6QB, UK c School of Agriculture, Policy and Development, University of Reading, Reading RG6 6AR, UK article info Article history: Received 21 August 2014 Revised 5 January 2015 Accepted 9 February 2015 Keywords: Urban environment Atmospheric pollutants Quercus ilex NO 2 O 3 Uptake abstract The evergreen Quercus ilex L. is one of the most common trees in Italian urban environments and is considered effective in the uptake of particulate and gaseous atmospheric pollutants. Howev- er, the few available estimates on O 3 and NO 2 removal by urban Q. ilex originate from model-based studies (which indicate NO 2 /O 3 removal capacity of Q. ilex) and not from direct measurements of air pollutant concentrations. Thus, in the urban area of Siena (cen- tral Italy) we began long-term monitoring of O 3 /NO 2 concentrations using passive samplers at a distance of 1, 5, 10 m from a busy road, under the canopies of Q. ilex and in a nearby open-field. Measure- ments performed in the period June 2011–October 2013 showed always a greater decrease of NO 2 concentrations under the Q. ilex canopy than in the open-field transect. Conversely, a decrease of average O 3 concentrations under the tree canopy was found only in autumn after the typical Mediterranean post-summer rainfalls. Our results indicate that interactions between O 3 /NO 2 concentra- tions and trees in Mediterranean urban ecosystems are affected by temporal variations in climatic conditions. We argue therefore that the direct measurement of atmospheric pollutant concentra- tions should be chosen to describe local changes of aerial pollution. Ó 2015 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.uclim.2015.02.001 2212-0955/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +39 0577 232828; fax: +39 0577 232930. E-mail address: fantozzi3@unisi.it (F. Fantozzi). Urban Climate 12 (2015) 119–127 Contents lists available at ScienceDirect Urban Climate journal homepage: www.elsevier.com/locate/uclim