The influence of roadside vegetation barriers on airborne nanoparticles and pedestrians exposure under varying wind conditions

Roadside vegetation barriers are used in many urban areas to restrict air and noise pollution from reaching roadside pedestrians, but their effectiveness in limiting the movement of nanoparticles is not yet known. This study investigates the influence of a roadside vegetation barrier on particle number distribution (PND) and concentration (PNC) and associated exposure under different wind directions. Size-resolved particles in the 5-560 nm size range were measured along a busy roadside in Guildford (Surrey, UK) using a fast response differential mobility spectrometer (DMS50). A custom-built solenoid switching system, together with the DMS50, was used to make sequential measurements at the front (L-2), middle (L-3) and back (L-4) of the vegetation barrier; L-1 was in parallel to L-2 at a vegetation-free location. Measured data were divided into the three predominant wind directions: cross-road (NW-SW), cross-footpath (NE-SE) and along-road (NW-NE). The consistency in the shape of PNDs and the corresponding geometric mean diameters at the three sites (L-2, L-3, L-4) indicate an identical removal effect of vegetation barrier for all sizes of particles. Comparison of the PNCs at two parallel locations (with and without the vegetation barrier) showed similar to 11% higher PNCs (1.99 +/- 1.77 x 10(5) cm(-3)) at L-2 than those at L-1 during cross-road winds, showing the impeding effect of the vegetation barrier. Such differences were insignificant during the remaining wind directions. Cross-road winds indicate the effect of vegetation barrier; the PNCs were decreased by 14 and 37% at L-3 and L-4, respectively, compared with L-2. During cross-footpath winds, particles were carried away by the wind from the sampling location. Significant decrease in PNCs were consequently seen at L-3 (1.80 +/- 1.01 x 10(4) cm(-3)) and L-4 (1.49 +/- 0.91 x 10(4) cm(-3)) compared with L-2 (6.26 +/- 3.31 x 10(4) cm(-3)). The PNCs at these locations showed modest differences during the cross-footpath and along-road winds. Respiratory deposited doses (RDD) at 1.4 were found to be the lowest during all wind directions compared with the 1.1 L-3. The vegetation barrier efficiently reduced the RDD by similar to 36% during cross-road winds. Our results show the mitigation potential of vegetation barriers in limiting near-road nanoparticles exposure and the measured data can facilitate performance evaluation of theoretical models. (C) 2014 Elsevier Ltd. All rights reserved.