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 (L2), middle (L3) and back (L4) of the vegetation barrier; L1 was in parallel to L2 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 (L2, L3, L4) 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 ∼11% higher PNCs (1.99 ± 1.77 × 105 cm−3) at L2 than those at L1 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 L3 and L4, respectively, compared with L2. During cross-footpath winds, particles were carried away by the wind from the sampling location. Significant decrease in PNCs were consequently seen at L3 (1.80 ± 1.01 × 104 cm−3) and L4 (1.49 ± 0.91 × 104 cm−3) compared with L2 (6.26 ± 3.31 × 104 cm−3). The PNCs at these locations showed modest differences during the cross-footpath and along-road winds. Respiratory deposited doses (RDD) at L4 were found to be the lowest during all wind directions compared with the L1-L3. The vegetation barrier efficiently reduced the RDD by ∼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.