Modelling the size-dependent collection efficiency of hedgerows for ambient aerosols

A model for estimating the size-segregated particle collection efficiencies of three hedgerow species of different aerodynamic porosities has been developed and tested in the particle size range 0.5–20μm. Collection efficiency (CE) has been described in terms of the coupled effects of the deviation of the approach flow (CEflow)(CEflow) and the filtration through the foliage elements (CEfiltration)(CEfiltration). Variations in barrier porosities with wind speed, which govern both the flow and the particle collection mechanism, have been modelled using an empirical parameter called the “rigidity factor” (r)   for representative leaf-sizes of the three hedge types. Computational fluid dynamics (CFD) allows simulation of velocity fields and turbulence around the barriers, leading to identification of prominent flow separation zones in the wake of denser hedges. Application of different turbulence closure schemes in the CFD model influences the predicted CE only for particles above (5μm). Field validations obtained for a porous (hawthorn) and a denser (yew) hedge indicate that CEfiltrationCEfiltration ranges between 10 and 35% for “coarse” particles (10–20μm) and 1–5% for “fine” particles (0.5–3.5μm), the porous hedge showing higher values in the former case and vice versa in the latter. Model predictions of CE over the particle size range studied for the porous hedge lie within 1–30% and for the denser hedge within 0.4–3%.