Investigation of Ultrafine Particle Deposition to Vegetation Branches in a Wind Tunnel

Vegetation is an important sink for atmospheric ultrafine particles (UFP). Prediction of UFP deposition to vegetation, however, is still problematic. In this study, we have investigated size-dependent removal of UFP by two conifer species: pine and juniper. The experiments were performed by placing freshly cut branches into a wind tunnel and measuring UFP size distributions upstream and downstream of the branches. Five air velocities ranging from 0.3 to 1.5 m/s, two packing densities (volume fraction occupied by the branches) and two branch orientations were tested to assess their effect on particle removal. The UFP removal efficiency was found to decrease with increasing particle size, increasing wind speed, and decreasing packing density. The branch orientation did not affect the removal efficiency within the uncertainty of the measurements. To facilitate extrapolation of the measurement results to real-life conditions, we have tested the applicability of filtration theory to particle removal by vegetation branches. The filtration theory predictions agreed well with the experimental data. The representative effective fiber diameter derived from the experimental data was found to be 0.054 (+/- 0.001) cm and 0.065 (+/- 0.001) cm for pine and juniper, respectively. For pine, the derived effective diameters were very close to the physical diameter of pine needles. In accordance with the filtration theory, the removal efficiency was found to be correlated with the pressure drop across the vegetation branches. This finding suggests that the UFP removal can be linked to aerodynamic properties of vegetative barriers, such as their drag coefficient. This could potentially facilitate modeling of UFP removal and dispersion by vegetative barriers using fluid dynamics models. The applicability