The simultaneous effects of image force and diffusion on ultrafine particle deposition onto vegetation: A wind tunnel study

Atmospheric ultrafine particles (UFP) and their associated sources and sinks continue to attract significant research attention in climate and air pollution science. Vegetation is an important sink for UFP given its large area coverage. What remains a subject of inquiry are the aerodynamic and electromagnetic processes governing the aforementioned vegetation sink. Single fiber theory can explain deposition of zero charged UFP onto vegetation by treating vegetation as filter media. However, the ability of single fiber theory to predict deposition of charged UFP onto vegetation remains to be explored and frames the scope here. Wind tunnel experiments were used to investigate UFP dry deposition onto Juniper branches (Juniperus chinesis) under three different wind speeds (0.3, 0.6, and 0.9 m/s). Results indicate that the single fiber theory can describe the deposition of singly charged particles onto vegetation if both the image force and Brownian diffusion are simultaneously considered. The image force can be expressed by 24.5√K _IM when the image force dimensionless number (K _IM ) is smaller than 10 ⁻⁸ . It is shown that the single fiber filtration model and the wind tunnel measurements mostly agree to within 20%. The main finding here is that the image force at smaller K _IM (10 ⁻¹⁰ ∼ 10 ⁻⁸ ) increases deposition of charged UFP onto vegetation. Because filtration theory can be readily accommodated in fluid transport formulations as a boundary condition, the findings regarding charge enhance UFP deposition schemes that can be used in air quality and climate models.