Wind Tunnel Experiment of Particulate Matter Resuspension by Different Surface Types and Environmental Factors

Abstract:

Urban forests play a role in reducing air pollution and their role in mitigating particulate matters (PM) has been recognized (Yang et al., 2005; Nguyen et al., 2015). The mechanisms of PM mitigation by urban forests are cut-off, adsorption, sedimentation, and absorption. However, except the absorption process by leaf stomata, they are not the complete reduction mechanisms. The resuspension of PM would occur by different ground surfaces, the change in micrometeorology, etc., resulting in re-increase in PM concentrations of urban atmosphere.

To understand main factors influencing resuspension, we conducted a simulation experiment using a wind tunnel (1.5X0.5X0.5m), which was blocked back and forth by HEPA filters. To simulate the effect of different ground surface on resuspension, we prepared three types of ground surface plates (100X300X50mm): concrete, bare soil, and grass. The 1 ml of polystyrene particles (Spherotech, US) with the size from 0.7um to 5.0um were evenly distributed on the top of each plate. The resuspension rate of PM particles was calculated as the ratio of the amounts of suspended and initial particles. The spatial distribution of resuspension was identified at three horizontal points with two heights. The amount of suspended particles was measured by the light scattering detector (Grimm, EDM 164, Germany). We analyzed the effects of wind speed, temperature, and relative humidity on resuspension rate. The wind speed was regulated by the pump connected to the outlet of the tunnel from 1m/s to 9m/s. The temperature and relative humidity inside the wind tunnel were adjusted into 18℃ and 25℃, 40% and 60% respectively.

Overall, the resuspension rate of PM varied with plate types and wind speed. The resuspension rate for concrete and bare soil plates showed relatively similar pattern compared to that for grass plate. The threshold, minimum wind speed inducing the resuspension, was highest for the grass plate. Due to the hydrophobic characteristic of polystyrene particles, the effect of relative humidity and temperature in this study was insignificant. Our results imply that the management of surface could influence air quality by changing resuspension of PM in urban environment. Further, we emphasized the importance of understory vegetation in urban greenery.