Vertical distribution characteristics of particulate matter beside an elevated expressway by unmanned aerial vehicle measurements

Increasing knowledge in the law of particulate matter (PM) distributions beside elevated expressways could assist in addressing the deterioration of urban air quality. However, the vertical distribution patterns of PM concentration have not been clearly recognized due to limited measurement methods. Hence, unmanned aerial vehicle (UAV) measurements are carried out at three typical sites near an elevated expressway to investigate the effects of the noise barrier, vegetation, and building on PM vertical distribution patterns. Results show that the order of mean PM concentration in three trial sites from low to high is: 86.3 μg/m³ at VN (with vegetation and noise barrier), 90.2 μg/m³ at CT (control, without vegetation, noise barrier, and building), and 93.2 μg/m³ at VNB (with vegetation, noise barrier, and building) site. Additionally, the PM₁ concentrations present a unimodal distribution, peaking at 5–7 m and 7–9 m at CT and VN sites, respectively. PM₁₀ concentrations decrease continuously with height at CT site and show a bimodal distribution at VN site, with the maximum and the upper peak occurring at 0–3 m and 5–7 m, respectively. High-density vegetation is more conducive to remove PM downwind than low-density vegetation in the open road, while the opposite is true in the street canyon. Moreover, statistically significant positive correlations are obtained between roadside concentration and traffic volume, as well as between roadside concentration and background concentration, with Spearman correlation coefficients of 0.61 and 0.93, respectively. This study could provide insights in mitigating traffic-related PM pollution into urban areas.