Mathematical models for traffic-source PM_2.5 dispersion in an urban street canyon considering the capture capability...

Rapid urbanization increases the densely built-up blocks, the population and vehicles. Large amounts of particulate matter (PM), especially PM_2.5 (PM with an aerodynamic diameter of 2.5μm or less), from vehicle exhaust are critical to human health. In typical street canyons in hot and humid regions, traffic-source PM usually diffuses to the densely built-up blocks through roadside trees. Roadside trees are a double-edged sword, serving as "guards" to absorb PM_2.5 while may lead to PM_2.5 gathering in street levels, thereby influencing the PM_2.5 dispersion in the densely built-up blocks. To quantify the dispersion process, this study proposed traffic-source PM_2.5 dynamic dispersion models considering the capture capability of roadside trees and built-up blocks based on the OSPM model. Due to the difficulty in obtaining the adsorption and deposition rate of the proposed models, the numerical simulations by ENVI-met software were used to solve and obtain the relationship between capture capability and characteristic index of roadside trees. Subsequently, The accuracy and effectiveness of the proposed traffic-source PM_2.5 dynamic dispersion models were verified through field experimental data. Results show that the calculated PM_2.5 concentration significantly linearly increased with the measured values with the determined coefficient (R2) of 0.98, and the first-order coefficient close to 1. It indicates that the proposed traffic-source PM_2.5 dispersion model accurately quantified the impact of roadside trees on PM_2.5 and its concentration dispersion process to the built-up blocks. This study provides suggestions for designing characteristic indexes of roadside trees and built-up blocks to improve the air quality of urban street canyons.