Transport mechanisms of coarse, fine, and very fine particulate matter in urban street canopies with different building layouts

A particulate matter (PM) transport model is developed to investigate coarse PM (PM₁₀), fine PM (PM_2.5), and very fine PM (PM₁) transport mechanisms in urban street canopies under low-wind conditions. Two common building layouts (i.e., the open and staggered street canopies) are considered. Large eddy simulations with the subgrid-scale stress model and the wall function are used to simulate urban streetcanopy flows. The Lagrangian particle tracking approach, considering the effects of the drag force, gravitational force, Brownian motion, and Saffman lift force on particles is adopted to study PM transport behaviors in urban street canopies. The box counting method is used to calculate the canopy-averaged PM₁₀/PM _2.5/PM₁ mass concentrations and transport mechanisms at each tracking time. The simulated results show that the removal efficiencies of PM₁₀, PM_2.5, and PM₁ in the open street canopies are all better than those in the staggered street canopies. As a result, the open street canopies having higher PM removal ability lead to a swifter shift of the particle size distributions towards smaller size and less deviation than the staggered street canopies. The major particle removal mechanism for the open street canopies is particle escape, whereas wall deposition plays the most important role for the staggered street canopies. In comparison with the effectiveness of PM₁₀/PM_2.5/PM ₁ removal for both building layouts, PM10 particles are easier to overcome the root mean square vertical turbulent velocity and need less time to deposit. Fine particles would follow airflow paths and need longer time to deposit. As a result, PM_2.5 and PM1 are more difficult to be removed than PM₁₀.