Designing water spray systems to mitigate particulate matter pollution in an urban area

Water spray systems can mitigate particulate matter (PM) pollution by spraying water droplets to capture PM in the air. This study evaluates the PM removal using water spray systems in an urban site in Tsuen Wan, Hong Kong, through computational fluid dynamics (CFD) simulations. A droplet capturing model considering the relative motion of droplet and PM was integrated into the CFD simulations to model the mechanism of the droplet in removing PM in a realistic urban area. CFD simulations evaluated three design parameters: installation location, spray pressure, and water flow rate. The results show that installing the water spray system at the road center is the most efficient and quickest way to remove PM in the urban site. However, increasing spray pressure has limited improvements in PM removal efficiency because the background wind speeds deter the droplet movements. Instead, increasing the water flow rate significantly increases the droplet concentration in the air, thereby reducing personal intake of PM by pedestrians up to 16.14%. In addition, this study revealed that CFD simulations that omit the droplets’ capturing effect can result in 7.88%–15.44% underestimation of PM removal by water spray systems. Finally, this study recommends designing water spray systems with high flow rates in road centers and evaluating their performance in removing PM by using CFD simulations with the droplets’ capturing effect to mitigate PM pollution in urban areas.