Green Roofs Mitigate Particulate Matter Under Stable Nocturnal Boundary Layers in Dense Urban Canyons

Abstract

The mitigation of particulate matter in dense urban environments remains a critical challenge for modern environmental science and urban planning. While the aerodynamic dispersion and dry deposition of aerosols during daytime convective conditions have been extensively documented, the behavior of fine and coarse particulate matter during calm, stable nocturnal periods in urban canyons is less understood. Under these specific conditions, strong temperature inversions and reduced wind speeds severely limit vertical mixing, leading to the accumulation of hazardous pollutants at the street level and roof level. This comprehensive study investigates the mechanisms through which extensive green roofs facilitate the removal of particulate matter during calm nights in urban canyons. By utilizing a high-resolution sensor network deployed across paired building roofs one featuring a mature sedum-based green roof and the other a conventional bare bituminous surface we analyze the localized concentration gradients and deposition velocities of suspended particles. The findings indicate that while aerodynamic impaction is minimized due to low wind speeds, mechanisms such as gravitational settling, Brownian diffusion, and hygroscopic growth under high nocturnal humidity significantly enhance particulate deposition onto the micro-structured biological surfaces of the vegetation. The green roof demonstrated a substantially higher retention capacity for both fine and coarse fractions compared to the control surface, thereby acting as a localized sink during periods of peak atmospheric stability. These results underscore the importance of integrating green infrastructure into urban design not merely for thermal regulation, but as a passive, continuous filtration mechanism for improving nocturnal air quality in densely built environments.