Influence of depressed road configuration on downwind pollutant concentrations: A CFD study under various thermal stability conditions

Abstract

Outdoor air quality is a major concern worldwide, especially in urban areas. In this paper, the influence of depressed roads (specific road designs where the road's surface is lower than the surrounding ground level) on the downwind pollutant concentration was assessed using a validated Computational Fluid Dynamics (CFD) solver, for eight road depths (D) and thirteen stability conditions (Richardson numbers, Ri). Depressed roads can decrease downwind pollutant concentrations compared to classical roads, but only under neutral and unstable thermal conditions. Under neutral thermal condition, a threshold is reached for D = 0.375, leading to a maximal pollution reduction of around 10% at pedestrian level and around 5% at the first-floor level. In such stability conditions, pollutant concentrations are lower as D increases and Ri decreases. Under stable conditions, such roads lead to higher pollutant concentrations. Four equations allowing to predict the downwind pollutant concentrations are given depending on the distance from the road centerline, the road depth, and the thermal stability condition. The results of this study provide pre-construction guidance on whether a depressed road should be considered to protect human health, as well as predictive tools to assess the beneficial or adverse impact of such structures on air quality.