The Influence of Urban Morphology on PM2.5 Accumulation at the Pedestrian Level: A Case Study of Siam Square, Bangkok

Abstract

This study examines the impact of urban morphological factors on outdoor wind speed and the accumulation of air pollution at pedestrian level in high-density urban areas. The aim is to identify strategies for building design and urban planning that enhance ventilation and reduce the accumulation of pollutants. An experimental framework was employed to investigate the effects of various building layout configurations within the case study area, covering the Rama 1 Road Valley and Siam Square in the Pathum Wan District of Bangkok. The study is situated within the context of maximum urban growth. Outdoor wind velocity was simulated using the External CFD function of DesignBuilder version 6.1.0.6 and compared against a target wind speed required to mitigate the accumulation of fine particulate matter (PM2.5), defined as at least 2 m/s. Input data were based on ten-year average hourly wind speed records (2010–2019) and prevailing wind directions obtained from the Bangkok meteorological monitoring station. The reference wind speed was set at 1 m/s at a height of 10 meters above ground level (U_ref), with two primary wind directions—south and west—applied in the analysis. The study focused on three main variables: 1) the influence of Building shape, using three layouts—rectangular, oval, and triangular—at a BCR of 35-36%; 2) the influence of three BCR levels (35-36%, 50%, and 70%) with a rectangular building layout; and 3) the influence of Open space positioning, with five configurations using a rectangular layout and a BCR of 50%.

The findings reveal that 1) Building shape exerts a marginal influence on airflow in the Rama 1 Road Valley case study area. Among the tested configurations, the rectangular layout produced the highest proportion of areas achieving the target wind speed, followed by the oval and triangular layouts, respectively. 2) Building Coverage Ratio (BCR) exerts a clear influence, with the lowest BCR level (35–36%) resulting in the greatest proportion of target wind speed areas, whereas higher BCR levels (50–70%) were ineffective in reducing PM2.5 accumulation under the study’s initial wind conditions. 3) Open space positioning also exerts a marginal influence. Configurations in which open space was placed on one side of the building block facilitated better airflow than those with peripheral open space. However, alternating the location of open spaces did not contribute to reduced particulate accumulation. In addition, The average aspect ratio of the street canyon (H/W), defined as the ratio of building height to street width in the Siam Square case study area, demonstrates a positive relationship with outdoor wind speed. When buildings are significantly taller than the adjacent street width, resulting in a higher H/W ratio, airflow within the street canyon is enhanced, thereby effectively reducing the accumulation of PM2.5. In contrast, the average building length-to-width ratio (L/W) exhibits a negative relationship with airflow. As building length increases relative to width, leading to a higher L/W ratio, airflow tends to diminish, which may contribute to increased PM2.5 accumulation. These results underscore the importance of urban morphology in influencing airflow and PM2.5 accumulation at pedestrian level, particularly in cities like Bangkok, where low baseline wind speeds prevail year-round. Effective building design and urban space planning are crucial for improving urban environmental conditions.