Green Walls in Hot Arid Street Canyons: Thermal Benefits vs. Particulate Limitations

Urban sprawl and traffic are intensifying air pollution and heat stress, harming human health, thermal comfort, and urban living environments. Vegetation, specifically green walls, offers a compelling strategy to reduce pollutant concentrations and pollution-related hospitalization, in addition to enhancing the microclimate—specifically, human thermal comfort. Its impact is strongly associated with well-planned strategic distribution. However, this pollutant-reduction benefit is largely theorized from studies conducted under different traffic, wind, and coverage conditions, and its applicability to dense, high-traffic corridors in hot arid climates has not been tested. The scientific novelty of this study is the first coupled design assessment of thermal comfort and PM benefits of vertical greening under hot arid conditions in retrofitting contexts—focus on structure-mounted elements. This configuration and climate combination solely is limited and present in temperature and humid climates in the existing literature. In this study, green walls are integrated into a vibrant street with limited urban spaces to examine their effects on particulate matter (PM) and thermal comfort. The current street status, 100% vertical green, and 200% vertical green area coverage on structure-mounted columns scenarios were simulated and compared using ENVI-met 5.9.0. The study is conducted in an urban street in Cairo, Egypt. The street faces severe air pollution and high thermal discomfort. The results showed that vertical greening increased user thermal comfort while having a low effect on PM dispersal and reduction under the specific hot-arid, low-wind, and single-day design of this study. At the maximum 200% coverage, air temperature dropped by up to 0.93 °C and PET by up to 1.71 °C; 100% coverage alone was not sufficient to produce a significant improvement, suggesting a possible coverage response threshold that intermediate scenarios were not tested to confirm. The plant with the higher leaf area index (LAI = 3) produced the greatest thermal benefit, though LAI had no measurable effect on PM reduction. This study helps policymakers and urban planners design retrofitting solutions to improve air quality and thermal comfort by emphasizing the need to examine both variables simultaneously. It should also be noted that the effect is influenced by multiple factors—street characteristics, microclimate conditions, local activities, plant characteristics, and the available space—and that the findings are specific to the single hot arid street canyon and summer design day simulated in this study.