Citywide quantification of street-canyon contributions to near-road NO₂ in Hong Kong using a coupled WRF-CMAQ-ADMS...

Street canyons in high-density urban areas impede pollutant dispersion and intensify human exposure to traffic-related air pollution. Quantifying these effects at the urban scale remains difficult in high-rise coastal megacities such as Hong Kong because of challenges in multiscale coupling, street-level emissions, urban morphology data, and representation of canyon flow and chemistry. We developed a high-resolution WRF-CMAQ-ADMS modeling framework to quantify street-canyon contributions to near-road NO₂ concentrations across Hong Kong. A street-scale traffic emission inventory was constructed, and paired simulations with and without the street-canyon module were performed to isolate the canyon-induced increment (ΔC). Hotspots were defined as road segments with ΔC > 5 μg m^-3, approximately corresponding to a 1% increment in long-term NO₂-attributable added-risk equivalent. Four hotspot districts (Yau Tsim Mong, Kowloon City, Central and Western, and Wan Chai) were identified for detailed analysis, with hotspot road segments accounting for 12.2-18.8% of roads within each district. Canyon effects showed clear seasonal variability, with mean ΔC at hotspots increasing by 19-26% from summer to winter, consistent with reduced boundary-layer mixing and weaker canyon ventilation under more stable winter meteorological conditions. Population-related assessment further indicated that about 184,000 residents in winter were exposed to substantial canyon-induced NO₂ increments associated with at least a 1% additional health risk. These findings demonstrate the importance of street-canyon effects in dense urban environments and provide a practical basis for targeted air-quality management and ventilation-oriented urban planning.