Toward Sustainable Commercial Streets in Severe-Cold Cities: Co-Optimizing Pedestrian Thermal Comfort and PM2.5 Exposure Through Landscape Configuration—A Combined Field Measurement and ENVI-Met Modeling Study in Shenyang, China

Dense commercial pedestrian streets in severe-cold cities simultaneously suffer from summer heat stress and traffic-related fine particulate matter (PM2.5), yet street greening that cools pedestrians can also suppress ventilation and trap pollutants. This study combines field measurement with microclimate modeling to identify configurations that co-optimize thermal comfort and PM2.5 exposure. On two Shenyang commercial streets (the 1300 m east–west Middle Street and the 800 m northeast–southwest Taiyuan Street), air temperature, relative humidity, wind speed, solar radiation, and PM2.5 were measured hourly on two clear summer days in 2023 and used to calibrate ENVI-met models of five street segments. A three-factor, three-level orthogonal design generated nine configurations combining tree canopy cover (30–50%), lawn cover (10–25%), and water cover (10–25%), all planted with the native Siberian elm (Ulmus pumila). A thermal-pollution synergy index was built by reverse range-standardizing the physiological equivalent temperature (PET) and PM2.5, and multiple linear regression quantified the contribution of each element. All configurations lowered daytime air temperature by 0.43–0.94 °C and raised humidity by 1.75–3.28%, but dense canopy cover reduced wind speed and increased PM2.5, whereas moderate cover reduced both heat stress and pollutants. The index identified 30% tree cover with 10–15% lawn and 10–25% water as the most robust compromise; tree cover was the dominant and highly significant factor (p = 0.004), lawn was secondary (p = 0.040), and water was not significant. The findings provide an evidence-based configuration rule for the measured summer conditions of the two studied streets, without transferring heat problems into air-quality problems; the rule is specific to the tested streets and should be re-validated before transfer to other settings. As a nature-based solution that jointly addresses climate adaptation and air-quality protection, this approach supports the sustainable renewal of commercial streets in severe-cold cities and contributes to the United Nations Sustainable Development Goals (SDG 3 on health, 11 on sustainable cities, and 13 on climate action).