Impact of ENVI-met-Based Road Greening Design on Thermal Comfort and PM 2.5 Concentration in Hot–Humid Areas

Road greening markedly impacts road thermal comfort and air quality. However, previous studies have primarily focused on thermal comfort or PM 2.5 individually, with relatively few addressing both aspects comprehensively, particularly in humid regions. This study combined field measurements and simulations. It employed physiological equivalent temperature (PET) and quantified the horizontal distribution of particulate matter 2.5 (PM 2.5 ). The research examines the effects of planting spacing, tree species, and tree–shrub combinations on pedestrian walkways in humid climates during both summer and winter. Using measured tree data and road PM 2.5 , a plant model was established and pollution emission parameters were set to validate the effectiveness of the ENVI-met through fitting simulations under various scenarios. The results indicated that (1) plant spacing for trees influenced both the road thermal environment and PM 2.5 levels. Smaller spacing improved thermal conditions but increased PM 2.5 . (2) trees with large canopies and high leaf area indices (LAIs) notably enhanced thermal comfort, while those with smaller canopies and dense understories facilitated PM 2.5 dispersion. The 3 m spacing resulted in a maximum absolute PM 2.5 concentration difference (C) of 5.05 μg/m 3 in summer and a maximum mean absolute PM 2.5 concentration difference (M) in the downwind region of 2.13 μg/m 3 in winter. (3) Combining trees with shrubs moderately improved pedestrian thermal comfort. However, taller shrubs elevated PM 2.5 concentrations on walkways; heights ranging from 1.5 m to 2 m in summer showed higher C values of 5.38 μg/m 3 and 5.37 μg/m 3 . This study provides references and new perspectives for the optimization of roadway greening design in humid areas in China.