*2.1. Study Area and Neighborhoods*

Five megacities (Wuhan, Hefei, Nanjing, Shanghai, and Hangzhou) in the middle and lower reaches of the Yangtze River were selected as the study area (Figure 1); they have the same climate conditions. Due to the extreme high-density construction, these cities experience more serious PM2.5 pollution than other cities in this region [31]. The similar

features of the five cities provide sufficient comparability for this study, including urban form, landform, population, and PM2.5 pollution characteristics.

**Figure 1.** Location of five megacities and distribution of monitoring stations [32].

On average, each city has 10 national PM2.5 monitoring stations that present a relatively uniform distribution throughout the constructed areas. A neighborhood was then defined as a square domain with an area of 1 km2 (1 km × 1 km) centered at a monitoring station because 1 km is a common size for a *neighborhood* division in China and was frequently used in previous studies [32,33]. In addition, the air environment within this size plays an important role in people's quality of daily life [34]. In consideration of the unique locations of 13 neighborhoods and subsequent special land use, such as close to pollution sources (construction sites) and natural land (urban parks or scenic regions), 37 neighborhoods were selected for analysis in this study. Detailed information on the 37 cities is shown in Table S1.
