3.1.1. Overall Characteristics of PCA

PCA had pollution level differences due to the difference in meteorological factors included at the different pollution levels. At the overall pollution level, the KMO value of 22 standardized factors was 0.630, and the significance of the Bartlett spherical test was 0.000, which met the requirements of PCA. As shown in Figure 3a, with the increase in the number of principal factors, the eigenvalues and variance showed a downwards trend. The eigenvalue of principal factors reflects their contribution. The eigenvalues of the first six main factors were greater than 1, and the cumulative contribution of these six principal factors was 85.7%, which can explain most of the information of the initial factors. When the number of principal factors reached 12, it could basically contain 98% of the information of the initial factors. The contribution of the first principal factor (*P*1) to the total variance was 35.2%. There was little difference between the loadings of *P*1, most of which were between 0.2 and 0.3. *P*<sup>1</sup> was negatively correlated with the green space factor and positively correlated with the gray space factor. Therefore, *P*<sup>1</sup> reflects the great difference between the green space and the gray space of the neighborhood. The contribution of *P*<sup>2</sup> to the total variance was 18.7%. The higher loading of *P*<sup>2</sup> was higher than 0.3, which was mainly positively correlated with TCR (*x*1) and the Edge (*x*6), reflecting the large-scale green space of the neighborhood. The contribution of *P*<sup>3</sup> to the total variance was 11.2%. The higher loading of *P*<sup>3</sup> was more than 0.4, which was significantly positively correlated with H (*x*15) and H<sup>σ</sup> (*x*16), reflecting the higher building height in the neighborhood and their greater height differences. *P*<sup>4</sup> contributed 9.2% to the total variance. Relative humidity (*x*21) and wind speed (*x*22) were two principal factors for *P*4, representing meteorological factors. *P*<sup>5</sup> and *P*<sup>6</sup> contributed 6.4% and 5% to the total variance, respectively, and GCR (*x*2) and Loop (*x*7) were principal factors.

The variance and eigenvalue showed similar trends at different pollution levels, which were similar to those of the overall pollution (Figure 3b). The KMO values of 22 standardized factors were 0.621, 0.615, and 0.612 for slight, moderate, and heavy pollution, respectively, and the significance of the Bartlett spherical test was 0.000. The eigenvalues of the first six principal factors were greater than 1, and the cumulative contributions of these six main factors were 85.3%, 86.0%, and 83.9%. When the number of principal factors reached 12, they could contain 98% of the information of the initial factors.

**Figure 3.** Variance and eigenvalue of principal factors at: (**a**) Overall pollution level; (**b**) Slight, moderate, and heavy pollution level.
