## 3 Spatial Correlation Network Analysis of Air Quality in China
### Spatial Correlation Network Characteristics of Air Quality in China  
The Air Quality Index (AQI) is objective, and most scholars use this indicator to represent air quality [42; 43; 44]. Like most scholars, this paper uses the Air Quality Index to measure China's air quality. The network nodes are 30 provinces in China (excluding Xizang, Hong Kong, Macao and Taiwan), and the time span of the samples is 2014-2019. The air quality index comes from the China Meteorological website. According to the value calculated by Formula (1), this paper uses Gephi0.9.2 to draw the spatial correlation network of air quality. The results are shown in Figures 1-4. This paper only shows the spatial correlation network of air quality in 2014, 2016, 2018, and 2019. The larger the network node is, the greater the connectivity. That is, the air quality of a province with this characteristic has more influence on the air quality of other provinces. The average continuous output degree in 2014 was 9.069, the average continuous output degree in 2018 was 4.917, and the average continuous output degree in 2019 was 4.174. From Figures 1-4, it can be found that the spatial correlation of air quality presents a typical network structure. Figure 1 shows more provinces and relationships, and the network structure represented in this figure is the most complex. With the continuous control of air pollution, the number of provinces and relationships in Figure 4 is greatly reduced, and the network structure is gradually simplified in China. Figure 1: AQI spatial correlation network in 2014. Theoretically, there are 29 relationships between the air quality of a province and the air quality of other provinces, and the maximum number of relationships is 870 (30 \(\times\) 29). From Figure 1, the total number of spatial relationships of air quality in China in 2014 was 263. From the size of the nodes, the provinces with the most serious air pollution started from Beijing, Tianjin, Hebei, Shanxi and Shandong and had gone all the way south, involving provinces such as Hubei, Anhui, Jiangsu and Shanghai. Among them, the air quality of Hebei was the worst. According to the measurement of the gravity model, its air pollution had spread to 22 provinces. In addition to Hebei Province, air pollution in Henan and Hubei had also spread to 20 or more provinces. The air quality of Beijing, Tianjin, Shanxi, Jiangsu, Anhui, Shandong, Hunan and Shaanxi had affected more than 10 provinces. From Figure 2, the total number of spatial relationships of air quality in China in 2016 was 153. Compared with 2014, the overall air quality index began to decline in 2016, and the air pollution index had dropped by an average of 16.73%. The impact of a province on air pollution in surrounding provinces had reached 20 provinces, but this was no longer the case. The air quality of Beijing, Tianjin, Hebei, Jiangsu, Zhejiang, and  
Figure 3: AQI spatial correlation network in 2018. Figure 2: AQI spatial correlation network in 2016. Anhui had affected more than 10 provinces, but the number of affected provinces had also dropped considerably. For example, the air quality in Beijing affected 18 provinces in 2014, but in 2016 it affected only 12 provinces. The most obvious change was with Hebei, where the air quality in 2014 affected 22 provinces, but in 2016 had dropped to 12 provinces. From Figures 3 and 4, compared with 2014, China's air pollution had further decreased in 2018 and 2019. In 2018, China's air quality index had dropped by 22.71%, and in 2019, China's air quality index had dropped by 28.05%. In 2018, the total number of the largest spatial correlations of air quality in China was 118. Air quality in Henan had affected 16 provinces. The air quality of Hebei and Shanxi had affected 10 provinces, and the air quality of other provinces had affected fewer than 10 provinces. In 2019, the total number of the largest spatial correlations of air quality was 96, and the number of air quality correlations had been further reduced. China's air quality had improved.