## 4 Results  
### Spatial and Temporal Features of PM\({}_{2.5}\) Concentration  
As shown in Figure 4, in different seasons and months, the PM\({}_{2.5}\) concentration varied considerably. The PM\({}_{2.5}\) concentration was low and stable in summer, higher in autumn and spring, and most severe in the winter. This is because the high temperature in summer can lead to atmospheric instability, triggering an increase in rainfall and high humidity, leading to large-scale wet deposition of suspended particles [35]. PM\({}_{2.5}\) concentration was higher during spring than summer but showed a decreasing trend. In March, dry weather and high winds produced more soil dust, but air pollution eased as temperatures rose and rainfall increased in April and May. In autumn, seasonal factors such as weak winds, steady climate, and more frequent burning of biomass during harvest season [36] contributed to an increase in pollutants. In Beijing, the high PM\({}_{2.5}\) concentration mainly occurred in winter, from November to February, with the peak occurring in December. The highest PM\({}_{2.5}\) concentration in winter was mainly due to coal heating, burning of biomass, and fireworks during the Spring Festival, which has a great adverse effect on the atmospheric environment [37]. Furthermore, in winter, when the dry climate is unfavorable for air dispersion, suspended particles, organic or inorganic, could also lead to large amounts of pollutants in the air. From the spatial dimension, the Kriging interpolation model was conducted for the average PM\({}_{2.5}\) concentration at twelve sites in Beijing from 1 December 2016 to 31 December 2016 (the winter season with the highest PM\({}_{2.5}\) concentration and the greatest variation). This indicated that the PM\({}_{2.5}\) concentration of twelve stations in Beijing showed a clear spatial aggregation. As shown in Figure 5, PM\({}_{2.5}\) concentration was higher in the southeastern part  
Figure 4: PM\({}_{2.5}\) concentration in Beijing from 3 December 2016–28 February 2017. Seasonal variation of PM\({}_{2.5}\) concentration (The picture on the left), Monthly variation of PM\({}_{2.5}\) concentration (The picture on the right). of Beijing and lower in the northwestern part, varying between 85 and 121 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\). The areas with higher PM\({}_{2.5}\) concentration were mainly concentrated in the main urban areas of Beijing, where the highest value is 121 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\). As the main urban area of Beijing, southeastern Beijing was a transportation and scenic area as well as a mixed area with high emissions of vehicle exhaust, which led to high PM\({}_{2.5}\) concentration [38, 39]. As the distance from the central city increases, the PM\({}_{2.5}\) concentration gradually decreases, and the atmospheric environment is improved (Figure 5). The areas with low PM\({}_{2.5}\) concentrations are mainly concentrated in the countryside of Beijing, where the lowest PM\({}_{2.5}\) concentration is 85 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\). Generally speaking, for various reasons, PM\({}_{2.5}\) concentration in Beijing shows a trend of gradually decreasing in the southeast to the northwest. The main reason was that Beijing was surrounded by mountains in the northwest, north, and northeast, and pollutants from the main urban area were blocked by the Taihang Mountains and Yanshan Mountains when they dispersed, resulting in large differences in the spatial distribution of PM\({}_{2.5}\) concentration in Beijing [40] (In this study, data from 12 monitoring stations were used to fit the spatial variation of PM\({}_{2.5}\) concentration in the entire Beijing region, resulting in incorrectly high PM\({}_{2.5}\) concentration values in the northeast of Beijing. However, when we analyze the spatial variation of PM\({}_{2.5}\) concentration, we choose to ignore it).