## 2 Materials and Methods
### Overview of the Study Area  
Urumqi, the capital of the Xinjiang Uyghur Autonomous Region of China, is located in the northern foothills of the Tianshan Mountains in central-northern Xinjiang. It serves as the political, economic, cultural, and transportation hub of the region, as shown in Figure 1. Urumqi City (42\({}^{\circ}\)45\({}^{\prime}\)-45\({}^{\circ}\)20\({}^{\prime}\) N, 86\({}^{\circ}\)37\({}^{\prime}\)-88\({}^{\circ}\)58\({}^{\prime}\) E) covers an area of about 180 km east-west  
Figure 1: General map of the study area. and 153 km north-south, with the Urumqi River crossing in the center [22]. Figure 2a,b, provide an overview of the study area, where Figure 2a is the general map of the study area; Figure 2b is a schematic diagram that illustrates the research area. All types of landforms exist in the region, including plains, valleys, and hills. It is located in the Junggar Basin in the north and the Tianshan Mountains in the south [23]. It is in a \"T\"-shaped distribution, also known as the \"funnel\" city. To gain a deeper understanding of the relationship between the evolution of winter environmental pollution and topography in Urumqi, several representative urban agglomerations were carefully selected as the core study areas. These include the New Urban District, Shuimoqou District, Midong District, Tianshan District, and Shayibak District, all of which feature relatively high population densities and vibrant economic zones within the city of Urumqi. The selected study area is characterized by a topography that slopes downward from the south to the north [24]. Figure 3 provides a slope map of this area, offering a visual representation of its terrain features. In the northern part of Midong (43\({}^{\circ}\)58\({}^{\prime}\)23.2\({}^{\prime\prime}\) N, 87\({}^{\circ}\)39\({}^{\prime}\)12.0\({}^{\prime\prime}\) E), the landscape rises in the southeast and descends towards the northwest. The lowest point in the region lies at the southern edge of the Gurbantunggut Desert (also known as the Junggar Basin Desert), located in the northern part of the city. Dongdaohaizi, a notable geographical point here, has an elevation of 418 m. Conversely, the highest point in Urumqi is Bogda Peak in the Tianshan Mountains, soaring to an elevation of 5445 m. The southeastern part of Tianshan District (43\({}^{\circ}\)47\({}^{\prime}\)32.9\({}^{\prime\prime}\) N, 87\({}^{\circ}\)37\({}^{\prime}\)48.6\({}^{\prime\prime}\) E) serves as the southeast gateway to Urumqi, connecting the city with the southern border and inland cities. It has a highest elevation of 1669 m, a lowest elevation of 826 m, and an average elevation of 1108 m. The southwestern Shayibak District (43\({}^{\circ}\)48\({}^{\prime}\) N, 87\({}^{\circ}\)35\({}^{\prime}\) E) presents a more elevated profile, with a highest elevation of 3744 m, a lowest elevation of 606 m, and an average elevation of 1552 m. Table 1 is a summary table of key geographical information and meteorological factors. In particular, it meticulously elaborates on the unique characteristics of slope aspect and gradient, which are pivotal for understanding the local geographical context and its potential influence on environmental pollution. Figure 2: Overview of the study area: (**a**) general map of the study area; (**b**) schematic diagram of the study area. Figure 4 shows the three-dimensional contour lines of the geographic environment of the study area. It can be seen from the figure that the area is surrounded by mountains on three sides, and the terrain slopes from south to north. Among them, the terrain in the south is narrower and undulating. At the same time, the north is a vast alluvial plain area with flat terrain, which is favorable for the development and urban construction of the city. Influenced by the terrain, the city's construction land mainly extends from the south to the north, and in the northern plains extends to the east and west [25; 26].