## 3 Results and Discussion
### Source Analysis  
#### 3.2.1 Path Source  
The sources of contaminants in snow were mainly classified into local polluting source emissions and remote transmission of pollutants. The remote transmission of pollutants in this study was primarily impacted by the air mass coming from the northwest (Figure 4). The distribution of organic  
Figure 3: Comparisons of the percentages of various PhACs in snow (**a**) each substance; (**b**) species.  
pollutants in environments is affected by various factors such as NH\({}_{4}\)-N, NO\({}_{3}\)-N, PO\({}_{4}\)-P, SiO\({}_{3}\)-Si and TOC [26]. To understand the relationship between environmental factors and the concentration of PhACs in snow, we analyzed the correlation. The results are shown in Table S4.  
Generally, NH\({}_{4}\)-N mainly comes from agricultural activities [52; 53], including volatilization of nitrogen fertilizer, animal husbandry, application of human and animal exceta, and other organic fertilizers [54; 55], and the relatively high content of NH\({}_{4}\)-N in snow is associated with the well-developed production of agriculture and animal husbandry near JZB [26]. NO\({}_{3}\)-N is considered to be closely associated with industrial and traffic emissions [56; 57], and NO\({}_{3}\)-N in snow is mainly in connection with well-developed local traffic and coal-fired heating in winter. The extremely remarkable positive correlation (\(p<0.01\)) among the PO\({}_{4}\)-P, NO\({}_{3}\)-N, and SiO\({}_{3}\)-Si concentrations indicated that they may have similar sources (local pollutant emission). The remarkable positive correlation (\(p<0.05\)) between TOC concentration and SiO\({}_{3}\)-Si indicated that they may also  
Figure 4: Typical air mass backward trajectories and their source directions during the sampling periods.  
Figure 5: Source directions of typical precipitation air masses during the sampling periods.  
have similar sources. Thus, NH\({}_{4}\)-N, NO\({}_{3}\)-N, PO\({}_{4}\)-P, SiO\({}_{3}\)-Si and TOC in snow were mainly local pollutant emissions.  
As shown in Table S4, 14 kinds of PhACs (chloramadinone acetate, trenbolone, indomethacin, sulfararazine, desacetylcefotaxime, sulfaromonethoxine, nitrazepam, tilmicosin, prednisone, budesonide, trimicinolone diacetate, sulfarameter, lorazepam, and tetracycline) had a significant positive correlation with one or more environmental factors (NH\({}_{4}\)-N, NO\({}_{3}\)-N, PO\({}_{4}\)-P, SiO\({}_{3}\)-Si, TOC) (\(p<0.05\)); thus, the 14 above-mentioned substances may come from local pollutant emissions.  
There is a significant or extremely significant negative correlation between the substances of nonidazole and salbutamol and NO\({}_{3}\)-N, PO\({}_{4}\)-P, and TOC (\(p<0.05\) or \(p<0.01\)). Oxolinic acid shows a significant negative correlation with NO\({}_{3}\)-N (\(p<0.05\)), and both salbutamol and nonidazole have a significant positive correlation with oxolinic acid (\(p<0.05\)). These results indicate that nonidazole, salbutamol, and oxolinic acid are not from local pollutant emissions but rather may be from remote transmission in the atmosphere. The backward trajectory chart of the air masses during the sampling period (Figure 4) shows that the airflow above JZB during the sampling period was mainly affected by the air masses coming from the northwest direction and the northwest desert area. The rainy cloud cluster was partially affected by the Loess Plateau in addition to the impact from the northwest cloud cluster (Figure 5). Therefore, Asian sand and dust particles brought strongly by winter northwest wind with low precipitation and dry weather increased the particle contents in the atmosphere and are beneficial to the adsorption of some PhACs with stable properties. In addition, due to the small molecular weight and stable properties of salbutamol, nonidazole, and oxolinic acid, these compounds can be easily adsorbed on particles for remote transmission.