3.4.1. PCA analysis
PCA was performed to estimate the potential sources of PAHs in Shihezi. The results of the PCA are shown in Figure 4. Three principal components explained 64.3% of the total variance, with proportions of 44.2% for PC1, 12.4% for PC2, and 7.7% for PC3. Chrysene (Chr), benzo(b)fluoranthene (BbF), and benzo(k)fluoranthene (BkF) are indicators of coal combustion(Hamid et al. 2018), whereas acenaphthene (Ace), pyrene (Pyr), and BaP are mainly produced from vehicle emissions(Dhital et al. 2021, Smit et al. 2017). Thus, PC1 reflects a coal combustion and vehicle emissions source. PC2 was dominated by acenaphthylene (Acy), fluoranthene (Flu), and phenanthrene (Phe), which have a strong correlation with the source of fugitive dust(Cao et al. 2021, Han et al. 2009). Therefore, PC2 could explain fugitive dust. PC3 was mainly defined by fluoranthene (Flu), BkF, and dibenzo(a,h)anthracene (DBA), which are interpreted as industrial emissions(Wu et al. 2021).
3.4.2. Source apportionment of PAHs by PMF
Figure S2 illustrates the correlations between the measured and model-predicted total PAH mass concentrations. Based on this, the PMF model was able to perfectly predict the total PAH mass concentrations, indicating that this model is suitable for PAH source apportionment.
Factor 1 was mainly defined by naphthalene (Nap), Pyr, and benzo(a)anthracene (BaA), which contributed 22.9% to the total PAH concentration, interpreted as natural gas emissions, as shown in Figure 5. Previous studies have shown that Pyr and BaA are typical tracers of natural gas(Li et al. 2019b). Xinjiang is rich in natural gas resources, which have been used as fuel for automobiles and households. Shihezi City has achieved full coverage of natural gas installations, which means that every household uses natural gas. In addition, the use of LNG in vehicles is trending because of low prices. Generally, the high-load low-ring aromatic hydrocarbons also verify that the combustion of LNG produces PAHs with lower and intermediate volatility (Mehmood et al. 2020).
Factor 2 explained 13.6% of the total PAH concentrations, and significant loadings of Ace, Fla, Pyr, Chr, BbF, DBA, and BaP are associated with this factor. According to previous studies, Pyr and Chr are chemical markers for industrial oil burning (Taghvaee et al. 2018). There are many factories in the north of Shihezi, most of which are chemical factories, such as factories producing acetylene, ethylene glycol, formaldehyde, and polyethylene, leading to an increase in gaseous and PM2.5 pollutants. DBA, BaP, and BkF have been previously observed as the dominant species in the ambient air of industrial sites (Rajput &Lakhani 2009).
Factor 3 accounted for 14.6% of the total PAH concentrations and it had a high loading of 3–4 ring PAHs, composed of Nap, Acy, Flu, Ace, Phe, anthracene (Ant), Pyr, and Chr. Phe, Pyr, and Chr are tracers for fugitive dust (Wang et al. 2020a), including coal, road floating soil, farmland soil, and construction waste. Shihezi has a temperate continental climate with sparse precipitation throughout the year and a dry climate, which causes the dust to enter the atmosphere for a long time, making it difficult to stabilize. In addition, the contribution of road dust and farmland soil to this source is not negligible as traffic flow has been increasing.
For factor 4, the main PAHs included BbF, BkF, BaP, DBA, indeno(1,2,3-cd)pyrene (Icdp), and benzo(ghi)perylene (Bghip), which are mainly generated from traffic emissions (Jia et al. 2018), accounting for 30.9%. Previous studies (Arellano et al. 2018, Mehmood et al. 2020, Rajput &Lakhani 2009, Sei et al. 2021) have shown that BbF, BkF, dibenzo(a,h)anthracene (DahA), and BghiP are specific tracers for traffic emissions, among which BbF, BaP, and BghiP are typical emission markers of gasoline vehicles. The contribution of automobile sources to PAHs is mainly due to the substantial increase in the number of automobiles in recent years, and there were obvious arterial roads near the sampling points. The lowest contribution to this source occurs in winter, but there is no seasonal change in traffic flow near the sampling point. This may be due to the poor performance of the car engine exhaust control system at low temperatures (Pham et al. 2013, Zhou et al. 2005).
Factor 5 was primarily contributed by Nap, Acy, Flu, Ace, Phe, Ant, Fla, and Pyr, accounting for 17.9%, interpreted as a coal combustion source. Studies by Zou et al. (Zou et al. 2015) and Jamhari et al. (Jamhari et al. 2014) showed that these substances are direct markers of coal combustion. The contribution of coal-fired sources to PAHs is less than that of automobile sources and natural gas sources, due to the local government’s achievements in coal substitution. However, the contribution of coal sources to PAHs in winter has significantly increased because coal is still the primary fuel source in this area, which is used in power plants and central heating system (Zhang et al. 2021). In addition, the contribution of coal sources in spring was also relatively high, which is related to the six-month heating season in Shihezi.