3.1. Concentrations and Compositions of PAHs
The PM
2.5-bound PAH concentrations from Beihuangcheng Island in the Bohai Sea from August 2017 to March 2018 are listed in
Table 1. The average concentration of PM
2.5-bound Σ
16PAHs was 35.3 ± 41.8 ng/m
3. Among which, the concentrations of two-ring, three-ring, four-ring, five-ring, and six-ring PAHs were 0.66 ± 0.19 ng/m
3, 6.97 ± 4.44 ng/m
3, 16.2 ± 17.9 ng/m
3, 9.33 ± 14.7 ng/m
3, and 2.15 ± 9.10 ng/m
3, accounting for 1.88%, 19.7%, 45.9%, 26.4%, and 6.10% of Σ
16PAH concentration, respectively. The most abundant PAH components were Flua, Pyr, and Phe, with average concentrations of 6.46 ± 6.39 ng/m
3, 5.30 ± 5.99 ng/m
3, and 5.04 ± 3.59 ng/m
3, respectively.
The PAH concentrations in PM
2.5 were classified according to the sampling time in seasons: August 2017 was summer (5 samples), September to November 2017 was autumn (29 samples), December 2017 to February 2018 was winter (18 samples), and March 2018 was spring (8 samples). As listed in
Table 1, the seasonal concentrations of Σ
16PAHs in PM
2.5 in summer, autumn, winter of 2017, and spring of 2018 were 9.93 ± 3.89 ng/m
3, 30.8 ± 48.9 ng/m
3, 45.9 ± 38.0 ng/m
3, and 43.7 ± 19.8 ng/m
3, respectively. Obviously, the highest concentration of Σ
16PAHs was in winter, followed by spring and autumn, and the lowest concentration was in summer. This seasonal feature is very similar to the previous observations in northern China [
5,
30,
31,
32,
33], although there might be some deviation because only one month’s samples were detected in both spring and summer. As shown in
Figure 2 and
Table 2, the air mass carrying high concentrations of PAHs were mainly from the northwest region with high-intensity emissions, proved by previous studies [
8,
11]. Generally, the elevated concentrations in the cold season were mainly attributed to PAH outflow carried by the East Asian winter monsoon from the northwest source region, where PAH concentrations increased significantly due to combustion emission of heating fuels [
5,
23,
34].
Table 3 lists concentrations of PM
2.5-bound PAHs measured in the present study and reported in previous studies for comparison. The atmospheric concentration of PAHs on Beihuangcheng Island was lower than that in cities of northern China, such as Beijing, Tangshan, Dalian, and Zhengzhou, but was higher than that at a national station for background atmospheric monitoring located at Tuoji Island, which is the closest location where PAH concentrations were reported (see
Figure 1). The concentration level was still higher than that in the Yangtze River Delta (e.g., Shanghai, Hangzhou, Hangzhou, Ningbo, Jinhua, Lishui, Zhoushan, and Nanjing) and the Pearl River Delta (e.g., Guangzhou). The comparison indicates that the pollution burden of atmospheric PAHs around the Bohai Sea was severe, at a relatively high level.
3.2. Source Apportionment of PAHs in the Atmosphere
PCA analysis showed that there were three eigenvalues greater than 1 and the three principal components (PC) could explain 83.7% of total variance in PM2.5-bound PAH concentrations. The pre-estimation results indicated that the PM2.5-bound PAH concentrations were mainly contributed to by more than three potential sources. Based on the results, model exercises with factors from three to six were performed. Among the model exercises, the model scenarios with four factors could provide the most physically reasonable source profiles. The Q and Q(robust) values in the PMF model were 2642.3 and 2529.6, respectively. The BS-DISP estimation of the modeling showed that the largest decreases in Q(robust) were <1%, indicating that the results of the base run of the model scenario could be treated as the global optimum solution. The Fpeak model runs with strengths of −1.0, −0.5, 0.5, 1.0, and 1.5 showed increases of Q > 3.9% in Q(robust) from the base run of the model scenario, suggesting that the Fpeak solution should be removed from further consideration.
The source profiles identified by the base run of the PMF modeling are shown in
Figure 3. Factor One was considered as biomass burning, typically characterized by relatively high loadings of low molecular weight PAHs such as Acy, Ace, Flu, and Phe [
15]. Factor Two showed high proportions to the moderate molecular weight PAHs of Ant, Flua, Pyr, BaA, and Chr, which were treated as coal combustion [
44,
45]. Factor Three was considered as traffic emission, characterized by relatively high load on high-molecular-weight PAHs such as BghiP, BaP, BkF and BbF [
4]. Factor Four was mainly loaded by Nap, Ace, Acy, and Ant, which indicated spills of petroleum or petroleum-related products [
14,
45].
Table 4 lists measured and modeled concentrations of PAHs in PM
2.5 and
Figure 4 displays the contributions of the four source factors identified by the PMF modeling to PAHs. The summer, autumn, winter, spring, and average PAH concentrations contributed by the four source factors identified by the PMF modeling were 12.1 ng/m
3, 29.2 ng/m
3, 45.9 ng/m
3, 43.8 ng/m
3, and 34.1 ng/m
3, which could explain 78.5%, 94.6%, 94.2%, 96.8%, and 96.6% of measured PAH concentrations, respectively. The averaged contributions of biomass burning, coal combustion, traffic emission, and spills of petroleum or petroleum-related products to PM
2.5-bound PAH concentrations were 25.8%, 43.1%, 24.7%, and 6.39%, respectively. This result suggested that coal combustion was the primary contributor of PAHs, followed by biomass burning, traffic emission, and petroleum release during the sampling period. The molecular diagnostic ratios of PAHs, such as Ant/(Ant + Phe), Flua/(Flua + Pyr), BaA/(BaA + Chr), and InP/(InP + BghiP), were often used to qualitatively differentiate sources of PAHs in the past studies [
17,
46]. These diagnostic ratios of PM
2.5-bound PAH components in the present study were calculated and are listed in
Table 5. The averaged ratios of Ant/(Ant + Phe), Flua/(Flua + Pyr), BaA/(BaA + Chr), and IDP/(IDP + BghiP) of PAHs in this study were 0.22 ± 0.18, 0.55 ± 0.07, 0.35 ± 0.17, and 0.36 ± 0.23, respectively. Among them, the ratio of Ant/(Ant + Phe) was greater than 0.1, indicating dominative contribution of combustion sources, such as biomass burning, coal combustion, and traffic emission [
16]. The ratio of Flua/(Flua + Pyr) was greater than 0.5, indicating primary contribution of coal combustion and biomass burning [
17]. The ratios of BaA/(BaA + Chr) and InP/(InP + BghiP) were in the range of 0.2 and 0.35 and 0.2 and 0.5, suggesting the dominant contribution of petroleum-based fuel burning [
47]. The similar sources identified by the PMF model and the method of diagnostic ratios indicate that the model used in this study can comprehensively apportion PAH sources.
The seasonal contributions of the four source factors identified by the PMF modeling are displayed in
Figure 4. The contributions of biomass burning were the highest in summer (45.8%), followed by spring (31.4%), winter (27.6%), and autumn (18.6%). The contribution pattern was similar to that for carbonaceous aerosols and nitrate in PM
2.5 on the Bohai Sea because of extensive agricultural waste open burning in fields in the Shandong Peninsula and PAH outflow carried by the East Asian summer monsoon from the emission region [
22,
23]. The coal combustion contributed 23.8%, 36.1%, 49.4, and 43.8% of PM
2.5-bound PAH concentrations in summer, autumn, winter, and spring, respectively. The strongest and weakest contributions were in winter and summer, agreeing extensively with the seasonal source apportionment of atmospheric pollutants over the region [
14,
22,
48]. Traffic emissions accounted for 6.28%, 36.5%, 19.3%, and 21.4% of PM
2.5-bound PAH concentrations in summer, autumn, winter, and spring, respectively. Petroleum release contributed 24.2%, 8.82%, 3.61%, and 3.30% of the PAH concentrations in summer, autumn, winter, and spring, respectively. Previous studies reported that petroleum release of river outflow and oil leakage from ships and offshore oil fields (such as Da-Gang and Sheng-Li oil fields) were the important contributors of PAHs in the sediment and air of the Bohai Sea [
14,
49]. High contribution of the source to PAHs in PM
2.5 in the warm season was probably due to volatilization from seawater [
14]. The temporal patterns of the source contributions were in agreement with that reported in previous studies [
5,
14]. As listed in
Table 5, the seasonal molecular diagnostic ratios of Ant/(Ant + Phe), Flua/(Flua + Pyr), BaA/(BaA + Chr), and InP/(InP + BghiP) were within defined ranges, indicating that the combustion sources were the main contributors to PM
2.5-bound PAH concentrations [
16,
17].
It is already clear that the contribution of biomass burning increased obviously in summer and the emission source became the preponderant contributor of PM
2.5-bound PAH concentrations in the season on the Bohai Sea. Previous studies reported that wheat straw burning usually becomes frequent in summer for quickly eliminating agricultural wastes for the next planting in Shandong [
5,
23,
48,
50]. The finding suggests that more attention should be paid to open burning of agricultural waste in the Shandong Peninsula. The contribution of coal combustion emissions increased significantly in winter due to heating needs. This suggests that controlling emissions from coal combustion, especially for civil use, is a more effective means for mitigating atmospheric PAH concentrations in winter on the Bohai Sea.
3.3. Toxic Potency of PAHs and Source Contribution
As listed in
Table 1, the average concentration of ΣBaPeq during the sampling period was 2.32 ± 4.95 ng/m
3. The concentration was higher than the legal limit (1 ng/m
3) of ambient air quality standards established by the Chinese government (GB 3095—2012) and the reference level (0.12 ng/m
3) recommended by the World Health Organization (WHO) [
5,
51]. BaP was the largest contributor of the ΣBaPeq concentration due to the dominant toxicity equivalent factor [
3]. The PAH component contributed 37.9% of ΣBaPeq concentration, followed by BbF and BkF (21.1% and 18.5% of the totals) over the sampling period. The finding was similar to that measured at a national background atmospheric monitoring station at Tuoji Island in the Bohai Sea during November 2011 and January 2013 [
5].
The seasonal ΣBaPeq concentrations were comparable in spring (2.60 ± 1.84 ng/m
3), winter (2.54 ± 3.31 ng/m
3), and autumn (2.47 ± 6.49 ng/m
3) and were significantly higher than that in summer (0.18 ± 0.16 ng/m
3). The average concentration of ΣBaPeq from August 2017 to March 2018 (2.32 ± 4.95 ng/m
3) was slightly lower than that monitored at the national background atmospheric monitoring station in the Bohai Sea from November 2011 to January 2013 (2.70 ± 1.88 ng/m
3) [
5]. The finding suggests that PAH concentration in PM
2.5 over the Bohai Sea has slightly decreased.
Table 6 lists measured and modeled concentrations of ΣBaPeq in PM
2.5 and
Figure 5 displays the contribution of four source factors to the ΣBaPeq concentration in PM
2.5 identified by the PMF modeling. The averaged contribution proportions of biomass burning, coal combustion, traffic emissions, and spills of petroleum or petroleum-related products were 8.71%, 31.0%, 58.0%, and 2.36%, respectively. The contributions indicate that vehicle exhaust was the largest contributor to potential health risks of exposure to PM
2.5-bound PAHs, although the source contributed weakly to PAH concentration in PM
2.5 as mentioned above. The difference in the contribution of traffic emissions to PAH concentration and ΣBaPeq concentration is mainly due to the fact that this source emits more PAH components with high toxic potential, such as BaP, than the other sources apportioned by the PMF modeling [
3,
15,
18,
19]. As the largest contributor of PAH concentrations in PM
2.5 during the sampling period, coal combustion contributed 12.1% less to ΣBaPeq concentration (31.0%) than to PAH concentrations (43.1%). Many previous studies have found that motor vehicles emit a higher proportion of highly toxic PAH components than other sources, e.g., coal combustion, due to the higher temperature of combustion of motor vehicle fuel [
11,
32,
52].
As shown in
Figure 5, the seasonal contributions of vehicle exhaust (Factor 3) were 26.2%, 70.7%, 49.5%, and 53.7% in summer, autumn, winter, and spring. The largest contribution of vehicle exhaust occurred in autumn, attributed mainly to the prevailing northwest wind carrying PAHs from the upwind Beijing-Tianjin-Hebei region (see
Figure 2) to the Bohai Sea [
5,
34]. The weakest impact of vehicle exhaust on ΣBaPeq concentration in PM
2.5 was in summer, when it was mainly dominated by prevailing southeasterly winds, that is, air masses coming from relatively clean seas [
53,
54].