## 3 Results and Discussion
### Source Apportionment  
Calculation of different molecular diagnostic ratios (MDRs) has been applied in many studies for source apportionment of PAHs in the atmosphere [65; 73; 74]. The approach relies on the fact that emission of certain PAHs from characteristic sources tends to be constant, and thus the ratio receptor/source remains stable [67; 68]. However, MDRs should be used with caution, as PAH components may differ in reactivity and atmospheric residence times [67]. In Table 6, the selected MDRs are presented for the whole period and for each season separately. All four MDRs conclude to the same outcome, suggesting strong pyrogenic source contributions. Moreover, there was no observed difference between the cold and warm periods, assuming that the sources are constant and strongly related with combustions in the area. The MDRs calculated here are parallel with those reported previously for Aspropirgos and Eleusis [36; 47], which also suggest effects from pyrogenic sources. One of the tools to estimate possible common sources among PCDD/Fs is the calculation of PCDD to PCDF (D/F) ratio. In general, these ratios in ambient air indicate the degree of contamination from combustion sources. Values above 1.0 suggest less contribution, while values <0.5 show increased contribution of combustion sources [75]. In our study, the D/F ratio was found in an average value of 0.45, a value ranging from 0.49 during the cold period to 0.42 during the warm period. The values suggest that in the whole sampling period, the PCDD/Fs emissions were influenced dominantly by thermal processes. The same results were obtained from other studies in industrial sites in satellite cities of Seoul, Korea [64]. Steel and iron plants' emissions can also be considered as a continuous source of PCDD/Fs in Thiassion Plain, as reported also by [PERSON] et al. (2011) in northeast China [38]. In order to evaluate any potential associations and gain an overview of the relationships among the pollutants investigated, principal component analysis (PCA) was performed in the entire dataset (Table 7). PCA was also applied for PAHs, PCDD/Fs, and PCBs separately, but the results were overlapping so the combined PCA was preferred, including all the studied compounds. Three factors explained the 83.4% of the total variance. PC1 was heavily loaded with high molecular weight PAHs like BaP (0.805), IPY (0.821), and BbkF (0.759) together with most of the PCDFs and some of PCDDs. This factor agrees that PAHs and PCDD/Fs have common sources related with combustions, both controlled and illegal. Indeed, open burning for waste incineration in the studied area is a common practice, and these emissions combined with industrial ones could compose a constant source of both pollutants. Another possible common source for these POPs could be cement kiln factory where the conditions of the furnace; very high temperature, mixing and excess of oxygen; make it an ideal place for a 'perfect' combustion [39; 76]. The PC2 (24.3% of variance) was tightly clustered with PCBs loadings (around 0.7) and is a factor indicating the common source of PCBs, both dlPCBs and indPCBs. PCBs were clearly distinct from PCDD/Fs and PAHs as also reported in other studies [23]. PCBs emissions may be affected of the industries and chemical plants of Thiassion Plain while uncontrolled combustions of plastic may also enrich the aerosols with PCBs. The third factor (22.9% of variance) was associated with petrogenic, low molecular weight PAHs and DBaA, suggesting a