## 3 Results and Discussion
### Chemometric Approach  
A chemometric investigation has been carried out on the average deposition rates of annual collecting data, for a better knowledge of the relationships among the main species occurring in the deposition rates. Cluster analysis (CA) and principal component analysis (PCA) were used to achieve similarities among the different species as well as for understanding how the dataset can be divided [77; 78; 79]. First, CA among the data of each location was run. As it can be seen in Figure 5, data collected at the two locations such as Rocca Respampani and Allumiere show high similarities whereas ones collected at S. Agostino site show very high dissimilarity in comparison with the previous ones. Roccaccia has an intermediate behavior among the sites. This behavior can be explained taking into account both the four sampling points (Figure 1) and the direction of the prevailing winds, i.e., North-West and South-East (Figure 2). The S. Agostino site is located both close to the power plant and close to the port-industrial area in Civitavecchia [67] whereas Allumiere and Rocca Respampani are in the inner part of the region, far from the anthropogenic sources. Roccaccia is located north of S. Agostino, along the same direction as the prevailing winds but farther from anthropogenic sources, i.e., power plant and port-industrial area. These occurrences can explain the Allumiere and Rocca Respampani profiles cluster as well as Roccaccia one whereas S. Agostino is totally different. The authors would like to investigate the overall data (inorganic and organic fractions) both for finding out similarities among the different species and for evidencing possible presence of outliers that could affect the analysis. First, the CA shows the presence of three clusters:  
cluster # 1 1 compound: Sn;  
cluster # 2 5 compounds: 118-CB, Ba, Mn, Sr, Zn;  
cluster # 3 62 compounds: all other compounds/elements. For a better representation, the authors would like to show the PCA (Figure 6): the three clusters are well evidenced with the related species. Figure 5: Cluster Analysis (CA) related to the different sampling points. As it can be seen, the only different species is Sn, which makes group by itself and very distant from the other clusters. In these conditions, Sn could be considered an outlier: this consideration is confirmed looking at the data reported in Table 8 where Sn concentration value determined at S. Agostino site reaches high annual average level (113 \(\upmu\)g m\({}^{-2}\) d\({}^{-1}\)). The data reported in Table S8 of the Supplementary Material confirm and stress this consideration. The data determined in summer and autumn seasons are 100 to 12,000 times higher than those determined in the other seasons; further, the seasonal values greatly range among them, with a very high level in autumn (449 \(\upmu\)g m\({}^{-2}\) d\({}^{-1}\)) at S. Agostino site. These factors lead to the hypothesis that Sn could be considered an outlier. Following this assumption, the PCA among sites was performed using all the data but omitting the Sn values, as shown in Figure 7. Component 1 represents the organic fraction, distinguished in the positive values associated with PCBs (mainly, 118-CB, 105-CB, 123-CB, 114-CB) and PCDD/Fs (mainly, OCDD) and the negative values associated with PAHs: Roccaccia is the station with the highest content of PCDD/Fs and PCBs, while Allumiere is the station with the highest values of PAHs and S. Agostino is the one with the lowest contribution of organic compounds. Component 2 represents the inorganic fraction (mainly, Ba, Zn, Sr, Mn): once again, Allumiere shows the highest metal content.