## 3 Results
### PM\({}_{10}\), PM\({}_{2.5}\), and PM\({}_{1}\) Analysis
In European legislation, As, Cd, and Ni should be at levels below 6.0 ng m\({}^{-3}\), 5 ng m\({}^{-3}\), and 20 ng m\({}^{-3}\) (actually, these target values will be entered into force on December 2020) whereas the World Health Organization (WHO) suggests [48] a guide value for Cd of 5 ng m\({}^{-3}\): in all cases, our data were much lower than those fixed; even the maximum values were significantly below these levels. Particular attention should be given to the CV% whose meaning has just been reported above: it ranged between 9% and 164% in PM\({}_{10}\), between 18% and 91% in PM\({}_{2.5}\), and between 7% and 114% in PM\({}_{1}\). In particular, except for a few cases in PM\({}_{10}\) (Au, CV% 164%; Co, 92.9%), PM\({}_{2.5}\) (K, 71,8%; Na, 90.9%; Ta, 81.5), and PM\({}_{1}\) (Fe, 88.1%; Sc, 114.7), the CV% in these three fractions were below 60%, which means that the elements in these fractions showed slight variations and that not many sources influenced the element levels. This last information was also confirmed by the good correlation reported in Figure 1 between the various fractions (PM\({}_{10}\) vs. PM\({}_{2.5}\): equation curve y = 0.328x + 7.252, r\({}^{2}\) = 0.635; PM\({}_{2.5}\) vs. PM\({}_{1}\): y = 0.521x + 2.817, r\({}^{2}\) = 0.551; PM\({}_{10}\) vs. PM\({}_{1}\): y = 0.112x + 9.139, r\({}^{2}\) = 0.150). The last correlation (PM\({}_{10}\)/PM\({}_{1}\)) was not so good, which is not surprising considering the large difference between these two sizes. On the other hand, the correlations between elements in each fraction (Table 4) confirmed the common sources for all elements, both natural (crustal origin) and anthropogenic (plant emissions and auto-vehicular traffic): the elements showing \(r\)-values above 0.6 were considered well correlated [1; 2; 49] such as Br, Sm, Au, Th, Ce, Ni, Eu, and Ag, (\(r>0.8\)). Regarding the toxic elements, it should be underlined that the correlation between Ni and the other elements was not good, except with the rare earth elements (REEs), meaning a main component of natural sources for this element. For Cr and As, which are largely diffused in nature, the situation was slightly different: over the natural origin, their presence in the atmosphere could be due to auto-vehicular traffic and industrial emissions (e.g., metallurgical industries, coal-fired power plants, galvanic industries, waste incinerators). Figure 1: Relationship between the concentration elements in the PM fraction: correlation curves, linear equations, and regression coefficients (R\({}^{2}\)). Studies performed in European Union (EU) countries have shown chromium values in ambient air ranging between 4 and 70 ng m\({}^{-3}\) in urban areas, and between 5 and 200 ng m\({}^{-3}\) in industrial areas. Arsenic values in ambient air, on the other hand, vary between 1 and 3 ng m\({}^{-3}\) in urban areas and between 20 and 30 ng m\({}^{-3}\) in industrial areas [48; 50; 51]. In this study, these two elements (i.e., Cr and As) defined as carcinogenic compounds for human health (Group 1) by the International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA) [48; 50], showed good correlations in all cases, meaning that they have common origins. This consideration was strengthened by their correlations with Fe, an element widely present in the crustal composition. The correlation is really important when analyzing the correlation data for the REEs: their good values (most of them showed a \(r>0.8\)) allow to take information about the evaluation of data quality, to understand the mass transport events because their presence affects the sample homogeneity, and the particulate matter composition and aging [52; 53].