Through cluster analysis, a dendrogram was produced clustering the variables (Figure 6a). Confirming the results of the factor analysis, we observed a cluster including the “seasonal“ factors ozone, temperature and solar radiation. 1–OHPYR and 2– OHPH appeared in the same group close to another group with the “pollution” factors nitrogen and sulfur oxides. Finally, a fourth group emerged containing the different particles PM10–PM2.5 but also encompassing NO2. Although the latter is difficult to explain, it confirms our previous observations.
When clusters were constructed on the observations using the Ward method and Euclidean distances, the same four clusters appeared (Figure 6b). These clusters were practically identical to the ones provided in Figure 5. Thus, to the left (group 1) appear the samples most affected by the OH–PAHs and the chemical pollutants. It may also be observed that the least polluted samples group together according to the season during which they were collected.