1. The classification of the concentration according to wind direction, divided into 16 categories (sorted clockwise from N to NNW), allows to visualize which site influences another as well as to illustrate whether the pollution measured at the 13 stations comes from the same wind directions or not. In the former case, it would indicate remote sources; in the latter case, local source. Rather than a pollution rose, in which all directions are shown, we plotted arrows on the stations for the concentration above the threshold of two and three standard deviation above the average value. In this way, we highlighted the significant influences from one site to another and discarded the insignificant variations.