Fig. 1.

Air masses arriving at the MM site at 500 m above ground level (see text). Back-trajectories were calculated hourly (grey lines) for the sampling time corresponding to each sample and grouped in four cases: Saharan (SH, lefthand upper panel), Northeastern (NE, righthand upper panel), Northwestern (NW, lefthand lower panel), and regional (RG, righthand lower panel). Red lines are plots of the average of each group of BTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)