Long-term trends in the chemistry of atmospheric deposition in Northwestern Italy: the role of increasing Saharan dust deposition

The Lake Maggiore watershed is one of the areas most affected by acid deposition in Italy. Decreasing sulfate concentration and increasing pH values of precipitation have been observed in this area in the last 15–20 yr. Besides atmospheric deposition, an important factor controlling water acidification and recovery is the episodic deposition of Saharan calcareous dust, which contributes significantly to alkalinity and base cation deposition. At the sampling site at Pallanza, rain chemistry has been investigated since 1975. The frequency and chemical characteristics of alkaline events (pH > 5.6) and Saharan events in particular (distinguished by an exceptionally high calcium and alkalinity content and by the visible presence of red dust) were evaluated at this site. Saharan events, despite their small contribution to total annual precipitation (3–17%), carry a considerable fraction of the annual total calcium and bicarbonate deposition (up to 70 and 100% respectively). The frequency and intensity of alkaline events affect the acidity/alkalinity budget on an annual basis. Apart from the major effect of sulfate decline, the increasing pH of precipitation recorded at the study site since the 1980s might also be influenced by an increased occurrence and intensity of alkaline events. An increase in the export of Saharan dust from arid regions together with changes in the circulation pattern over the Mediterranean area has been hypothesized as the main causes of this trend.