Wet and dry atmospheric deposition at prealpine and alpine sites in northern Italy

Modified wet and dry samplers that use a water surface to collect dry deposition were used in a pluriannual field campaign in five sites in alpine and prealpine areas of Lombardy (Italy). SO42−, NO3−, Cl− and NH4+ concentrations in rainfall increased from north to south; the southern area being closest to industrialised areas of the Po Valley. In dry deposition the spatial pattern of sulphur and nitrogen species was ostensibly linked to local pollution sources. In winter, the SO42− concentration in dry deposition samples from Morbegno (urban) and Longone (semi-urban) was about 3 times that in summer in association with low pH, explained as due to a higher rate of SO2 dissolution in winter followed by oxidation to sulphuric acid. The dry deposition of Ca2+, Mg2+ and K+ formed 41–60, 35–64 and 57–67% of the total flux, respectively; furthermore 25–50% of total SO42−, 21–37% of total NH4+, and 23–31% of total NO3− came from dry deposition. Only at the remote high-altitude site was the dry deposition of these species insignificant. The sum of the wet and dry deposition rates indicated that current nitrogen loads exceed the nutrient critical loads by several kg N ha−1 at all investigated sites. Measured nitrogen loads at Longone and Castione (rural) were about 4 times higher than critical values.