Relative contribution of nitrogen and sulphur to deposition acidity and regional modelling in Lake Maggiore watershed (Southern Alps, Switzerland and Italy)

Over the last 20 years wet deposition chemistry in the Lake Maggiore watershed has been studied at 13 sites distributed at different altitudes throughout the territory. At the end of the 1990s, 55% of actual acidity values were derived from sulphur oxide compounds, and 45% from nitrogen oxides. Of potential acidity, 30%was attributable to sulphur oxide compounds, 25% to nitrogen oxides and 45% to the oxidation of reduced nitrogen compounds. Largely due to a reduction in the sulphur content of fuel oils, actual acidity and sulphate concentrations fell by 70 and 50%, respectively. Unlike sulphate, nitrogen compound concentrations did not show any major variations during this period, so that the potential acidity values which also take account of the acidity released by ammonium at environmental receptor level, decreased by only 35%. Principal Component Analysis highlighted the existence of an ionic concentration gradient along a south-north axis and with altitude. This made it possible to construct some empirical regression models to estimate concentrations of the major ions in function of geographical variables. These represent the distance from the main sources of pollutant emissions (Po Valley, Northern Italy). The models obtained are highly descriptive and predictive, and constitute an effective tool in calculating high definition maps.