Precipitation chemistry at a rural Mediterranean site: Between anthropogenic pollution and natural emissions

The chemistry of weekly rainwater collected at a rural site (La Castanya, Montseny Mountains, northeast Spain), exposed both to anthropogenic pollutants and natural African dust, was analyzed for the period 1983-2000. On the basis of back trajectories and synoptic charts we distinguished four source regions: (1) African, (2) European, (3) Oceanic, and (4) Local. The chemical characteristics of each provenance were interpreted in relation to the emission processes at each source region. The temporal trends between 1983 and 2000 in the volume weighted mean (VWM) concentrations showed an upward trend for alkalinity in European, Local, and Oceanic provenances, while they showed a slight (nonsignificant) downward trend for African rains. All provenances exhibited a significant decline in excSO-24, which was most pronounced for European rains (b = -5.7 μeqL-1 yr-1). Because NO-3 remained about constant (except for European rains), NO-3 acquired a more important role in the acid anion contribution. A reduction of the acid deposition for European, Local, and Oceanic rains was observed, as well as an increase in the global deposition of alkalinity. The role of African rains appeared as a determinant in the accumulated alkalinity during the period. A quantification of the transport of pollutant species (SO-24, NO3-, and NH+4) from eight contributing European subregions was obtained on the basis of the residence time of the trajectories in each subregion. It suggested that not only subregions closer to the receptor point, such as the Iberian Peninsula, France, and the United Kingdom, were important contributors to the deposition of SO-24, NO-3 and NH+4 but also distant sites such as Scandinavia contributed considerably.