CREAF

To study the relationship between rainwater chemistry and transport and deposition processes at Montseny (eastern Catalonia) we analysed air mass back-trajectories and synoptic maps of rain events and classified them according to their geographical origin as follows: (1) African, (2) European, (3) oceanic (Atlantic and Mediterranean) or (4) local. African rains had high pHs and ionic concentrations, especially of Ca²⁺ and alkalinity. European rains were acidic and presented high concentrations of non-sea-salt SO₄², NO₃^- , and NH₄⁺. Local rains were moderately alkaline with intermediate concentrations. Oceanic rains were the most dilute, with slightly acidic pHs. The fact that the composition of rainwater varies according to origin is related to the emission processes in each source area and can be observed in a two-axis plot of a principal component analysis.

Fig. Scatterplot of rain events at La Castanya (Montseny) according to the two main factors extracted from a principal component analysis based on rainwater concentrations. L = local, O = oceanic, A = African, E = European. 
 

As a consequence of reductions in anthropogenic S emissions, a sharp decline in rainwater SO₄^2-concentrations was observed at Montseny. A time-trend analysis showed declining SO₄^2-concentrations for all provenances, the greatest decrease being found in European rains. The fact that SO₄^2- in African rains also decreased significantly would indicate that, in part, these rains also carry European-derived air pollution. Oceanic rains showed the lowest decline in SO₄^2- concentrations although, since they are the most abundant rains on an annual basis (amounting to 52 % of total annual precipitation), they in fact showed the most important quantitative reductions in terms of SO₄^2-deposition (calculated as the product of element concentration and the amount of precipitation).

Fig. Mean annual bulk atmospheric deposition of non-seasalt SO₄-S at La Castanya (Montseny), by rain provenance (A = African rains, E = European rains, L = local rains, and O = oceanic rains; Total= all rains), averaged over the initial and final 5-year periods of the study.

A quantification of the potential transport of the main pollutants based on source-receptor models incorporating the residence time of trajectories in various European sub-regions suggests that the closest regions, such as the Iberian Peninsula, France and the United Kingdom, can potentially contribute heavily to the annual deposition of S and N at Montseny. Nevertheless, more distant zones can also provide a non-negligible contribution, thereby confirming the transboundary character of atmospheric pollution.