Chemical composition of acid rain in the North of Spain: the EPOCA program

Abstract The EPOCA program (Estudios en el Pirineo Occidental sobre Contaminacion Acida), whose main goal is to show the possible effects of an “acid rain” type pollution on the forests of the Spanish Basque country, is presented in this paper. A measurement network comprising two automatic precipitation collectors located at Vitoria and at Llodio has been operating since December 1985. 30% of the events of rainfall were acidic with a pH ⩽5. However, rain with an alkaline character (pH>6) is found in 25% (Vitoria) and 40% (Lldio) of the cases. The statistical analysis of the chemical components confirms that the chemical contents of the acid rain have an anthropogenic character and those of the alkaline rain a soil origin. Both sources play an important part in the temporal trend of pH values and precipitation chemistry.