Acknowlegements
Marcelo Izquierdo (Instituto de Ciencias de la Construcción, CSIC, Madrid) supplied us with meteorological data since 1991. Sampling and analyses were carried out by: Miguel Alvarez-Cobelas, Angel Rubio Olmo, Matilde Segura, Salvador Sánchez-Carrillo, José Luis Ayala, José Luis Velasco, María Valladolid, Oscar Soriano, Angel Baltanás, Mercedes Arauzo, Julián Vicioso, María Verdugo, Julio Camargo, David Angeler, Palmira Riolobos, Javier García-Avilés, Juana Hidalgo, Youssef Himi, José María Alonso, Eva López Delgado, María José Ortiz and Alejandro Abadía.