title: The evolution of the pH in Europe (1986-1997) using panel data.
abstract: The temporal evolution of pH values in precipitation over Europe during the period 1986-1997 is examined using panel data. The use of panel data techniques allows us to determine the temporal evolution of groups of stations rather than analyzing the temporal behavior of each of them. The analysis reveals three different temporal patterns: Peripheral, Central and French. We find a significant increasing trend (P < 0.00001) in both Peripheral and Central patterns. The annual increases are +0.057 pH-units yr(-1) and +0.022 pH-units yr(-1) respectively. However the French pattern is characterized by a significant decreasing trend (P < 0.004) and the annual decrease is -0.022 pH-units yr(-1). The standard errors of panel data estimates are around 47% smaller than those of classical pooling and 32% smaller than aggregate time-series regression. The use of panel data produces higher R2 values than classical pooling and aggregate data. This technique takes into account the individual heterogeneity, allows a larger number of data points and improves the efficiency of the estimates. In general, the policies of governments to reduce pollutant emissions seem to be effective.
authors.affiliation: Dpto. Física del Aire, Facultad de Físicas Avda, Universidad Complutense, Madrid, Spain.
authors.forename: E
authors.initials: E
authors.lastname: Marín
authors.forename: T
authors.initials: T
authors.lastname: Pérez-Amaral
authors.forename: A
authors.initials: A
authors.lastname: Rúa
authors.forename: E
authors.initials: E
authors.lastname: Hernández
mesh_terms: D015258:Acid Rain; D003625:Data Collection; D004784:Environmental Monitoring; D005060:Europe; D006863:Hydrogen-Ion Concentration; D013997:Time Factors
chemical_list: D015258:Acid Rain