Spatial variability and source identification of rural precipitation chemistry in France

The precipitation chemistry in France has been examined for large-scale spatial variability using the MERA/WMO-GAW network of 13 rural sites equipped with wet-only collectors. Three classes of mean chemical compositions of MERA sites, corresponding to three different geographical regions in France have been identified by applying hierarchical clustering analysis. Factor analysis has also been performed on the deposition and concentration data sets for the three MERA sites. All the factors could be interpreted as falling into one of four categories: Acid, Sea, Neutralization, or an Agriculture/Soil association. Both sulfate and nitrate were found to be significant components of the acid factor for two stations (Morvan and Donon). These individual site analyses indicate that the sea has a strong influence on the precipitation chemistry at these continental stations. A comparison between factor analysis of concentrations and factor analyses of depositions suggests on the one hand that the acid forms, H₂SO₄ and HNO₃, seem preferentially present in cloud and on the other hand that the neutralization of precipitation acidity by soil-derived particles and by ammonia emissions seems to depend on the geographical and climatic characteristics of a site (altitude, proximity to human activity and precipitation amount).