Trends in Chemical Composition of Wet-only Precipitation at Rural French Monitoring Stations Over the 1990–2003 Period

The long-term monitoring of precipitation and its chemical composition are important for identifying trends in rain quality and for assessing the effectiveness of pollution control strategies. A statistical test has been used to the atmospheric concentrations measured in the French rural monitoringnetwork (MERA) in order to bring out spatiotemporal trends in precipitation quality in France over the period 1990–2003. The non-parametric Mann—Kendall test which has been developed for detecting and estimating monotonic trends in the time series was used and applied in our study at annual values of wet-only precipitation concentrations. The emission data suggest that SO₂ and NO_x emissions decreased (−3.3 and −2.0% year⁻¹, respectively) contrary to NH₃ emissions that increased slightly (+0.2% year⁻¹) over the period 1990–2002 in France. On the national scale, the pH values have a significant decreasing trend of −0.025±0.02 unit pH year⁻¹. SO²⁻ ₄ and nss — SO²⁻ ₄ concentrations in precipitation have a significant decreasing trend, −3.0±1.6 and −3.3±0.6% year⁻¹, respectively, corresponding with the downward trends in SO₂ emissions in France (−3.3% year⁻¹). A good correlation (R ²=0.84) between SO₂ emissions and nss — SO²⁻ ₄ concentrations was obtained. The decreasing trend of NH⁺ ₄ was more significant (−5.4±5.2% year⁻¹) than that of NO⁻ ₃ (−1.3±2.4% year⁻¹). Globally, the concentration of the major ions showed a clear downward trend including marine and alkaline ions. In addition, the relative contribution of HNO₃ to acidity precipitation increased by 51% over the studied period.