Seasonality of the Na/Cl ratio in precipitation and implications of canopy leaching in validating chemical analyses of throughfall samples

The temporal variation of the Na/Cl ratio in bulk precipitation and throughfall samples was analysed using an extensive data set based on measurements over several years at 11 sites of the Swiss Long-Term Forest Ecosystem Research Programme (LWF). This analysis was prompted by the results of the application of the criteria recommended for validating chemical analyses of precipitation samples in the Integrated Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). One of these criteria involves verifying that the sodium to chloride (Na/Cl) ratio of the analysed sample is within a restricted range, assuming that most of the chloride originates from sea-spray, and that the contribution of marine ions in atmospheric deposition is in the same molar ratio (0.86) as in sea-water. The range of possible Na/Cl values was defined between 0.5 and 1.5 by ICP Forests in order to account for other possible sources of Na+ and Cl− (natural or anthropogenic). When all sites were considered, approx. 85% of our Na/Cl values were within the proposed range, both for bulk precipitation and throughfall samples. In some cases, low Na+ or Cl− concentrations close to the detection limit were responsible for the Na/Cl ratios outside the range of acceptance. Plotting the Na/Cl ratio versus time revealed a seasonal pattern, which was clearer in the throughfall than in the bulk precipitation samples. This could also account for Na/Cl values higher or lower than the defined limits. The seasonality of the Na/Cl ratio and its components (Na+ and Cl− fluxes) was tested using a regression model. For throughfall, the seasonal pattern of Na/Cl could be ascribed to the seasonally driven canopy leaching of Na+ and Cl−, the intensity of which depended on the tree species.