The deposition of base cations in the Nordic countries

Base cation deposition from the atmosphere provides, together with weathering, ions for neutralising acidity and, after sufficient reductions of acidifying deposition, a source for replenishing lost base cation pools. In the integrated assessment modelling to support the strategies of pollution abatement, a need for base cation data over Europe has been expressed. Data are needed by the scientists working with acidification effects and recovery of ecosystems. A study has been made on a procedure for mapping base cation deposition over the Nordic countries, with an additional aim of recommendations for base cation mapping on a European scale. The mapping over the Nordic countries was made using the data assimilation part of the MATCH model, run with a resolution of 11x11 km. The mapping was based on monitoring data for concentrations of base cations in air and precipitation in the Nordic and surrounding countries.The influence of base cation deposition in the calculation of critical loads and their exceedance as well as for the dynamic modelling of recovery processes was analysed and base cation deposition was found to be of significant importance. For dynamic modelling, the influence of base cation deposition is less important when using models, which are calibrated with monitoring data. However, in prognoses for the future, it is still very with accurate estimates of the base cation deposition. The procedure for mapping base cation deposition proposed in this project represents an improvement with respect to many previous mapping exercises, because it includes both wet and dry estimates of base cation deposition, and because it handles the decreasing sea salt gradients from the coast towards inland. It is recommended in the report to use the experiences from this Nordic mapping activitiy to initiate a process to compile necessary monitoring and emission data in order to carry out a mapping procedure of base cation deposition at European level.