Mapping of Concentrations in Europe Combining Measurements and Acid Deposition Models

Measurements of air and precipitation quality have been carried out within the EMEP programme under the Convention on Long Range Transboundary Air Pollution (LRTAP) since 1978. Approximately 100 rural sites are currently in operation. The Meteorological Synthesising Centre-West (MSC-W) operates two EMEP models estimating transboundary fluxes of air pollutants, a two-dimensional Lagrangian model and a three-dimensional Eulerian model. Traditionally kriging has been used to produce gridded concentration fields from observed data for comparison with modelled data. This paper describes a method for producing optimal fields based on both point measurements and. The difference between modelled and measured values in each measurement point is interpolated to give a smooth two-dimensional expression for the discrepancy between the two data sets. A combined map is derived by adjusting the modelled values with the interpolated difference weighted by the distance to the nearest measurement point. The method has been applied to sulphur and nitrogen measurements in air and precipitation from the EMEP network and modelled results in a 150x150 km grid from the EMEP Lagrangian model. The combined maps give improved regional concentration fields combining characteristics from both the measured and modelled data sets depending on the distance to the measurement points. Comparison with results from the higher resolution Eulerian model shows good agreement.