Comparison of modelled and monitored deposition fluxes of sulphur and nitrogen to ICP-forest sites in Europe

The EMEP MSC-W Eulerian chemical transport model, and its predictions of deposition of acidifying and eutrophying pollutants over Europe, play a key role in the development of emission control strategies for Europe. It is important that this model is tested against observational data. Here we compare the results of the EMEP model with measured data from 160 sites of the European Union/ICP Forest (Level II) monitoring network, for the years 1997 and 2000. This comparison comprises: (a) Precipitation amount, (b) Total deposition of SO42- to coniferous and deciduous forests, (c) Wet deposition of SO42-, NO3- and NH4+ in open field sites, and (d) Concentrations of SO42-, NO3- and NH4+ in precipitation. Concerning precipitation, the EMEP model and ICP network showed very similar overall levels (within 4% for 1997 and 11% for 2000). The correlation was, however, poor (r(2)= 0.15 - 0.23). This can be attributed largely to the influence of a few outliers, combined with a small range of rainfall amounts for most points. Correlations between modelled and observed deposition values in this study were rather high (r(2) values between 0.4 - 0.8 for most components and years), with mean values across all sites being within 30%. The EMEP model tends to give somewhat lower values for SO42-, NO3- and NH4+ wet deposition to ICP, but differences in mean values were within 20% in 1997 and 30% in 2000. Modelled and observed concentrations of SO42-, NO3- and NH4+ in precipitation are very similar on average (differences of 0 - 14%), with good correlation between modelled and observed data (r(2)= 0.50 - 0.78). Differences between the EMEP model and ICP measurements are thought to arise from a mixture of problems with both the observations and model. However, the overall conclusion is that the EMEP model performs rather well in reproducing patterns of S and N deposition to European forests.