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

O. Westling (1) , H. Fagerli (2) , S. Hellsten (1) , J. C. Knulst (1) , D. Simpson (2, 3)

1 IVL - Swedish Environmental Research Institute
2 MSC-W - EMEP Meteorological Synthesizing Centre-West
3 Department of Radio and Space Science [Göteborg]

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 the UNECE and EU. It is important that this model is tested against observational data. Such model evaluation is usually conducted using observations from the EMEP monitoring network, which includes around 70 sites with wet deposition measurements in a variety of landscapes. Here we compare the results of the EMEP model with a completely independent data-set, that of the EU/ICP Forest (Level II) monitoring network. Modelled data from 1997 and 2000 were compared with observed deposition data from 160 ICP-Forest plots.

In general, similarities between modelled and observed deposition in this study were reasonably good (r² values between 0.5-0.8 for most components and years, with mean values across all sites being within 30%), despite the uncertainty in comparing measured plot data with modelled grid data. EMEP tends to give somewhat lower values for the average, median and percentile SO^2-₄, NO^-₃ and NH⁺₄, wet deposition in the whole deposition gradient compared to ICP, but differences in mean values were within 20% in 1997 and 30% in 2000. Modelled and observed concentrations of SO^2-₄, NO ^-₃ and NH⁴₊ in precipitation are very similar as average (differences of 0-14%), median and percentiles, and the correlation between modelled and observed data is rather high r²=0.50-0.78). Many of the sites showing large discrepancies between EMEP and ICP data were found to have unusually high inter-annual variability in the precipitation amounts registered by ICP, suggesting that some differences may be due to sampling procedures or complex topographic effects. A simple precipitation variability index (PVI) was devised to flag sites with large variability, and many outliers in the chemical comparison were shown to be such sites. The largest discrepancies were seen between EMEP and ICP precipitation amounts and for this component the correlation was very poor (r²=0.04-0.23 with all data, and 0.23-0.27 with PVI-filtered data). Although more work is needed to understand these discrepancies, the overall conclusion is that the EMEP model performs rather well in reproducing patterns of S and N deposition to European forests.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

O. Westling, H. Fagerli, S. Hellsten, J. C. Knulst, D. Simpson. Comparison of modelled and monitored deposition fluxes of sulphur and nitrogen to ICP-forest sites in Europe. Biogeosciences Discussions, 2005, 2 (4), pp.933-975. ⟨hal-00297780⟩