An evaluation of European nitrogen and sulfur wet deposition and their trends estimated by six chemistry transport models for the period 1990–2010
Authors: Theobald Mark R. , Vivanco Marta G. , Aas Wenche , Andersson Camilla , Ciarelli Giancarlo , Couvidat Florian , Cuvelier Kees , Manders Astrid , Mircea Mihaela , Pay Maria-Teresa , Tsyro Svetlana , Adani Mario , Bergström Robert , Bessagnet Bertrand , Briganti Gino , Cappelletti Andrea , D'Isidoro Massimo , Fagerli Hilde , Mar Kathleen , Otero Noelia , Raffort Valentin , Roustan Yelva , Schaap Martijn , Wind Peter , Colette Augustin 
Published: January 2019

Abstract. The wet deposition of nitrogen and sulfur in Europe for the
period 1990–2010 was estimated by six atmospheric chemistry transport models
(CHIMERE, CMAQ, EMEP MSC-W, LOTOS-EUROS, MATCH and MINNI) within the
framework of the EURODELTA-Trends model intercomparison. The simulated wet
deposition and its trends for two 11-year periods (1990–2000 and
2000–2010) were evaluated using data from observations from the EMEP
European monitoring network. For annual wet deposition of oxidised nitrogen
(WNOx), model bias was within 30 % of the average of the observations for
most models. There was a tendency for most models to underestimate annual wet
deposition of reduced nitrogen (WNHx), although the model bias was within 40 %
of the average of the observations. Model bias for WNHx was inversely
correlated with model bias for atmospheric concentrations of NH3+NH4+, suggesting that an underestimation of wet deposition partially
contributed to an overestimation of atmospheric concentrations. Model bias
was also within about 40 % of the average of the observations for the
annual wet deposition of sulfur (WSOx) for most models. Decreasing trends in WNOx were observed at most sites for both 11-year
periods, with larger trends, on average, for the second period. The models
also estimated predominantly decreasing trends at the monitoring sites and
all but one of the models estimated larger trends, on average, for the second
period. Decreasing trends were also observed at most sites for WNHx, although
larger trends, on average, were observed for the first period. This pattern
was not reproduced by the models, which estimated smaller decreasing trends,
on average, than those observed or even small increasing trends. The largest
observed trends were for WSOx, with decreasing trends at more than 80 %
of the sites. On average, the observed trends were larger for the first
period. All models were able to reproduce this pattern, although some models
underestimated the trends (by up to a factor of 4) and others
overestimated them (by up to 40 %), on average. These biases in modelled
trends were directly related to the tendency of the models to under- or
overestimate annual wet deposition and were smaller for the relative trends
(expressed as % yr−1 relative to the deposition at the start of the
period). The fact that model biases were fairly constant throughout the time series
makes it possible to improve the predictions of wet deposition for future
scenarios by adjusting the model estimates using a bias correction calculated
from past observations. An analysis of the contributions of various factors
to the modelled trends suggests that the predominantly decreasing trends in
wet deposition are mostly due to reductions in emissions of the precursors
NOx, NH3 and SOx. However, changes
in meteorology (e.g. precipitation) and other (non-linear) interactions
partially offset the decreasing trends due to emission reductions during the
first period but not the second. This suggests that the emission reduction
measures had a relatively larger effect on wet deposition during the second
period, at least for the sites with observations.