Modeled deposition of nitrogen and sulfur in Europe estimated by 14 air quality model systems: evaluation, effects of changes in emissions and implications for habitat protection

Abstract

The evaluation and intercomparison of air quality models is key to reducing model errors and uncertainty. The projects AQMEII3 and EURODELTA-Trends, in the framework of the Task Force on Hemispheric Transport of Air Pollutants and the Task Force on Measurements and Modelling, respectively (both task forces under the UNECE Convention on the Long Range Transport of Air Pollution, LTRAP), have brought together various regional air quality models to analyze their performance in terms of air concentrations and wet deposition, as well as to address other specific objectives.This paper jointly examines the results from both project communities by intercomparing and evaluating the deposition estimates of reduced and oxidized nitrogen (N) and sulfur (S) in Europe simulated by 14 air quality model systems for the year 2010. An accurate estimate of deposition is key to an accurate simulation of atmospheric concentrations. In addition, deposition fluxes are increasingly being used to estimate ecological impacts. It is therefore important to know by how much model results differ and how well they agree with observed values, at least when comparison with observations is possible, such as in the case of wet deposition.This study reveals a large variability between the wet deposition estimates of the models, with some performing acceptably (according to previously defined criteria) and others underestimating wet deposition rates. For dry deposition, there are also considerable differences between the model estimates. An ensemble of the models with the best performance for N wet deposition was made and used to explore the implications of N deposition in the conservation of protected European habitats. Exceedances of empirical critical loads were calculated for the most common habitats at a resolution of 100 × 100 m2 within the Natura 2000 network, and the habitats with the largest areas showing exceedances are determined.Moreover, simulations with reduced emissions in selected source areas indicated a fairly linear relationship between reductions in emissions and changes in the deposition rates of N and S. An approximate 20 % reduction in N and S deposition in Europe is found when emissions at a global scale are reduced by the same amount. European emissions are by far the main contributor to deposition in Europe, whereas the reduction in deposition due to a decrease in emissions in North America is very small and confined to the western part of the domain. Reductions in European emissions led to substantial decreases in the protected habitat areas with critical load exceedances (halving the exceeded area for certain habitats), whereas no change was found, on average, when reducing North American emissions in terms of average values per habitat.

Model Performance Differences in Sulfate Aerosol in Winter over Japan Based on Regional Chemical Transport Models of CMAQ and CAMx

“…The results showed that the other important factor that explained the differences in SO4 2− concentration between CMAQ and CAMx was SO4 2− wet deposition at EANET observation sites and over domains 3 and 4. A model intercomparison in Europe also concluded that the accurate estimate of deposition is key to an accurate simulation of ambient concentrations [30]. Differences in SO 4 2− concentration were found at Nagoya and Tokyo compared with the high temporal resolution observations; therefore, the analysis for SO 4 2− wet deposition was done over J-STREAM domains 3 and 4, and the results are shown in Figure 11.…”

“…The results showed that the other important factor that explained the differences in SO 4 2− concentration between CMAQ and CAMx was SO4 2− wet deposition at EANET observation sites and over domains 3 and 4. A model intercomparison in Europe also concluded that the accurate estimate of deposition is key to an accurate simulation of ambient concentrations[30].…”

An evaluation of European nitrogen and sulfur wet deposition and their trends estimated by six chemistry transport models for the period 1990–2010

“…The resulting gaseous and particulate nitrogen-and sulfur-containing compounds in the atmosphere can be subsequently deposited to the surface through the mechanisms of wet (in rain, fog or snow) and dry deposition. Within Europe, of the N and S deposition is estimated to come from European emissions (Sanderson et al, 2008;Tan et al, 2018;Vivanco et al, 2018). Over the last 3 decades there has been a reported decrease in European emissions of NO x , NH 3 and SO 2 by approximately 50 %, 15 % and 90 %, respectively (EEA, 2017).…”

“…In Europe, this evaluation can currently only be done for wet deposition, since measurements of dry deposition of N and S are sparse, incomplete and intermittent. Various studies have compared N and S wet deposition estimated by regional and global CTMs with observed values in Europe, giving varying results (Dentener et al, 2006;Simpson et al, 2006Simpson et al, , 2014Vet et al, 2014;Vivanco et al, 2017Vivanco et al, , 2018. Compiling the results of these studies shows that model bias for wet deposition of oxidised ni-trogen ranges from −86 % to +72 % of the average of the observed values with a median bias of −19 %.…”

Response to anonymous reviewer #2

“…Knowledge on the mixing state is more important in the transition from fresh to aged aerosol. The difference be-tween external and internal mixing becomes small when hydrophobic material 100 nm in diameter has a coating of soluble material of 3 nm or more, which may be achieved in a few hours in photo-chemically active environments, such as urban areas during daytime (Wang et al, 2010). The total ERFaci is currently estimated as −0.45 (−1.2 to 0.0) W m −2 (Myhre et al, 2013).…”