EMEP Eulerian model for atmospheric transport and deposition of nitrogen species over Europe

As a part of EMEP, MSC-W in Oslo has the task to compute atmospheric transport and deposition of sulphur and nitrogen species over Europe. To perform this task, a three-dimensional Eulerian multilayer model was developed at MSC-W. This model has a 50 km horizontal resolution with 20 vertical layers, of which 10 are below 2 km level. Meteorological input, from a dedicated weather prediction model, is updated at 6-hour intervals. Emission inventories for NOx (NO + NO2), NH3 (and SO2), submitted officially by the individual countries, are used as input data. In the model, NOx may be converted to PAN by the CH3COO2 radical, or to nitrate (as HNO3 in the gas phase and ammonium nitrate as particulate matter). The oxidation of NOx is assumed to proceed either by the OH radical in the gas phase, or as a night-time reaction via N2O5 on deliquescent particles. Both OH and CH3COO2 are prescribed as a function of solar zenith angle and cloud cover. Furthermore, ammonium sulphate is formed by NH3 and sulphate. Excess NH3 may react with HNO3, forming ammonium nitrate in an equilibrium reaction. Nitrogen is removed from the atmosphere by dry and wet deposition. A description of the model, as well as the model results, are presented and discussed. We also present a comparison of calculated concentrations and deposition fields for nitrogen with available measurements of concentrations and wet deposition for 1992.