THE PARAMETRIZATION OF THE SULFATE-NITRATE-AMMONIA AEROSOL SYSTEM IN THE LONG-RANGE TRANSPORT MODEL EURAD

Because of the obvious importance of aerosols for atmospheric modelling, an aerosol module has been developed as part of the European Acid Deposition Model (EURAD). The chemical and physical processes have been restricted to the sulfate-nitrate-ammonia system to study the influence of aerosols on acid deposition. Special consideration is given to the reversible formation process of NH4NO3 aerosols and the influence of the dissociation constant on the aerosol and gas phase concentrations. This component appears to be important in particular for regions where large ammonia emissions are found as in Europe. Concentration and deposition fields as predicted by EURAD are analysed for a simulation with and without the new aerosol module. Due to their smaller dry deposition velocities in the aerosol phase, the amount of nitrogen species in air is larger and the spatial distribution is considerably different if aerosols are taken into account. On the other hand wet deposition of nitrogen is enhanced due to the presence of easily soluble ammonium nitrate aerosols. A comparison with observations from the EMEP network has been conducted to evaluate the performance of the aerosol module. Modelling with the aerosol phase leads to a much better agreement between modelled and measured concentration fields.