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 modellin g, an aerosol module has been developed as part of the European Acid D eposition Model (EURAD). The chemical and physical processes have been restricted to the sulfate-nitrate-ammonia system to study the influen ce of aerosols on acid deposition. Special consideration is given to t he reversible formation process of NH4NO3 aerosols and the influence o f the dissociation constant on the aerosol and gas phase concentration s. This component appears to be important in particular for regions wh ere 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 depo sition velocities in the aerosol phase, the amount of nitrogen species in air is larger and the spatial distribution is considerably differe nt if aerosols are taken into account. On the other hand wet depositio n of nitrogen is enhanced due to the presence of easily soluble ammoni um nitrate aerosols. A comparison with observations from the EMEP netw ork has been conducted to evaluate the performance of the aerosol modu le. Modelling with the aerosol phase leads to a much better agreement between modelled and measured concentration fields.