A LONG-RANGE TRANSPORT MODEL FOR AMMONIA AND AMMONIUM FOR EUROPE

This paper reviews available data on ammonia (NH3) emission and other parameters used in the model, viz. dry deposition velocities, scavenging coefficients and the pseudo-first order reaction rate. There are some striking differences in emission and subsequent behaviour of NH3 and sulphur dioxide (SO2). NH3 is predominantly emitted near or at ground level, whereas SO2 is mainly emitted from higher sources. Moreover, the conversion rate of NH3 to ammonium (NH4+) is higher than for SO2 to sulphate (SO42-). The approximate treatment in the Lagrangian model of turbulent mixing (instantaneous homogeneous mixing over the whole mixing layer) had to be modified by the introduction of several correction factors derived from detailed diffusion calculations. Contrary to SO2, the concentration at ground-level of NH3 will be determined to a large extent by local sources, which hinder model verification. For this purpose only measurements in the Netherlands could be used. The model has been run for the year 1980. The computed ammonium concentration in air and precipitation are in agreement with the available measured data. The model results show that the dry deposition of NH3 in any country is mainly caused by inland sources, whereas the dry and wet deposition of ammonium is to a considerable extent caused by foreign sources. Moreover some numerical aspects of the model are discussed and the results of a sensitivity analysis are shown.