The transport distances of NH3 and the species derived from NH3 emissions are investigated as a function of the emissions of SO2 and NOx. This is done by Lagrangian model calculation of long-range transport to six Scandinavian European Monitoring and Evaluation Programme measurement sites in August--October 1985. In the presence of SO2 and NOx emissions, NH3 forms ammonium aerosols 4HSO4, (NH4)2SO4, and NH4NO3> which are transported over longer distances than NH3. When the SO2 emissions are set to zero in the model, the fraction of airborne ammonia and ammonium (defined as the ratio between the concentration at the receptor site averaged over 3 months divided by the source, i.e., the initial concentration plus the sources along the trajectories averaged over the same 3 months) at the six receptor sites drops from 9% at Keldsnor to 18% at Ut¿.

When both NOx and SO2 emissions are set to zero in the calculations, only the initial concentrations of NOy and S contribute to ammonium aerosol formation and most of the NH3 emissions then remain as airborne NH3 before dry deposition or rain out. In this case the fraction of NH3 or ammonium aerosols which remains airborne at the receptor sites dropped from 19% of the sources along the trajectories arriving at Keldsnor to 30% at Birkenes. This is a substantial change in the airborne fraction, and in the deposition calculation for the grid boxes where the receptor sites are located, a similar drop in NH3+NH+4 deposition is found. ¿ American Geophysical Union 1994