MULTI-POLLUTANT MODELING AND THE CRITICAL LOADS APPROACH FOR NITROGEN

Understanding of the processes driving the deposition of acidic sulphu r, oxidised-and reduced-nitrogen species has improved dramatically ove r recent years. A simple trajectory model approach is used here to des cribe the highly coupled behaviour of sulphur dioxide (SO2), oxides of nitrogen (NOx) and ammonia (NH3). The model is able to reproduce the salient features of the geographical distribution of the main wet-depo sited pollutants across the United Kingdom. A feature of the model is the important role played by the oxidation products of sulphur dioxide and nitrogen oxides in determining the distribution of wet-deposited ammonium. Scenarios employing substantial reductions in European sulph ur dioxide emissions (based on the 1994 UN-ECE Sulphur Protocol) also produce concomitant reductions in wet-ammonium deposition. This behavi our serves to emphasise the importance of establishing multi-pollutant strategies within the European application of the critical loads appr oach to nitrogen. (C) 1998 Elsevier Science Ltd. All rights reserved.