Multi-pollutant modelling and the critical loads approach for nitrogen

Understanding of the processes driving the deposition of acidic sulphur, oxidised- and reduced-nitrogen species has improved dramatically over recent years. A simple trajectory model approach is used here to describe 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-deposited 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 sulphur dioxide emissions (based on the 1994 UN-ECE Sulphur Protocol) also produce concomitant reductions in wet-ammonium deposition. This behaviour serves to emphasise the importance of establishing multi-pollutant strategies within the European application of the critical loads approach to nitrogen.