Problems of Modelling the Long-Range Transport of Reduced Nitrogen

As negotiations on a new sulphur protocol under the UN Economic Commission for Europe reach an advanced stage, increasing attention is being paid to the contribution of nitrogen compounds to acidification. In this context European emissions of nitrogen as ammonia (estimated at almost 8 million tonnes of N over the EMEP map area- EMEP 1993), originating mainly from livestock, are comparable with those as NOx (7 million tonnes as N over the same map area). However as a reactive gas emitted at or near ground level from a distribution of sources, there are special problems in modelling the subsequent dispersion and atmospheric transport out to long distances. There are also large uncertainties about the nature and magnitude of emissions, and the interaction of ammonia with other atmospheric species. This paper illustrates some of these difficulties by application of a Lagrangian model TERN (Transport over Europe of Reduced Nitorgen) to selected episodes.