A MULTILAYER MODEL TO DESCRIBE THE ATMOSPHERIC TRANSPORT AND DEPOSITION OF AMMONIA IN GREAT-BRITAIN

The large spatial variability of ammonia (NH3) emissions and depositio n makes it difficult to estimate the input of reduced nitrogen to diff erent ecosystems from measurements, since to quantify accurately the s patial variation in NH3 concentration would require a very large numbe r of monitoring stations. Such quantification is an important requirem ent for assessing the impacts of acidifying and nitrogen entrophicatin g deposition. These problems have been addressed in the current study by applying an existing multi-layer model of ammonia transport (TERN) (ApSimon et al., Atmospheric Environment 1994, 28, 665-678), together with a detailed NH3 emission field, to develop a long-term statistical trajectory model (FRAME) over Great Britain at a 5 km x 5 km grid res olution. The model treats the vertical concentration gradient explicit ly with 33 layers, as well as provides a description of the land-use d ependence of dry deposition using a resistance formulation dependent o n major surface landcover classes. By treating vertical diffusion expl icitly, the model provides the capability for examining the spatial va riability at a finer scale than models assuming instantaneous mixing o f emissions, and also avoids the need for air concentration and deposi tion correction factors. Model results are presented in the form of ma ps of annual mean NH3 concentration and net dry deposition, and a comp arison is made with NH3 concentrations measured by a national monitori ng network. A total annual budget for reduced nitrogen is given which shows the directional dependence of both total deposition and export o f reduced nitrogen. (C) 1998 Elsevier Science Ltd. All rights reserved .