A multi-layer model to describe the atmospheric transport and deposition of ammonia in Great Britain

The large spatial variability of ammonia (NH3) emissions and deposition makes it difficult to estimate the input of reduced nitrogen to different ecosystems from measurements, since to quantify accurately the spatial variation in NH3 concentration would require a very large number of monitoring stations. Such quantification is an important requirement for assessing the impacts of acidifying and nitrogen entrophicating 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 resolution. The model treats the vertical concentration gradient explicitly with 33 layers, as well as provides a description of the land-use dependence of dry deposition using a resistance formulation dependent on major surface landcover classes. By treating vertical diffusion explicitly, the model provides the capability for examining the spatial variability at a finer scale than models assuming instantaneous mixing of emissions, and also avoids the need for air concentration and deposition correction factors. Model results are presented in the form of maps of annual mean NH3 concentration and net dry deposition, and a comparison is made with NH3 concentrations measured by a national monitoring network. A total annual budget for reduced nitrogen is given which shows the directional dependence of both total deposition and export of reduced nitrogen. (C) 1998 Elsevier Science Ltd. All rights reserved.