Modelling the spatial distribution of agricultural ammonia emissions in the UK

Accurate models of the spatial distribution of ammonia (NH3) emissions are an essential input to models of atmospheric transport and deposition. This is especially important when resulting deposition maps are used to calculate patterns of critical loads exceedance or to determine suitable abatement measures. A new methodology has been developed to model the distribution of agricultural ammonia emissions and is applied here for the UK. The model employs a specific spatial weighted redistribution of NH3 emission sources onto suitable landcover types at a 1-km grid level. Key input data to the model are agricultural census data, a satellite-based landcover map and estimates of NH3 emission source strength. The model provides more realistic spatial NH3 emission estimates than previous models, especially for semi-natural/natural areas by relocating emission sources from extensively used upland areas to the more intensively farmed lowland areas within each parish. At present the model results are summarised as maps at a 5-km grid resolution to reduce uncertainty in the spatial location of NH3 sources. Compared with coarser resolution estimates this also provides a more accurate link to critical load exceedances. The more accurate redistribution also reduces the apparent critical loads exceedance on upland areas. Results are presented and compared for 1988 and 1996. These show broadly similar patterns between the years, although substantial local changes have occurred, particularly for intensive livestock farming. The model has been used to generate initial spatially resolved abatement scenarios and provides a general tool for locating NH3 emissions that could be applied to other regions.