The Importance of Modelling Fine-Scale Spatial Variability and Local Deposition in the Assessment of Ammonia Emissions A contribution to subproject GENEMIS

Atmospheric ammonia (NHg) is a pollutant with effects both from long-range atmospheric transport and local dispersion away from sources. While coursescale modelled NHs emission inventories (e.g. 50 km grid) might be sufficient for long-range transport models (e.g. Barrett and Berge, 1996), a problem arises in that the sources, primarily agriculture, occur in the rural environment, closely intermixed with the sink areas, mostly sensitive semi-natural ecosystems and forests. On the one hand, critical loads estimates, representing the sensitivity of ecosystems, are very spatially variable (often mapped to the nearest 1 km), while in many rural areas the spatial gradients of NHg emission vary on landscape scales (e.g. 5 km) and field scales (sub-1 km). Where NHs emissions are mapped at with low resolution, the discrepancy of scales results in a misleading assessment of the ecological impacts. Maps may apparently show fine detail (through the critical loads), but much of the variability due to NHg emissions is actually lost.