Comparison of spatial patterns of ammonia concentration and dry deposition flux between a regional Eulerian chemistry-transport model and a local Gaussian plume model Abstract Agricultural activities are the principal sources of ammonia (NH3) emitted into the atmosphere. High ammonia deposition flux may impact sensitive ecosystems. Regional models of NH3 dispersion, transport and deposition may under- or overestimate NH3 fluxes. We compared NH3 dry deposition fluxes simulated with local and regional models on different theoretical scenarios characterised by varying the values of several input factors: grid cell sizes, characteristics of the NH3 sources such as location and emission rate, characteristics such as canopy resistance (Rc) or roughness length (z0) at the NH3 sinks, and meteorological conditions such as wind speed and direction. Our results showed that, for a given grid cell size, both models provide similar predictions of average NH3 concentration and dry deposition flux over the whole simulation domain. A sensitivity analysis of NH3concentration and dry deposition flux to wind speed and to surface resistance also showed a similar behaviour between both models. However, the differences of model formalism and changes in the values of the input factors, especially grid cell size and vertical resolution, provide different spatial patterns of NH3 dry deposition flux and concentration. Our results would suggest that regional models operating with large grid cell sizes (e.g. larger than 1 km) could not predict accurately patterns of NH3 dry deposition fluxes close to the sources (e.g. a few tens or hundreds of metres) on heterogeneous landscapes in terms of NH3 fluxes.