Exchange of atmospheric ammonia (NH3) with vegetation is characterized by the juxtaposition of sources and sinks at a landscape level. Such situations lead to a large fraction of the landscape being exposed to local advection effects that if not accounted for, introduce errors in standard micrometeorological measurements of NH3 exchange with the surface. In this study, a simplified dispersion-exchange model for NH3 (Flux Interpretation by Dispersion and Exchange over Short Range, FIDES) is evaluated and used to assess the advection fluxes at 260 m downwind of an isolated pasture, grazed with sheep, using the measurements of a classical three-point NH3 gradient system located on adjacent moorland. The method consists of fitting the measured and modeled concentration profile by adjusting at the same time the emission strength of the local source and the exchange rate of NH3 to the moorland area downwind. A local dispersion and surface exchange model such as FIDES has proved to be a valuable tool to estimate advection corrections, given sound estimates of background NH3 concentrations, source location, and standard meteorological parameters. According to the model results the advection fluxes at the moorland measurement site, at 1.0 m height and 260 m downwind of the grazed pasture, were positive. For 80% of the situations they ranged between 30% and 60% of the vertical fluxes. In stable conditions the advection fluxes were large and more sensitive to the surface exchange parameters. These results demonstrate that if not accounted for, advection fluxes may lead to a severe underestimate of the NH3 deposition to seminatural ecosystems, such as moorland, in the vicinity of ground level agricultural sources. ¿ 2001 American Geophysical Union