Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion-exchange model

Exchange of atmospheric ammonia (NH3) with vegetation is characterized by t he 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 micrometeo rological measurements of NH3 exchange with the surface. In this study, a s implified dispersion - exchange model for NH3 (Flux Interpretation by Dispe rsion and Exchange over Short Range, FIDES) is evaluated and used to assess the advection fluxes at 260 in downwind of an isolated pasture, grazed wit h sheep, using the measurements of a classical three-point NH3 gradient sys tem located on adjacent moorland. The method consists of fitting the measur ed and modeled concentration profile by adjusting at the same time the emis sion strength of the local source and the exchange rate of NH3 to the moorl and area downwind. A local dispersion and surface exchange model such as FI DES has proved to be a valuable tool to estimate advection corrections, giv en sound estimates of background NH3 concentrations, source location, and s tandard meteorological parameters. According to the model results the advec tion fluxes at the moorland measurement site, at 1.0 m height and 260 in do wnwind of the grazed pasture, were positive. For 80% of the situations they ranged between 30% and 60% of the vertical fluxes. In stable conditions th e advection fluxes were large and more sensitive to the surface exchange pa rameters. These results demonstrate that if not accounted for, advection fl uxes may lead to a severe underestimate of the NH3 deposition to seminatura l ecosystems, such as moorland, in the vicinity of ground level agricultura l sources.