Vertical distribution and fluxes of ammonia at Great Dun Fell

As part of the study of the ammonia budget over Great Dun Fell, measurements of fluxes of gaseous ammonia (NH3) with the hill surface (grass moorland and blanket bog) were made using micrometeorological techniques, to provide information on NH3 removal by the hill surface and on vertical concentration gradients. Measurements of vertical concentration, χ, profiles of NH3 concentration were coupled with turbulent diffusivities to determine fluxes, Fg deposition velocities, and canopy resistances, Rc to uptake by the ground. Consistent with published measurements for this site, NH3 was generally found to deposit efficiently to the vegetation canopy, with mean Rc of 5 and 27 s m− for example days shown. However, short periods of NH3 emission from the moorland were also observed at small χ (< 0.3 μgm−3). Under these conditions of bidirectional exchange, the Rc model does not adequately describe the exchange process, and an alternative model was applied, which treats both the resistance for deposition to leaf cuticles, Rw, and exchange with a “stomatal compensation point,” ξs. This link between y and direction of Fg may result in an important effect of cloud processing: depletion of χ by in-cloud reaction would be expected to favour NH3 emission from down-wind agricultural land and moorland, though emission from the hill itself during immersion in cloud is unlikely. Comparison of two measurement techniques to determine air concentrations (batch wet rotating denuder, inlet 0.5 m height; continuous wet denuder, inlets 0.3, 2 m heights) showed acceptable agreement, although because vertical concentration gradients were large (small Rc) the height of sampling had a substantial effect. Vertical gradients are also relevant to the use of the measured concentrations as estimates of NH3 in the air mass passing over the hill, for modelling atmospheric budgets. Where NH3 deposition occurs at the maximum rate, concentrations measured at 1 m require a 35% correction in neutral conditions when scaling to a reference height of 10 m.