The mass budget of atmospheric ammonia in woodland within 1 km of livestock buildings

The emissions of ammonia from point sources close to the ground and the rap id deposition to vegetation lead to very large horizontal gradients in both concentration and deposition close to sources. The sources are primarily l ivestock related and some of the largest terrestrial N inputs occur in the proximity of intensive production facilities. This study quantifies the loc al fate of livestock NH3 emissions from a poultry farm using measured NH3 c oncentrations and the relationship between canopy resistance (r(c)) and amb ient NH3 concentration from intensive flux measurements. The results of the measured concentrations and deposition are compared with those obtained us ing a dispersion model of the emission, transport and deposition close to p oint sources. The results of the measurements showed annual mean concentrat ions in the range 23 mu g m(-3) to 63 mu g m(-3) at a distance of 15 m from the source, declining to background concentrations for the region of 1 to 2 mu g m(-3) at a distance of 276 m and in reasonable agreement with the mo del. The deposition of NH3-N estimated from the measurements, declined from 42 kg N ha(-1) at 15 m to 5 kg N ha(-1) at 270 m and was smaller than the deposition estimated using the dispersion model by about a factor of two. A nnual deposition within 270 m of the source to the woodland amounted to 155 kg N, and represented 3.2% of annual emissions from the poultry unit. The comparison between measurements and the model indicated substantial uncerta inty in the deposition budget values, but supports the overall conclusion t hat local deposition of NH3 to woodland within 300 m of the source represen ts a small fraction (3% to 10%) of the local emission source.