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 rapid deposition to vegetation lead to very large horizontal gradients in both concentration and deposition close to sources. The sources are primarily livestock related and some of the largest terrestrial N inputs occur in the proximity of intensive production facilities. This study quantifies the local fate of livestock NH3 emissions from a poultry farm using measured NH3 concentrations and the relationship between canopy resistance (rc) and ambient NH3 concentration from intensive flux measurements. The results of the measured concentrations and deposition are compared with those obtained using a dispersion model of the emission, transport and deposition close to point sources. The results of the measurements showed annual mean concentrations in the range 23 μg m−3 to 63 μg m−3 at a distance of 15 m from the source, declining to background concentrations for the region of 1 to 2 μg m−3 at a distance of 276 m and in reasonable agreement with the model. 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. Annual 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 uncertainty in the deposition budget values, but supports the overall conclusion that local deposition of NH3 to woodland within 300 m of the source represents a small fraction (3% to 10%) of the local emission source.