Emission, dispersion and local deposition of ammonia volatilised from farm buildings and following the application of cattle slurry to grassland
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Emissions of ammonia (NH3) into the atmosphere, principally from agricultural sources, have been implicated in the pollution of forests, moorlands and grasslands, through the subsequent deposition of reduced nitrogen (NHx -N). Consequently, legislation has been implemented to control both the transboundary transport and local environmental impacts of NHx. This thesis investigates the emission, dispersion and local deposition of NH3 from two sources that are major components of national NH3 emissions inventories, slurry applied to grassland and naturally ventilated cattle buildings. A N balance method was identified for determining the time-average deposition of NH3 downwind of a farm building, whilst an adapted micrometeorological flux-gradient technique was developed for estimating local deposition downwind of slurry spreading. This method used an analytical atmospheric dispersion model to provide advection corrections to the standard flux-gradient method. The UK-ADMS model, which incorporates a reasonably detailed treatment of building effects, was identified for use in determining the near-field dispersion from naturally ventilated farm buildings. Eight field experiments were conducted to determine the emission, dispersion and local deposition of NH3 volatilised from slurry applications. Emission fluxes during the initial runs following slurry spreading were found to depend on friction velocity, relative humidity and rainfall. Local deposition, at sufficient rates to affect local deposition budgets, was not found to occur during near-freezing conditions or following the application of fertilisers. Local deposition velocities during other periods were found to depend on the latent heat flux, temperature and the roughness length. During such periods, 14 - 18 % of the emitted NH3 was estimated to deposit within 50 m of the source. Experiments were also conducted at two naturally ventilated farm buildings, the Silsoe Research Institute Structures Building and a working dairy farm. Ammonia emission factors were determined for the main building and slurry lagoon at the dairy farm. A novel application of a model back-calculation method was applied to determine the emission from the lagoon. Dispersion of NH3 from both sites was found to be adequately modelled using UK-ADMS. Approximately 2 % of the emitted NH3 deposited within 100 - 150 m of each building. Time averaged deposition velocities calculated from the farm building studies confirmed that NH3 was deposited to the leaf surfaces and uptaken across the leaf cuticle. Temperature dependent exchange rates were also indicated by the results of the farm building experiments, with NH3 uptake being regulated by the assimilation potential of the plant. The experimental results demonstrated that deposition around both sources could lead to local critical load exceedances. These were only estimated to occur within a few tens of metres downwind of slurry spreading whilst critical load exceedances were predicted at distances of up to 100 m or more downwind of the farm building. The temporal variability in local recapture found in these experiments, particularly for farm buildings, suggests that seasonal variability in the treatment of NH3 emission and deposition should be included in atmospheric transport models. Furthermore, it is possible that transboundary transport of NHx may increase during winter periods with peak housing emissions.
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