Modelling effects of wind speed and surface cover on ammonia volatilization from stored pig slurry

A mechanistic model for calculating ammonia losses from stored slurry is developed. The model is tested against ammonia loss data determined with wind tunnels covering pilot slurry tanks. Wind speed within the tunnels was adjusted from 1.8 to 10 m s ⁻¹. There was agreement between the predicted and measured ammonia loss rates from uncovered slurry and slurry covered by surface crust or a 15 cm straw layer. A surface resistance parameter $\text{(r}{}_{\mathrm{<mtext>c</mtext>}}\text{)}$ is included in the transfer coefficient of the model. It is shown that surface resistance is of great importance if the slurry is covered by a surface crust or a 15 cm straw layer. Ammonia losses increased curvilinearly with wind speed approaching a maximum at about 6 m s⁻¹. The transport coefficient for ammoniacal nitrogen in the stored slurry was found to be about 10-times higher than the diffusion coefficient of ammonium in water. This is probably caused by convection.