Detailed modelling of ammonia emissions stage 2 (GN 036)
When Stage one predictions, without the inclusion of ammonia deposition, indicate that the relevant critical levels are exceeded, appropriate concentration dependent deposition velocities covering the range of predicted concentrations (including background) need to be selected from the figures below.
These can then be included into the model, for example using the spatially varying deposition feature in the ADMS software (see the ADMS user manual for details), and the model re-run. The figures below present a summarised relationship between ammonia dry deposition velocity and long-term ground level concentration based on information from a number of studies.
Total ammonia concentration of 10 micrograms per cubic metre or less
Where the total ammonia concentration, which will include the Ammonia from your own and other development and background Ammonia, is 10 micrograms per cubic metre or below, the deposition velocity for short vegetation is 0.02 microgram per second and for tall vegetation the deposition velocity is 0.03 micrograms per second.
Total ammonia concentration of between 10 and 20 micrograms per cubic metre
Where the total Ammonia concentration which will include the Ammonia from your own and other development and background Ammonia is between 10 and 20 micrograms per cubic metre, the deposition velocity for all vegetation is 0.015 metre per second.
Total ammonia concentration of between 20 and 30 micrograms per cubic metre
Where the total Ammonia concentration, which will include the Ammonia from your own and other development and background Ammonia, is between 20 and 30 micrograms per cubic metre, the deposition velocity for all vegetation is 0.01 metre per second.
Total ammonia concentration of between 30 and 80 micrograms per cubic metre
Where the total Ammonia concentration, which will include the Ammonia from your own and other development and background Ammonia, is between 30 and 80 micrograms per cubic metre, the deposition velocity for all vegetation is 0.005 metre per second.
Total ammonia concentration above 80 micrograms per cubic metre
Where the total Ammonia concentration, which will include the Ammonia from your own and other development and background Ammonia, is above 80 micrograms per cubic metre, the Deposition Velocity for all vegetation is 0.0035 metre per second
Wet deposition of ammonia
Wet deposition of ammonia is not significant compared to dry deposition close to the source. It is recommended that wet deposition of ammonia emitted at the development is not considered in the assessment. Consultants may adopt an alternative approach to estimating ammonia concentration and deposition to that recommended above but this will need to be properly justified in the modelling report.
Uncertainty in the recommended approach
Modelling uncertainty may arise from a number of different sources:
- atmospheric turbulence
- the model itself which describes the transport of pollutants through the atmosphere
- input data such as meteorological information, surface roughness, particularly estimates of emission rates.
The overall uncertainty in a model prediction of air concentration will depend strongly on the situation being modelled and the model being applied, but is typically within 50% of measured values for the annual average concentration. Estimating the deposition of ammonia adds significant additional uncertainty.
Stage one of the recommended approach is likely to be conservative in concentration or deposition flux because the depletion process is not considered. The recommended deposition velocities in the figures above are based on the limited data available.
There is currently insufficient information to do an uncertainty analysis, however the proposed empirical approach does allow both the concentration dependency of the deposition velocity and the ammonia depletion of the plume to be taken into account. The use of the long term average concentration also avoids the complexity encountered in short term (i.e. hourly) modelling. The recommended method does not consider ammonia wet deposition from a farm, although this is generally considered to be small.