Detailed modelling of ammonia emissions stage 1 (GN 036)
Use an appropriate dispersion model to predict the annual average ammonia concentration in air with the deposition module switched off. Where necessary the effects of building downwash and terrain should also be considered.
Once the modelling run is completed, process the concentration field using the following equation to calculate the ammonia dry deposition flux,
F = Vd x C
Where: C is predicted ammonia annual mean concentration in micrograms per cubic metre (µg/m3).
Vd is ammonia dry deposition velocity, (Vd= 0.02 metres per second for grassland and 0.03 metres per second for forest and woodland).
F is deposition flux (micrograms of ammonia per square metre per second - µg NH3 m-2s-1), which can be converted to the unit of kilograms of nitrogen per hectare per year (kg N ha-1year-1), by multiplying a factor of 259.7.
If there are insufficient computing resources available to process the complete concentration field as described above, the dispersion model can be used to provide concentration values at specific locations across the sensitive site (including the point of maximum ground level concentration) and the deposition flux estimated at these points.
Monitoring/modelling data on the edge of an ombrotrophic bog gives a concentration of 0.8 micrograms of ammonia per cubic metre (µg NH3m-3). The appropriate deposition velocity for this type of habitat is 0.02 metres per second (m s-1).
- 0.8 micrograms per cubic metre multiplied by 0.02 metres per second = 0.016 micrograms per square metre per second, or 0.8 µg m-3 x 0.02 m s-1 = 0.0160 µg m-2s-1
- 0.016 micrograms per square metre per second multiplied by 31,536,000 seconds multiplied by 10,000 square metres = 5,045,760,320 microgram per hectare per year, or 0.0160 µg m-2s-1 x 31,536,000 s x 10,000 m2 = 5,045,760,320 µg ha-1 y-1
- Divide by 10 to the power of 9 to get kilogram per hectare = 5.05 kilogram of Ammonia per hectare per year
- Correct NH3 (ammonia) to N (nitrogen): 5.05 kilogram Ammonia per hectare multiplied by 0.82 = 0.82 4.16 kilograms Nitrogen per hectare per year, or 5.05 kg NH3 ha-1 x 0.82 = 4.16 kg N ha-1y-1.
If the predictions from Stage one indicate that the critical level is not exceeded then no further work is required.