An air dispersion model is used to determine the impact of stack or other air emissions on the local ambient air quality.  Environmental Efficiency uses AERMOD and other software to investigate this air modeling.

 

Air Dispersion Model: Investigating the impact of air emissions

Air emissions from stacks, bag filters, waste water treatment plant and other sources can cause health and nuisance problems not only in the locality but sometimes 10’s of kms away.  These air emissions (termed the process contribution), when added to the ambient air (termed the background contribution) may cause the EU air quality standard to be breached.  This may prevent the proposed development proceeding.

To determine whether the proposed development’s air emissions will cause a breach of the EU air quality standards requires a prediction of the process contribution at various distances from the development.  This air modeling prediction is evaluated for a range of wind directions and atmospheric conditions. The predicted process contribution is then added to the background contribution to determine the total concentration.  This total is then compared to the air quality standard.

Air Modelling: Use of screening models

Running an air dispersion model is time consuming as typically three years of meteorological data is used together with data regarding the topography of the area.  Environmental Efficiency’s initial approach is to run a screening model.  This will assume worst case weather conditions.  If the screening model predicts no adverse impact on air quality then regulators will accept that.  This results in a significant cost saving.

If the screening model indicated that local air quality is breached, then a full air dispersion model is required.  Environmental Efficiency use AERMOD to perform air modeling.  This program is the industry standard. Our staff includes certified AERMOD and CALPUFF modellers.

What if the results are too high?

Where the results from an Air Dispersion model show high concentrations, the effect of increasing stack height, increasing gas velocity or other modifications intended to reduce concentrations can be investigated by our team.

What guidance is available?

The EPA Guidance Document AG4 is the key guidance document in Ireland for air dispersion modelling. In the UK the equivalent document is the Environmental Agency’s Air Dispersion Modelling Requirements.