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UKWIN’s Emissions Dispersion Modelling Appraisal revised

UKWIN’s Emissions Dispersion Modelling Appraisal focuses on modelling emissions from point sources and on emissions from incinerators in particular including the relative performance of competing modelling systems ADMS and AERMOD, and it has been updated.

Various UKWIN members are currently looking to carry out their own baseline monitoring of air quality and soil quality prior to incinerators coming on-line to allow for comparisons with future measurements once a facility becomes operational. Another interesting development is the launch of Plumeplotter which uses the AERMOD modelling system to predict emissions based on current weather patterns and assumed incinerator emission levels provided by the incinerator company as part of the Planning Application or Environmental Permit (EP) processes.

The conclusions of the Appraisal are that:

1) Emissions modelling is a highly complex process involving complex algorithms and requiring the input of large volumes of accurate data. Without the latter, and in the context of incinerator emissions, neither AERMOD nor ADMS can be expected to produce representative results. In any case, both models would appear to underestimate the effects of source contributions at some off site locations, often, but not always, by relatively small sources of magnitude.

2) One situation in which ADMS and AERMOD can produce significantly different results appears to be that in which ground levels, in the vicinity of an incinerator stack, can be higher than the top of the stack. It is recommended that, in such situations, Members apply pressure on the Planning Authority to agree to the to set up an Air Quality Monitoring Area covering the downwind side of the incinerator should an EP be granted and on the EA, as part of the EP consultation process, to ensure that EP applicants are not allowed simply to opt for the modelling that produces the most favourable results (for the applicant).

3) In the majority of UK cases, the modelled process contributions from incinerators at point locations are small relative to background pollution levels (although there are exceptions). This makes it difficult to prove or disprove modelling results other than in the medium term (eg over 12 months, at least, during which a full range of measured results must be taken). Even in relatively polluted areas, the modelled process contributions from incinerators, combined with background levels, are unlikely to reach EQS / EAL levels.

4) From UKWIN’s position, the basis on which modelling results are presented in Planning Applications and EP applications, and in Decision documents, can be argued as leaving much to be desired. The practice is to address pollution levels in ‘mean’ terms such that there is no indication of peaks and troughs as may result, for example, from variations in emission levels from incinerators themselves and through meteorological variations. It tends to follow that however accurate the results a model may be capable of, the way in which these are presented is such that they may lose much of their potential significance.

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