Løfstrøm, P. 2008: "OML-spredningsberegninger på basis af 10 års meteorologi i relation til Luftvejledningen" Danmarks Miljøundersøgelser, Aarhus Universitet. 35 s.- Faglig rapport fra DMU nr. 697.

 

Summary

In the Danish Air Quality Guideline (Luftvejledningen) the Danish Environmental Protection Agency recommends that the OML atmospheric dispersion model is used for compliance checking for industrial facilities. The current procedure to compare concentration levels with the limit value is based on the use of one years' worth of meteorological data.

 

In order to obtain a better statistically founded basis for assessing the level of air pollution, the present study examines how the current use of one year of meteorological data can be extended to 10 years.

 

The current use of only one year of data occasionally causes problems in the interpretation of the geographical distributions of the concentration levels. That is because different levels in different directions from the source might be due to random meteorological conditions, or due to the source configuration. The procedure is based on the statistical parameter known as the maximum monthly 99-percentile for one meteorological year (Kastrup Airport 1976).

 

The present study uses 10 years of meteorological data from two Danish airports in Kastrup and Ålborg. These data are used for dispersion calculations for different types of point sources with varying stack heights, plume rise and possible nearby buildings.

 

In order to obtain a new statistical parameter for the exposure - that corresponds to the current level of the maximum monthly 99-percentile and can be compared to the C-value (‘limit value’) of the Guideline – the new statistics are based on the 120 monthly 99-percentiles for the 10 years of calculations. The 99-percentiles are ranked, and selected statistics are assessed, e.g. the maximum of all monthly 99-percentiles, the 4th highest monthly 99-percentile, the 8th highest monthly 99-percentile, the 12th highest monthly 99-percentile, and the average monthly 99-percentile.

 

The study recommends that in future assessments the statistical parameter to be compared with the C-value of the Guideline is the 4th highest monthly 99-percentile based on 10 years of meteorological data from Ålborg Airport for the period 1974 to 1983.

 

The new percentile provides a more robust and simple assessment of directional impacts - an assessment that is not affected by random meteorological conditions. It assures that computed concentrations can be taken at face value, avoiding the need for interpretation of the causes of a particular concentration pattern.

 

As a spin-off of the study it is recommended that - for substances in the Guideline where the C-values are based on dosage - the C-value should be used in a different way than at present. It is recommended that 1/40 of the C-value should be compared to the long-term (10 year) mean value calculated with OML.

 

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