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FAQ 89 – HMIP D1 Stack Height Calculation

Where can I find details of the HMIP D1 Method?

HMIP 1993 ‘Guidelines on Discharge Stack Heights for Polluting Emission. Technical Guidance Note D1 (Dispersion)’ ISBN 0 11 752794 7 is now out-of-print, but is available from the British Library. It provides a simple but versatile method for calculating the minimum permissible chimney height to safeguard against short-term air quality impacts, for any pollutant species. It allows for building downwash effects but not terrain effects.

The D1 method uses the 98th percentile envelope of meteorological conditions. Therefore, the D1 method could under-estimate the chimney height required for compliance against the current short-term air quality objective for NO2 (which approximates to a 99.8th percentile) or the 15-minute mean SO2 objective (which approximates to a 99.9th percentile).

Care should be taken in using the D1 method, in terms of defining the local background (Bc) and the current air quality guideline value (Gd). The default values set out in the HMIP document (dated 1993) are out-of-date. For Gd, the current statutory short-term Air Quality Strategy objectives should be used instead of values provided in Table 1 of the D1 Guidance. For Bc, local measured or estimated relevant percentile of the short-term background concentrations should be used instead of values provided in Table 2 of the D1 Guidance. Typically, these can be calculated from hourly/daily monitoring data obtained from Automatic Urban and Rural Network (AURN) or London Air Quality Network (LAQN) monitoring stations, or other local monitoring stations.

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