| Dispersion Modelling Methodology | WNP Processing Plant Air Discharge Assessment | 4397169-66885702-113 | 16/06/2022 | 19 Sensitivity: General Figure 5-3. Buildings incorporated into the CALPUFF model predictions 5.10 Other Dispersion Modelling Parameters Other CALPUFF dispersion modelling parameters selected for the assessment are; ● Dispersion coefficients computed from micrometeorological variables ● Transitional plume rise (Briggs) ● The PDF (probability density function) method used for sigma-z in the convective boundary layer ● Partial plume penetration of the mixed layer for area and point sources. 5.11 99.9 Percentile 1-hour Average Concentrations The use of 99.9 percentile, 1-hour averages is a standard dispersion modelling convention used to assess predicted pollutant 1-hour average concentrations. The 99.9 percentile 1-hour average concentration corresponds to the ninth highest 1-hour average concentrations predicted at each receptor point over the simulated meteorological year. The use of the 99.9 percentile, 1-hour average is intended to filter out improbably high concentrations that may be predicted due to extreme meteorological events. The Good Practice Guide for Atmospheric Dispersion Modelling (MfE, 2004) advises that the predicted 99.9 percentile concentration is the maximum ground-level concentration that is likely to occur.
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