Acknowledging the underestimate of short term peak concentrations if these fluctuations are ignored, modellers have adopted a variety of approaches. Factors are sometimes applied to convert between different averaging times. This is a rather crude approach as the factors may be based on monitored data that reflect contributions from many different sources and yet the factors may be applied to a single stack. The factors should be functions of the height of the source, the statistic in question, receptor height, distance of the receptor from the source and meteorology. In practice just one factor is often used to cover all cases. Other modellers account for a decreased averaging time by adjusting the amount of meandering of the wind direction. In the ADMS 3 dispersion model the fluctuations in concentration are calculated and the results presented in statistical terms: the standard deviation of concentration sc, short term percentile concentrations and the probability of exceeding threshold concentrations. For long term calculations, probability distributions calculated for individual meteorological conditions are combined to predict the expected number of exceedences of a threshold concentration during one year.
Turbulent Fluctuations And Their Use in Estimating Compliance Standards And In Model Evaluation (PDF)
Most practical dispersion models used for regulatory purposes assume that the meteorological conditions are constant over the averaging period and calculate ensemble average concentrations. In practice, although the meteorological conditions may be constant, during the averaging period there will be short time scale variations due to boundary layer turbulence. The turbulent fluctuations will lead to variations in short term concentration.