Annex 5.3 Limitations and uncertainties in meteorological estimates using dispersion models

Page Last modified 20 Apr 2016

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Guidance Report on preliminary assessment under EC air quality directives

Limitations and uncertainties of model prediction, related to the meteorological data are mainly due to:

The special nature of those meteorological conditions and circumstances which occasionally lead to the worst pollution episodes;

The availability in required resolution of the essential meteorological information;

The basic inaccuracy and unrepresentativeness of the calculations of dispersion, even in the circumstances for which the dispersive action of the atmosphere is best understood.

Pasquill (1977) having in mind all the difficulties for collecting all the necessary meteorological information or the limitation of the dispersion relations only to idealised situations of air flow, constructed a very useful table with the best levels of accuracy which may be expected even in relatively simple circumstances. The values presented in this table (displayed on the next page) were based on comparison between measurements and simple dispersion formulas.

From this table an accuracy of 10% may be envisaged for ensemble averages in the most ideal combinations of circumstances, or perhaps 10-20% for certain long-term averages in less ideal circumstances (excluding the special cases of stagnant or confined airflow), but in many circumstances of practical interest the uncertainties may at best be several tens per cent statistically and factors of two or more individually.

Reference

Pasquill F. (1977), Atmospheric Diffusion. The dispersion of windborne material from industrial and other sources. 2nd edition John Wiley & sons

Table

Uncertainty in pollution estimates (for a passive gas) derived from Pasquill (1977). The figures are fractional deviations, from the mean or actual values, of:

(a) ensemble average estimates in particular conditions (e.g. of wind speed or stability).

(b) individual values (r.m.s.) of cases forming the ensemble averages.

(d) extreme ratios 'estimate/actual'

Nature of source and terrain	Conditions	Distance of travel (km)	Property estimated	Uncertainty
1. Ground level source on flat terrain	Overcast steady wind Generally unstable	0.1-1 <3	Peak of time-mean (few minutes) crosswind distribution	a <0.1 b 0.1 b 0.2
2. Moderately elevated plume (50-100m) over flat terrain	Generally unstable	1	Distance of maximum. Magnitude of maximum	b 0.3 b 0.35
3. Buoyant elevated plume over irregular terrain	Unstable or windy Any except stagnation	10	Distance of maximum hourly average. Maximum hourly average. Long term average maximum	a 0.3 b 0.45 a 0.35 b 0.5 c 0.1
4. Plume in mixing layer	Unstable with inversion, definite wind field	100	Peak of time-mean (1 hr) crosswind distribution.	d 2.
5. Multiple sources in urban industrial complex	Any except stagnation	10	Long-term spatial mean Individual site, value averaged over few hours Extreme (1% of occasions) of few-hour average at individual site	c 0.2 b 1. d 2.
6. As in 5, but source inventory not specified	Any except stagnation	10	Daily mean averaged over 100 sites at 1 site	b 0.25 b 0.35