Annex 5.3 Limitations and uncertainties in meteorological estimates using dispersion models
- 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.
(c) long-term average estimates.
(d) extreme ratios 'estimate/actual'
|
Conditions |
Distance of travel (km) |
Property estimated |
Uncertainty |
|
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 |
|
Generally unstable | 1 | Distance of
maximum.
Magnitude of maximum |
b 0.3
b 0.35 |
|
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 |
|
Unstable with
inversion,
definite wind field |
100 | Peak
of time-mean (1 hr) crosswind distribution. |
d 2. |
|
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. |
|
Any except stagnation | 10 | Daily mean averaged over 100 sites at 1 site | b 0.25
b 0.35 |
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