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 1020% for certain longterm 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.
Pasquill F. (1977), Atmospheric Diffusion. The dispersion of windborne material from industrial and other sources. 2nd edition John Wiley & sons
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) longterm average estimates.
(d) extreme ratios 'estimate/actual'

Conditions 
Distance of travel (km) 
Property estimated 
Uncertainty 

Overcast steady wind
Generally unstable 
0.11 <3 
Peak of timemean (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 timemean (1 hr) crosswind distribution. 
d 2. 

Any except stagnation  10  Longterm
spatial mean
Individual site, value averaged over few hours Extreme (1% of occasions) of fewhour 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 