Dispersion Modelling-2 2

[techv]

I have some basic doubt on the Stability Class. I know my understanding of it has gone wrong but would like to get it right with your help.

Pasquill Index (A-E), a mesure of Atm. Turbulence, controls a parameter (Sigma x and y) in Gaussian dispersion equation. It is noted that 'A' is for higher Turbulence giving highest dispersion. But from the index selection table (for stability class), it is noted that for daytime, Class-A is considered if wind speed is <2 m/s while Class-C is when it is >6 m/s. My question is "Why higher tubulance when the wind speed is lower"?

 

[Flareman]

Techv

Re your earlier post
The basic characteristic is given as X * U / Q
X = concentration
U = wind speed
Q = total flux rate of pollutant
so there is a notional proportionality and this characteristic can be used as an eigenvector

Regarding the Turner stability categories
A = unstable
through
D = neutral
through
F = stable
relate to the natural vertical movement of air upward so any wind turbulence moves both ends toward D (neutral). There is some debate about reorganising the categories but it seems to me that they are "iffy" at best and only useful as generalities rather than definitives.
There really is a lot of work available on the internet. Do searches for stability categories, Adiabatic lapse rate, and Potential temperature gradient.
I have a nomograph posted at

http://www.geocities.com/flareman_xs

|main index|downloads| which may help a little.

regards
David

 

[mbeychok]

techv:

I don't know what table of Pasquill stability classes you are using. But if you look at the most common one at:

http://www.air-dispersion.com/formulas.html#stability

You will note that although stability class A is defined as having a windspeed of <2 m/s, it is also defined as having "Strong" daytime incoming solar radiation. In plain English, that means a calm hot day ... which induces a great deal vertical motion that creates turbulence.

If it helps you to know, the Pasquill stability classes have been in use for over 50 years or more. Some of the latest, most complex air dispersion modeling use other methods of characterizing the atmospheric turbulence.

If you will look at the complete equation for Gaussian air dispersion modeling at:

http://www.air-dispersion.com/gaussian.html

You will note that the sigma y and sigma z parameters are

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[mbeychok]

techv:

I don't know what table of Pasquill stability classes you are using. But if you look at the most common one at:

http://www.air-dispersion.com/formulas.html#stability

You will note that although stability class A is defined as having a windspeed of <2 m/s, it is also defined as having "Strong" daytime incoming solar radiation. In plain English, that means a calm hot day ... which induces a great deal vertical motion that creates turbulence.

If it helps you to know, the Pasquill stability classes have been in use for over 50 years or more. Some of the latest, most complex air dispersion modeling use other methods of characterizing the atmospheric turbulence.

If you will also look at the complete equation for Gaussian air dispersion modeling at:

http://www.air-dispersion.com/gaussian.html

You will note that the sigma y and sigma z parameters are functions of the Pasquill stability class and therefore of the wind speed. So there really isn't a linear relation between windspeed and C.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[SylvestreW]

From “Air Pollution, An introduction,” by Colls

On cloud-free days, solar radiation will warm the ground during the day making the lowest air layers hotter and causing these layers to rise vertically, mixing with upper layers.
As cloud cover increases, daytime heating of the ground is reduced resulting in increasing stability.
A higher wind speed during the day suppresses the rising air which leads to increasing stability.

On cloud-free nights, net upwards thermal radiation cools the ground making the lowest air layers cooler and thus more stable.
As cloud cover increases, nighttime cooling of the ground is reduced resulting in decreasing stability.
At night, a higher wind speed reduces the upward thermal radiation, resulting in decreasing stability.

If you are working in this field, I'd suggest buying Milton's book. It's an excellent source that will answer almost any question you have on air dispersion.

-
Syl.

 

[mbeychok]

SylvestreW:

Wow! Thanks for the plug! You made my day.

Milton Beychok
(Visit me at www.air-dispersion.com)
.