Read the statements given below.
(i) Value of the wind profile exponent for the 'very unstable' atmosphere is smaller than the wind profile exponent for the 'neutral' atmosphere.
(ii) Downwind concentration of air pollutants due to an elevated point source will be inversely proportional to the wind speed.
(iii) Value of the wind profile exponent for the 'neutral' atmosphere is smaller than the wind profile exponent for the 'very unstable' atmosphere.
(iv) Downwind concentration of air pollutants due to an elevated point source will be directly proportional to the wind speed.
Select the correct option.
Explanation
a) For statement 4 & statement 2
According to Fick's law of diffusion
It is assumed that the plume has a Gaussian distribution in both z ( vertical ) and y (horizontal ) direction
The concentration of a gas or aerosol ( < 20 μ ) calculated at ground level for a distance downward x is given by
\({C_{x,y}} = \frac{Q}{{\pi \times U{\sigma _z} \times {\sigma _y}\;}}{e^{\left( {  \frac{1}{2}{{\left( {\frac{H}{{{\sigma _z}}}} \right)}^2}} \right)}} \times {e^{\left( {  \frac{1}{2}{{\left( {\frac{y}{{{\sigma _y}}}} \right)}^2}} \right)}}\)
Where,
C is the concentration of pollutant
U is mean wind speed
σ_{y }is the standard deviation of horizontal plume concentration, in the crosswind direction
σ_{x }is the standard deviation of horizontal plume concentration, in the vertical direction
H is effective stack height (m)
X is the downward distance along the plume mean centerline from a point source (m)
Y – crosswind distance from the centerline of the plume (m)
From the relation, we can see that the Downwind concentration of air pollutants due to an elevated point source is not directly proportional to the wind speed.
whereas it is Inversely proportional to Concentration.
Statement 4 is false. Statement 2 is true.
b)
Category
A – Extremely unstable
B – Moderately unstable
C – Slightly unstable
D – Neutral
E – Slightly stable
F – Moderately Stable
Surface Wind Speed 
Day 
Night 

(m/sec) 
Incoming Solar Radiation 




Strong 
Moderate 
Weak 
Mostly Overcast 
Mostly Clear 
< 2 
A 
A – B 
B 
 
 
2 
A – B 
B 
C 
E 
F 
4 
B 
B – C 
C 
D 
E 
6 
C 
C – D 
D 
D 
D 
> 6 
C 
D 
D 
D 
D 
For Neutral (D) Velocity > For Very unstable (A)
∴ The value of the wind profile exponent for the 'very unstable' atmosphere is smaller than the wind profile exponent for the 'neutral' atmosphere
Statement 4 is false whereas Statement 1 is true.