Wind speed is generally measured by Anemometer at a height say Z0, knowing the wind speed say U0, At a any height Z.
We can workout velocity U as
k = const. for large lapse rate
for marked inversions
(average normal value)
The emitted gases being known as a plume and their source of origin as stack.
(i) Looping plume
- Occurs in super adiabatic environment. Which produces highly unstable environment because of rapid mixing.
- Higher stacks are need.
(ii) Neutral plume
- Upward vertical rise
- When ELR = ALR
(iii) Coning plume
- When wind velocity > 32 km/hr and when clouds are present.
- Also occurs under sub-adiabatic condition. (ELR < ALR)
- Environment is slightly stable.
(iv) Fanning plume
- Under extreme inversion conditions
- Emission will spread only horizontally
- High rising stacks are needed.
(v) Lofting plume
- When there exists a strong super adiabatic L.R. above surface inversion.
- Such plume has minimum downward mixing as its downward motion is prevented by inversion but upward mixing will be rapid and turbulent.
(vi) Fumigating plume
- When inversion layer occurs at a short distance above the top of the stack and super adiabatic conditions prevail below the stack.
- Pollutants cannot escape above the top of the stack because of inversion layer.
(vii) Trapping plume
- When inversion layer exists above the emission source as well as below the source. Naturally the emitted plume will neither go up nor down.
Various mechanical devices used for removal of particulate matters
Controlling air pollution from Auto mobiles
Automobiles normally emit
CO - 0.5 to 6.4%
HC - 300 to 1000 ppm
NOX - 500 to 3000 ppm
depending upon type and condition of engine also.
Vanadium pentoxide: used for removing SO2
Platinum metal: for removing NOX
Activated alumina: for removing Hydrocarbons (impregnated with metallic compounds)
Pd++ & Cu++ for converting CO to CO2
Characteristics of Sound and its measurement
P = period
f = frequency
c = fλ(V=nλ)
λ = wave length, V = Velocity of sound
n = frequency, prms
p(t) = pressure at any time ‘t’, prms = sound pressure
Sound pressure = Atmospheric pressure – barometric pressure.
Power of Sound
The rate of doing work by a travelling sound wave in the direction of propagation of wave.
The energy transmitted by a sound wave in the direction of its propagation is thus defined as its power.
(i) Intensity of sound
‘Sound power average’ over the time per unit area normal to the direction of propagation of wave.
a = unit area ⊥ to the direction of wave motion.
P = density of medium
v = vel. In m/sec.
T = temperature in K
(ii) Level of Noise
- Sound pressure which can be heard by human
- Loudest sound produced by rocket ≈ 200 Pa.
- Level (Measurement)
Q = measured quantity,
Q0 = Reference standard.
If Reference is power (≈ 10-12 watts)
If Reference Q0 is sound Intensity (≈ 10-12 W/m2)
(iii) Averaging sound pressure levels
The average value of the various records at a place is given by
average sound pressured level in
N = No. of measurement reading
nth sound pressure level in dB(Re-20μPa) say for e.g. average of 4 measurement 40, 50, 62 and 72 dB(Re=20μPa)
Noise Rating systems
A noise may consist of different types of sounds with different pressure levels. Operating for different time intervals, the frequency may also vary. Hence to find combined effect there are two concepts Ln and Leq concepts.
(i) The Ln Concept
The value of Ln will represent the sound pressure level that will exceed for N% of the gauging time. e.g. 70 dB value of L60 means that the sound level will exceed 70 dB for 60% of the measuring time.
Graph between Ln and N (N = …%)
(ii) Leq Concept
(Equivalent NOISE level)
Leq is that statistical value of sound pressure level that can be equated to any fluctuating noise level.
Thus, Leq is defined as the const. noise level which over a given time expands the same amount of energy as is expanded by the fluctuating levels over the same time, this value is expressed by the equation
n = total no of sound samples
Li = noise level of the ith sample
ti = time duration of ith sample expressed as % total time.
Leq value for fluctuating noise level of 95 minutes indicated earlier (i.e. the one with 80 dB lasting for 10 minutes, followed by 60 dB for 80 minutes followed by 100 dB for 5 min. can be worked out as below.
The equivalent noise level (Leq) constitutes an important parameter for evaluating the impact of fluctuating noise of all kinds.
The duration in hours, over which Leq is worked out for a given site is further mentioned in bracket, such as Leq (8) which means that Leq is based on 8 hr., measurement, when, however no such time is mentioned, then Leq is always corresponds to one hr. measurement.
(iii) Noise levels of different sources
Air traffic = 90 – 110 dB
Rail traffic = 90 – 110 dB
(about 30 m away)
Heavy Road traffic = 80 – 90 dB
Medium Road traffic = 70 – 80 dB
Light Road traffic = 60 – 70 dB
(iv) Noise reduction
R = Distance between source and wall.
H = Height of barriers wall
λ = Wave length of sound
D = Distance between barrier and the receiving point.
CPCB standards of noise levels (dB) ⇒ dB(A)
Ambient air quality standards dB(A), Leq