Air and Sound Pollution Study Notes for Civil Engineering Exams

By Vishwajeet Sinha|Updated : January 15th, 2017

Air and Sound Pollution

Primary Pollutants

  1. Sulphur dioxide (especially SO2 ↑)
  2. CO ↑
  3. Nitrogen oxides No and NO2
  4. Lead (Pb)
  5. Hydrocarbons
  6. Allergic agents like pollens and spores
  7. Radioactive substances etc.
    Certain less important primary pollutants are H2S, H2F and other fluorides, methyl and Ethyl mercaptans. etc.

Secondary Pollutants

  1. (more toxic then SO2 ↑)
  2. Ozone (O3)
  3. Formaldehydes
  4. Peroxyacetyl-nitrate (PAN) etc.,


Wind speed

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

Posted by:

Vishwajeet SinhaVishwajeet SinhaMember since Jun 2016
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Jan 17, 2017

Can you explain about electrostatic precipitaors removing?!
Rubeel Naqvi

Rubeel NaqviJan 27, 2017

What is the difference btw cpcb standards and air quality standards? Please explain
Vinay Raj

Vinay RajFeb 9, 2017

Can u explain in detail about electrostatic precipitator and fabric filters

ArunApr 2, 2017

Why new posts are not seen in this group? No articles and materials for ESE MAINS?🤔
Jayesh Patil

Jayesh PatilSep 10, 2017

Ambient air quality standards are wrong
Jayesh Patil

Jayesh PatilSep 10, 2017

Day n night are altered
Shruti Bansal
Is there an option to download this content as pdf

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