Environmental Engineering

Air Pollution

Primary and secondary pollutants, lapse rates and atmospheric stability — the dispersion physics behind air-quality control.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Primary pollutants are emitted directly (CO, SO₂, NOₓ, particulates); secondary pollutants form in the atmosphere (ozone, PAN, photochemical smog). Vertical mixing is governed by the environmental lapse rate compared with the dry adiabatic lapse rate (

\approx 9.8 ° C / km

): a steeper (super-adiabatic) profile is unstable and disperses pollutants, while an inversion traps them.

The Core Formula Matrix

Dry adiabatic lapse rate:

Γ_{d} \approx 9.8 ° C / km

Stability: ELR

> Γ_{d}

unstable; ELR

< Γ_{d}

stable; inversion (ELR

< 0

) very stable

Temperature at height:

T_{z} = T_{0} - Γ z

Effective stack height = physical height + plume rise.

The ‘IIT Traps’

⚠
Ozone at ground level is a secondary pollutant, formed photochemically — not emitted directly.
⚠
An inversion is the worst case for dispersion: warm air above cold traps pollutants near the ground.
⚠
Compare ELR with the adiabatic rate to judge stability — the absolute temperature alone tells you nothing.

📚 Standard references

Environmental Pollution Control Engineering — C.S. Rao
Environmental Engineering Vol. II — S.K. Garg

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

Ground-level ozone in photochemical smog is classified as a:

Q2MEDIUM2 Marks · NAT

With a dry adiabatic lapse rate of

9.8 ° C / km

and surface temperature

25 ° C

, the air temperature at

500 m

is _____ °C.

Q3HARD2 Marks · MCQ

An atmosphere with environmental lapse rate greater than the dry adiabatic lapse rate is: