Air & Noise Pollution

Atmospheric Dispersion & Stack Plumes

Effective stack height, lapse rates, stability classes and the Gaussian plume — predicting ground-level pollutant concentrations.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Pollutants released from a stack disperse by advection (wind) and turbulent diffusion. The effective stack height

H

is the physical height plus plume rise. The Gaussian plume model predicts concentration assuming normal spreading in the horizontal and vertical. Atmospheric stability — set by comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate (DALR ≈ 9.8 °C/km) — controls plume shape (looping, coning, fanning).

The Core Formula Matrix

Ground-level concentration (plume centreline, Gaussian):

C = \frac{Q}{π u σ _{y} σ _{z}} exp (- \frac{H ^{2}}{2 σ _{z}^{2}})

Maximum GLC

\propto \frac{Q}{u H ^{2}}

— doubling H quarters the peak concentration.

Lapse-rate stability: ELR > DALR ⇒ unstable (superadiabatic); ELR < DALR ⇒ stable; inversion ⇒ very stable.

DALR ≈ 9.8 °C/km (≈ 1 °C per 100 m).

The ‘IIT Traps’

⚠
Ground-level concentration ∝ 1/H². Taller stacks cut peaks sharply.
⚠
Inversion (temperature rising with height) traps pollutants — worst dispersion.
⚠
Effective height = physical height + plume rise, not just the chimney height.

📚 Standard references

Air Pollution Control Engineering — Noel de Nevers
Environmental Engineering Vol. II — S.K. Garg · Air Pollution

PART 2

Progressive 3-Tier Question Suite

Q1MEDIUM2 Marks · MCQ

If the effective stack height is doubled (all else constant), the maximum ground-level concentration becomes approximately:

Q2BASIC1 Mark · MCQ

A temperature inversion in the atmosphere produces conditions that are:

Q3HARD2 Marks · MCQ

The atmosphere is classified as unstable (superadiabatic) when the environmental lapse rate is: