How do stability modifiers affect dispersion calculations?

• A. They increase evaporation rates.
• B. They alter turbulent diffusion coefficients; stable atmosphere reduces mixing and yields higher ground-level concentrations.
• C. They only affect chemical reactivity.
• D. They have no effect on plume dispersion.

Answer: (B)

Stability modifiers change how much the air mixes, which directly governs how far and how fast a plume spreads. In dispersion models, the spread is governed by turbulent diffusion coefficients that quantify horizontal and vertical mixing. Under stable conditions, turbulence is suppressed, so these diffusion coefficients are smaller and the plume remains more concentrated, often leading to higher ground-level concentrations near the source. When the atmosphere is unstable, mixing is enhanced, the plume spreads more, and peak concentrations downwind are diluted. This is why stability modifiers matter: they adjust the diffusion coefficients that control dilution and spread, not evaporation or chemical reactions, and they do have a meaningful effect on plume dispersion.