How does atmospheric stability influence the accumulation of air pollutants in cities?

• A. Stable atmospheric conditions suppress vertical mixing, causing pollutants to accumulate near the ground.
• B. Unstable conditions cause more accumulation.
• C. Stability has no effect.
• D. Wind alone determines accumulation.

Answer: (A)

The main idea is that atmospheric stability controls how well air mixes vertically. When the air is stable, vertical motions are suppressed, so pollutants emitted near the ground stay confined to a shallow layer and build up at surface levels. This is especially pronounced during temperature inversions or calm, cool nights, when the air near the ground is denser and the layer of mixing is very shallow. As a result, pollutants accumulate where people breathe air in cities.

In contrast, unstable conditions promote convection and strong mixing, which loft pollutants higher and dilute them through a deeper vertical column, reducing ground-level concentrations. Wind can transport pollutants, but without sufficient vertical mixing due to stability, accumulation near the surface remains likely. So stability, not wind alone, determines how much pollution accumulates at human breathing height.