Which statement best explains why a thermal inversion tends to concentrate pollutants in the air layer closest to the Earth's surface?

• A. The air layer closest to the Earth's surface is cooler than the layer above it.
• B. The air layer closest to the Earth's surface is denser than the layer above it.
• C. The upper layer is more humid than the lower layer.
• D. The atmospheric pressure is higher at the surface.

Answer: (B)

Inversions create a stable layering of the atmosphere where a cool, dense air mass sits beneath a warmer, lighter air mass. Pollutants released near the ground get trapped in that bottom layer because rising into the lighter air above is not energetically favorable, so vertical mixing is strongly suppressed. This makes the pollutants accumulate in the layer closest to Earth. The statement that the surface layer is denser than the layer above directly explains this trapping mechanism.