Discussion
A city sits in a valley under a stagnant high pressure system for four consecutive days.
What is the likely air quality outcome?
Answer: A
Correct answer: A severe episode under a subsidence inversion
A stagnant high pressure system over a valley is the classic setting for a severe episode. Air descends within the high and warms adiabatically as it is compressed, forming a subsidence inversion that caps the mixed layer; the weak pressure gradients leave winds light; and the valley walls restrict horizontal ventilation further.
Mixing height and wind speed both collapse, so the ventilation coefficient falls to a small value while emissions continue, and concentrations build day after day. The descending air does not disperse pollution - it is precisely what creates the lid, since the sinking motion warms the air rather than mixing it downward.
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