What is a temperature inversion and can it trap smog/pollution?
Normally, temperature broadly decreases with altitude, and convection is effective: locally warmer air will rise, and cooler air will fall. A temperature inversion is where the air temperature rises with altitude. This means that convection is less effective because the air above is already warmer, and so there is less mixing of air between altitudes.

Since pollution is generally produced at ground level, temperature inversions can trap the pollution (e.g. smog) at ground level.