During the winter, a layer of cooler air is trapped by a layer of warmer air above, forming a temperature inversion. An inversion is an atmospheric condition caused by increasing temperature with elevation, resulting in a layer of warm air preventing the rise of cooler air trapped beneath. An inversion traps pollutants from vehicles, fireplaces, and other sources close to the ground, thus increasing the chances for carbon monoxide and particulates to reach unhealthy levels. At night, the ground level temperature cools faster than the air above it.
Pollutants become trapped under the layer of warm air.
As the sun rises in the morning, the ground level temperature warms up faster than the air above it, pushing the air upwards, which breaks up the warm air layer, allowing the pollutants to escape. However, if there is no wind, the air can become stagnant.