In winter the air can become loaded with the products of incomplete combustion -- various particles of ash and soot, carbon monoxide, and nitrogen oxides. When there is little wind to disperse these pollutants, or when there is a temperature inversion, a high level of winter smog can occur.
1.Normally, winds push and disperse pollutants horizontally. No wind means stagnant air and allows levels of pollutants to build up in the air (smog).
2. In normal situations, pollutants can disperse vertically high into the atmosphere. However during an inversion the pollutants are trapped at ground level where they can cause most harm. Cold air becomes trapped under the layer of warm air that acts as a lid. The pollutants in the cooler layer cannot be dispersed and the pollutants stay concentrated at ground level
Winter smog over Montreal
Text © Bruce Martin
Photo: Ministère du Développement durable, de l'Environnement et des Parcs du Québec
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