17.3.
Smog and Temperature Inversion
Smog.
Smog and air pollution are not the same thing. Smog is a form of air pollution, but air pollution is not necessarily smog. Each of the three major pollutants (HC, CO, NOx) is a by-product of combustion and is created in different ways as already discussed. But HC and NOx are the two principal materials that combine in the atmosphere to form smog.
Although smog can be created in a laboratory, but its mechanism of formation is not correctly understood. However, for the formation of smog in the atmosphere, three factors are necessary; these include sunlight, relatively still air, and a high concentration of hydrocarbons and oxides of nitrogen. When these are present, sunlight causes a chemical reaction between the HC and NOx producing smog.
Temperature Inversion.
Normally, air is cool at higher altitudes. Warm air near the ground rises, cooling itself by contact with the cooler air above it. When nature follows this normal flow pattern, smog and other pollutants are also carried away. But some areas experience a natural weather pattern called a temperature inversion. When this happens, a layer of warm air prevents the upward movement of air near the ground. This inversion acts as a “lid” over the stagnant air; preventing the air to rise, so that smog and pollution collect.
When the inversion layer is several thousand metres high, the smog raises enough to provide reasonable visibility. But when the inversion layer is within thousand metres from the ground, smog is trapped and visibility is reduced. The distant landscape is impossible to visualise and also eye irritation, headache, and difficulty in breathing take place. The Los Angeles area is the first place where temperature inversion was noticed. Surrounding mountains in this area form a natural basin in which temperature inversion is present to some extent for more than 300 days a year.
