The term smog was first used in 1905 by Dr. H A Des Voeux to describe the conditions of fog that had soot or smoke in it.
  • Smog is a combination of various gases with water vapor and dust.
  • There are 2 distinct types of smog:
  • Classical Smog/Sulfurous Smog/ London Smog:
- It is caused by high concentrations of sulphur oxides in the air due to use of sulphur-bearing fossil fuels like coal.
- Since sulphur oxides are reducing agents, it is also called as ‘reducing smog’.
- It is particularly formed in the morning hours of winter months when the temperatures are low. Hence it is known as ‘winter smog.
- This type of smog is aggravated by dampness and a high concentration of suspended particulate matter in the air.UPSC Prelims 2024 dynamic test series
  • Photochemical Smog/Los Angles Smog:
- Photochemical smog (smog) is a term used to describe air pollution that is a result of the interaction of sunlight with certain chemicals in the atmosphere.
- It requires neither smoke nor fog.
- The primary component of photochemical smog is ozone; thus it is also called as ‘oxidizing smog’.
- Ground-level ozone is formed when vehicle emissions containing nitrogen oxides (primarily from vehicle exhaust) and volatile organic compounds (from paints, solvents, printing inks, petroleum products, vehicles, etc.) interact in the presence of sunlight.
- Its occurrences are often linked to heavy traffic, high temperatures, and calm winds. During the winter, wind speeds are low and cause the smoke and fog to stagnate near the ground; hence pollution levels can increase near ground level.
- It is usually formed in summer months during afternoon when there is bright sunlight for photochemical reactions to take place. Hence it is also known as ‘summer smog’.

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