Smog is a complex cocktail of pollutants that degrades air quality. To manage it effectively, scientists categorize it based on how the pollutants are formed in our atmosphere.
Comparison Table
| Feature | Primary Smog (Sulfurous) | Secondary Smog (Photochemical) |
|---|---|---|
| Common Nickname | "London Smog" or Industrial Smog. | "Los Angeles Smog" or Brown Smog. |
| Formation | Emitted directly from a source. | Formed in the air via chemical reactions. |
| Weather Condition | Cold, humid, foggy weather (winter) | Sunny, dry, warm weather (summer) |
| Time of Occurrence | Early morning or night | Afternoon (peak around noon–3 PM) |
| Main Catalyst | Combustion of fossil fuels (coal). | Sunlight (UV radiation). |
| Key Components | Sulfur dioxide (SO2), Particulate Matter. | Ozone (O3), PANs, Aldehydes. |
| Chemical Nature | Reducing (has reducing agents like SO2) | Oxidizing (contains strong oxidants like ozone) |
| Visibility | Very low (dense fog + smoke) | Moderate to low (hazy brown air) |
| Health Effects | Respiratory irritation, bronchitis, lung diseases | Eye irritation, asthma, chest pain, lung damage |
| Examples / Famous Incidents | 1952 Great Smog of London | Los Angeles smog episodes (1940s–present) |
| Typical Timing | Often worse in winter/damp weather. | Worse on hot, sunny days. |
1. Primary Smog (Industrial Smog)
This type of smog consists of pollutants that are "ready-made." As soon as they leave a factory chimney or a car’s tailpipe, they are already harmful. It is characterized by high concentrations of sulfur oxides and soot.
- Source: Power plants, heavy industry, and home heating.
- Appearance: Often gray and thick.
2. Secondary Smog (Photochemical Smog)
Secondary smog is a bit more "creative." It happens when primary pollutants (like Nitrogen Oxides and Volatile Organic Compounds) bake in the sun. The solar energy triggers a reaction that creates new, often more toxic, chemicals.
NOx + VOCs + Sunlight → O3 + PANs
- Source: Interaction between vehicle exhaust and solar radiation.
- Appearance: Typically a brownish haze.
Related Topics
Ozone Depletion: Notes and MCQs
Ozone Layer Depletion / The Ozone Hole
How is ozone formed in the atmosphere
Global Warming and Ozone Depletion
Primary and Secondary Pollutants
Photochemical Smog