Smog, a portmanteau of “smoke” and “fog,” is a form of intense air pollution that reduces visibility and poses severe health risks. It is a complex mixture of primary pollutants (like SO2 and NOx) and secondary pollutants (like Ground-level Ozone).
Classification: Classical Smog vs. Photochemical Smog
Smog is broadly classified into two types based on its chemical nature and the environmental conditions under which it forms.
| Feature | Classical Smog (London Smog) | Photochemical Smog (Los Angeles Smog) |
| Climate | Cool, humid climate. | Warm, dry, and sunny climate. |
| Components | Smoke, fog, and Sulphur dioxide (SO2). | NOx, Volatile Organic Compounds (VOCs), and Ozone. |
| Chemical Nature | Reducing (due to high SO2 and carbon). | Oxidizing (due to high O3 and NO2 concentrations). |
| Primary Sources | Coal combustion, industrial emissions. | Vehicular exhaust, fossil fuel combustion. |
| Appearance | Dark, greyish soot-filled air. | Brownish haze (due to Nitrogen dioxide). |
The Chemistry of Photochemical Smog
Photochemical smog is formed through a series of complex reactions driven by sunlight.
- Step 1: Nitrogen Dioxide (NO2) absorbs sunlight and breaks down into Nitric Oxide (NO) and free Oxygen atoms (O).
- Step 2: The free Oxygen atoms react with atmospheric Oxygen (O2) to form Ground-level Ozone (O3).
- Step 3: O3 reacts with Volatile Organic Compounds (VOCs) and NOx to produce secondary pollutants like Peroxyacetyl Nitrate (PAN) and Aldehydes.
- Key Equation: NOx + VOCs + Sunlight → O3 + PAN + Smog
The Role of Temperature Inversion in Smog Formation
Under normal conditions, air temperature decreases with altitude, allowing pollutants to rise and disperse. However, smog is often exacerbated by Temperature Inversion.
- Mechanism: A layer of warm air settles over a layer of cooler air near the ground.
- Effect: This warm layer acts as a “lid,” trapping pollutants, smoke, and moisture close to the earth’s surface.
- Geography: This is particularly common in the Indo-Gangetic Plain during winter, where the Himalayan barrier and low wind speeds prevent the horizontal dispersion of pollutants.
Major Health and Environmental Impacts
- Respiratory Distress: Aggravates asthma, bronchitis, and leads to inflammation of the lung tissue.
- Eye Irritation: PAN and Ozone are potent eye irritants (lacrimators).
- Impact on Vegetation: Ozone in smog enters plant stomata and oxidizes plant tissue, leading to “bronzing” or “flecks” on leaves and reduced crop yields.
- Corrosion: Smog accelerates the deterioration of metals, stones, and painted surfaces (e.g., the yellowing of the Taj Mahal due to SO2 and particulate matter).
Regulatory Measures and Control Strategies
- Catalytic Converters: Used in vehicles to convert NOx and CO into less harmful N2 and CO2.
- Vapour Recovery Systems: Capturing VOCs at gasoline stations before they escape into the atmosphere.
- Green Belts: Planting certain tree species like Azadirachta indica (Neem) and Pinus that can metabolize or trap nitrogen oxides and dust.
- Smog Towers: Large-scale air purifiers installed in urban hotspots (e.g., Connaught Place, Delhi) to reduce particulate matter concentration in the immediate vicinity.
Trivia for UPSC Prelims
- Ground-level Ozone vs. Stratospheric Ozone: Stratospheric ozone is “good” as it protects against UV rays; Ground-level ozone is “bad” as it is a major constituent of smog and a potent greenhouse gas.
- Summer Smog: Though often associated with winter in India, Photochemical smog is actually most intense during summer afternoons when solar radiation is at its peak.
- Visibility Threshold: Smog is technically different from mist; if visibility is reduced to less than 1 km, it is often categorized under smog or fog depending on the pollutant load.