Air pollution is an important topic for the UPSC Prelims examination, especially concepts like smog, photochemical smog, ozone pollution, and NOx gases. Questions from environmental chemistry often test conceptual clarity rather than memorization.
Let’s understand smog in a simple way.
Contents
What is Smog?
The word smog comes from two words:
Smoke + Fog = Smog
Smog is a type of air pollution that reduces visibility and harms human health. It forms when pollutants mix with atmospheric conditions like moisture, temperature, and sunlight.
For UPSC, smog is broadly classified into two types:
- Winter Smog (Classical Smog)
- Summer Smog (Photochemical Smog)
Winter Smog (Classical Smog)
Winter smog usually forms in cold and humid conditions when dense air traps pollutants near the surface.
During winters, temperature inversion prevents vertical air movement. As a result, pollutants accumulate near the ground, creating a thick haze that reduces visibility.
This type of smog is commonly seen in:
- Northern Plains of India during winter
- Industrial cities with coal-based pollution
Key idea for UPSC:
Cold, dense air + moisture + trapped pollutants = Winter Smog
Summer Smog (Photochemical Smog)
Summer smog is also called photochemical smog, and it forms due to chemical reactions in the presence of sunlight.
This is a very important concept for the UPSC environment syllabus.
How Photochemical Smog Forms
The process involves three major components:
- NOx (Nitrogen Oxides) — nitrogen monoxide (NO) and nitrogen dioxide (NO₂)
- VOCs (Volatile Organic Compounds)
- Sunlight
In sunlight, nitrogen dioxide breaks down and releases oxygen atoms. These atoms react with atmospheric oxygen to form ozone (O₃).
Meanwhile, VOCs react with NOx to form PAN (Peroxyacetyl Nitrate).
Photochemical smog mainly contains:
- Ozone (tropospheric ozone)
- PAN (Peroxyacetyl nitrate)
- Nitric acid (HNO₃)
Together, these create photochemical smog, which is harmful to humans and plants.
Tropospheric Ozone vs Stratospheric Ozone
This distinction is important for UPSC.
Stratospheric ozone
- Protects Earth from UV radiation
- Beneficial
Tropospheric ozone
- Acts as a pollutant
- Causes eye irritation, breathing problems, and skin irritation
- Major component of photochemical smog
Quick UPSC Revision Points
- Smog = Smoke + Fog
- Two types: Winter smog and Photochemical smog
- Winter smog linked to temperature inversion
- Photochemical smog linked to sunlight-driven reactions
- NOx + VOCs + sunlight → PAN + Ozone
- Tropospheric ozone is a pollutant
This topic is important for UPSC Prelims environment questions and is often asked in conceptual MCQs.
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FAQs
- #UPSCEnvironment: What is smog and how is it formed in UPSC environment topics?
- #PhotochemicalSmogUPSC: How does photochemical smog form in the presence of sunlight?
- #WinterSmog: What is winter smog and why is it common in northern India?
- #TroposphericOzone: Why is tropospheric ozone considered a pollutant in UPSC environment studies?
