Secondary Pollutants — UPSC Notes

Secondary Pollutants — UPSC Notes

✅ Definition

Secondary pollutants are not emitted directly into the atmosphere.
They are formed when primary pollutants react chemically in the atmosphere — often in the presence of sunlight, water vapour, and oxidants.

🟩 Primary vs Secondary — Key Difference

🌤 How Secondary Pollutants Form

Common reaction processes include:

• Photochemical reactions → sunlight-driven
• Oxidation reactions
• Hydrolysis
• Combination reactions

⚠️ Major Secondary Pollutants (Very Important for Prelims & Mains)

1. Tropospheric (Ground-level) Ozone (O₃)
   Formed when NOx + VOCs react in sunlight
   → Key component of photochemical smog
2. PANs (Peroxyacetyl Nitrates)
   Formed from NOx + hydrocarbons in sunlight
3. Smog
   • Photochemical smog (Los Angeles-type) → Ozone + PANs + Aldehydes
   • Classical/London smog has both primary & secondary components
4. Sulphates & Nitrates
   • SO₂ → Sulphuric acid / sulphates
   • NOx → Nitric acid / nitrates
     → Lead to acid rain
5. Secondary Particulate Matter (PM₂.₅ & PM₁₀)
   • From reactions of SO₂, NOx, ammonia, VOCs

🌏 Sources of Precursor Gases

• Vehicles → NOx, VOCs
• Thermal power plants → SO₂, NOx
• Industries → VOCs, SO₂
• Agriculture → Ammonia (NH₃)
• Biomass burning

🚨 Why Secondary Pollutants Matter

• Often more toxic than the original pollutant
• Harder to control (since multiple sources involved)
• Cause:
  • Respiratory & cardiovascular disease
  • Eye irritation
  • Crop damage
  • Material corrosion
  • Visibility reduction
  • Climate forcing (e.g., ozone is a greenhouse gas)

📌 UPSC Key Lines for Writing

Secondary pollutants like tropospheric ozone and PANs form through photochemical reactions involving NOx and VOCs. They are major contributors to urban photochemical smog and have serious public-health and ecological impacts.

📝 Example UPSC-Style Questions

Prelims:
Tropospheric ozone is formed due to
(a) combustion of fossil fuels
(b) photochemical reaction involving NOx & VOCs
(c) volcanic eruption
(d) release from oceans
✔ Answer: (b)

Mains (GS-3):
Discuss the role of secondary pollutants in aggravating urban air pollution in India.