Secondary Pollutants and PM2.5: India’s Silent Air Crisis

Secondary Pollutants and PM2.5: India’s Silent Air Crisis

A Detailed UPSC-Focused Blog by Suryavanshi IAS

📅 June 28, 2025

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🔍 What Are Secondary Pollutants?

Air pollution is one of India’s gravest environmental challenges. To effectively understand it—especially for the UPSC Civil Services Exam—one must know the difference between primary and secondary pollutants.

Type	What it means	Example
Primary Pollutants	Directly emitted from a source into the atmosphere	SO₂, NOx, CO, PM10, PM2.5, VOCs
Secondary Pollutants	Formed when primary pollutants chemically react in the atmosphere with water vapor, sunlight, or other gases	Ozone (O₃), Ammonium Sulphate, Ammonium Nitrate, Peroxyacetyl nitrate

➡️ In short: Secondary pollutants are not emitted directly, but formed in the air after reactions.

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🧪 CREA Report Highlights: Ammonium Sulphate and PM2.5

According to a latest study by the Centre for Research on Energy and Clean Air (CREA):

• A large portion of India’s PM2.5 pollution is made up of ammonium sulphate, a secondary pollutant.
• It forms when sulphur dioxide (SO₂), mainly from coal-fired thermal power plants, reacts with ammonia (NH₃), largely from agriculture.

📌 Key Data:

• Ammonium sulphate makes up ~34% of India’s total PM2.5
• National average concentration: 11.9 μg/m³
• Near coal-fired plants: Up to 15 μg/m³, which is 2.5 times higher than other areas
• In 114 out of 130 NCAP cities, ammonium sulphate made up >30% of total PM2.5
• Only 8% of coal power plants have installed mandatory flue gas desulphurisation (FGD) systems

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🌍 Why This Is A Serious Concern

1. Health Impact

PM2.5 are ultra-fine particles (diameter <2.5 micrometers) that can:

• Penetrate deep into lungs and bloodstream
• Cause asthma, lung cancer, heart diseases
• Affect children and elderly more severely

2. Invisible and Long-Lasting

Unlike smog from vehicles, ammonium sulphate is not easily visible. But it spreads over long distances, even to cities far from power plants, making trans-boundary pollution harder to regulate.

3. Policy Gap

• Even though FGDs are legally required, installation is very low.
• Government is considering relaxing these requirements—a move that may worsen pollution levels.

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🏛️ Governance Angle: National Clean Air Programme (NCAP)

• Launched in 2019, NCAP aims to reduce PM2.5 and PM10 pollution by 20–30% by 2025–26 in 131 cities.
• Focus is more on primary pollutants so far—less attention to secondary pollution sources like SO₂ from coal plants or NH₃ from fertiliser misuse.

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🧠 UPSC Relevance: What You Need to Know

This topic connects to several parts of the syllabus:

Paper	Relevance
GS Paper I	Environmental geography, human impact on atmosphere
GS Paper III	Pollution control, environmental protection laws, energy sector
Essay Paper	Topics on sustainable development, clean air, health
Ethics Paper	Duties of government toward environment and public health

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📝 Previous UPSC Questions

Prelims 2020

Q. Which of the following are the reasons/factors for exposure to PM2.5 pollution?

1. Emissions from vehicles
2. Emissions from power plants
3. Use of biomass
4. Use of fossil fuels in industries
5. Construction activities
6. Waste burning

✅ Correct Answer: (d) All of the above

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Mains GS Paper III – 2022

"Air quality management goes beyond controlling the concentration of pollutants."

Discuss in the Indian context.

Expected Approach:

• Define primary vs secondary pollutants
• Show how current policies target only visible sources
• Recommend comprehensive emission control, agricultural reforms, and thermal power regulation

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🧩 What Should Aspirants Focus On?

✔ Chemistry behind secondary pollutant formation

✔ Role of SO₂ and NH₃ as precursor gases

✔ Function and importance of Flue Gas Desulphurisation (FGD)

✔ Why ammonia control in agriculture is vital

✔ Challenges in enforcing trans-boundary air pollution regulation

✔ Integration of satellite data in air pollution monitoring

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🛠️ What India Needs To Do

1. Strictly enforce FGD installation in all thermal power plants
2. Reduce ammonia emissions by promoting efficient fertiliser use
3. Promote cleaner alternatives to coal for energy production
4. Include secondary pollutants in NCAP targets
5. Strengthen satellite-based monitoring and real-time data sharing

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🎯 Mains Practice Question

Q. Explain the role of secondary pollutants in worsening India’s air quality. How can policy reform in the thermal power and agriculture sectors reduce the burden of PM2.5 pollution?

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📌 Final Takeaway

Secondary pollution, especially from ammonium sulphate, is one of India’s most underestimated pollution sources. As a UPSC aspirant, understanding the science, source, spread, and policy linkages of such topics will help you write well-rounded, data-backed answers in both Prelims and Mains.