Secondary Pollutants

A recent study has shed light on an important aspect of India's air pollution crisis: secondary pollutants, which now contribute to nearly one-third of PM2.5 pollution in the country. These pollutants are a significant and growing part of the problem, and their control is critical for improving air quality and public health in India.

What is Secondary Particulate Matter?

Secondary particulate matter refers to particles that don’t directly come from a source but instead form in the atmosphere when precursor gases react with other elements in the air. These precursor gases include:

• Sulfur dioxide (SO₂)

• Nitrogen oxides (NOₓ)

• Ammonia (NH₃)

Once emitted, these gases interact with other compounds, leading to the formation of pollutants such as:

• Ammonium sulfate (formed from the interaction of SO₂ and NH₃)

• Nitrates (from NOₓ)

• Ozone (a reactive gas that contributes to secondary pollution)

Why is India’s Role Critical?

India is one of the largest contributors to these precursor gases globally:

• Sulfur dioxide (SO₂): India emits the highest amount in the world—11.2 million tonnes annually.

• Nitrogen oxides (NOₓ): India is second only to China, emitting 9.4 million tonnes.

• Ammonia (NH₃): India ranks second globally, with 10.4 million tonnes of ammonia emissions.

These emissions create ideal conditions for the widespread formation of secondary pollutants, especially in areas with high coal consumption, such as thermal power plants, which are a major source of SO₂. The large amounts of sulfur in coal burn to produce SO₂, which, in turn, reacts with ammonia to form ammonium sulfate, a key contributor to PM2.5.

How Much of India’s PM2.5 Comes from Secondary Particles?

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

• Ammonium sulfate accounts for 34% of the average PM2.5 concentration nationwide.

• More than 60% of SO₂ emissions in India come from coal-fired thermal power plants, which directly influence the formation of ammonium sulfate.

• Near coal-fired power plants, ammonium sulfate levels are 2.5 times higher than areas farther away (within 10 km vs. beyond 10 km).

• In areas within 10 km of such plants, ammonium sulfate contributes to 36% of PM2.5 levels, while in other areas, it remains significant at 23%.

The impact is felt across the country:

• In 130 NCAP cities, ammonium sulfate concentrations ranged from 3.9 µg/m³ to 22.5 µg/m³.

• In 114 cities, ammonium sulfate made up over 30% of the PM2.5 levels.

The Transboundary Nature of Secondary Particulate Matter

One of the key challenges with secondary particulate matter is its transboundary nature—it can travel across city, state, and even national borders, affecting air quality in regions far from the original source of emissions. For example:

• Even cities without large local sources of emissions can experience high PM2.5 levels due to the movement of secondary pollutants across regions.

• This means that addressing air pollution in India requires more than just local action; it requires coordinated regional and national strategies.

Key Findings from the CREA Study

The Centre for Research on Energy and Clean Air (CREA) study highlights the rising importance of secondary pollutants in India’s air quality challenges:

1. Ammonium sulfate is responsible for up to 34% of India’s PM2.5 mass, with concentrations ranging from 3.9 to 22.5 μg/m³ across different cities.

2. Nationwide Impact: These pollutants are not confined to major urban centers or pollution hotspots. The transboundary nature of secondary pollution means that even distant cities experience high levels of ammonium sulfate, with an average concentration of 11.9 μg/m³ across the country.

3. Coal Power Plants: A major source of sulfur dioxide (SO₂), the precursor to ammonium sulfate, is coal-fired thermal power plants. These plants are responsible for over 60% of India’s SO₂ emissions, making them a key target for reducing secondary PM2.5 pollution.

   • Higher concentrations of ammonium sulfate are found within 10 km of coal power plants (15 µg/m³), compared to 6 µg/m³ in areas farther away.

   • Close to coal plants, ammonium sulfate contributes 36% of PM2.5, while it still accounts for 23% in more distant areas.

The Status of Emission Control Measures

Though Flue Gas Desulphurization (FGD) systems are mandated for coal thermal power plants to control SO₂ emissions, compliance has been poor—only about 8% of plants have installed the required systems. Furthermore, the government is considering rolling back the FGD installation requirement, which could be detrimental to efforts aimed at curbing secondary particulate pollution.

The Role of the National Clean Air Programme (NCAP)

The National Clean Air Programme (NCAP), launched by the Government of India, aims to reduce air pollution levels across the country. However, given that secondary particulate matter is such a significant contributor to PM2.5, strengthening the NCAP is essential to effectively tackle India’s air pollution crisis. Some suggested measures include:

1. Targeting Precursor Gases:

   • The primary sources of SO₂, NOₓ, and NH₃ must be addressed, especially coal-fired power plants. Implementing flue gas desulfurization (FGD) systems in these plants could significantly reduce the formation of ammonium sulfate and help lower PM2.5 concentrations.

2. Strengthening Emission Controls:

   • India’s emission standards for thermal power plants and industries must be improved to ensure that they meet stricter limits for SO₂, NOₓ, and NH₃ emissions.

3. Cross-State Cooperation:

   • The issue of transboundary pollution means that states and regions must work together to share data, implement coordinated measures, and ensure that pollution is addressed at the source and downwind areas.

4. Long-Term Strategies:

   • Policy reforms are needed to reduce coal dependency, transition to cleaner energy sources, and promote better waste management, especially in urban areas.

5. Public Awareness and Health:

   • Strengthening public awareness about the sources of pollution, including secondary pollutants, and its impact on health will be critical in fostering long-term changes

Strategies for Mitigation

To tackle the growing burden of secondary particulate matter, several strategies need to be implemented:

1. Strict Enforcement of Emission Norms:

   • Flue Gas Desulphurization (FGD) systems must be strictly enforced to curb SO₂ emissions from coal power plants.

   • A rollback of FGD requirements would undermine progress and compromise public health.

2. Agricultural and Industrial Reforms:

   • Efficient fertilizer management is essential to reduce ammonia emissions, which are a key precursor for secondary particles.

   • Transitioning industries towards cleaner technologies can help reduce NOₓ and SO₂ emissions.

3. Source-Specific Actions:

   • Pollution mitigation should target both direct emission sources and the precursor gases (SO₂ and NH₃) that form secondary pollutants.

    

4. Real-Time Monitoring and Research:

   • Enhanced air quality monitoring infrastructure should be developed to track secondary pollutants in real-time, helping policymakers adapt to evolving air quality challenges.

   • Continuous research into secondary pollutant formation and their impacts will inform future strategies.

Conclusion

India’s air pollution challenge is becoming increasingly complex due to the rising importance of secondary pollutants like ammonium sulfate, which now contribute up to one-third of the PM2.5 pollution in the country. With coal-fired power plants being a major source of these pollutants, effective strategies to tackle emissions—especially SO₂, NOₓ, and NH₃—are crucial.

For the National Clean Air Programme (NCAP) to succeed, policymakers must focus on stricter emission controls, effective pollution mitigation strategies, and regional collaboration. Additionally, real-time monitoring and research into the formation of secondary pollutants will be key to improving air quality in India’s cities.