Air pollutants can be categorized into two main types: primary and secondary pollutants. Understanding these classifications is crucial for addressing air quality issues effectively.
Primary pollutants are substances that are directly emitted into the atmosphere from identifiable sources. Common sources include:
These pollutants include sulphur dioxide (SO₂), nitrogen oxides (NOₓ), carbon monoxide (CO), and particulate matter (PM).
In contrast, secondary pollutants are not directly emitted. Instead, they form in the atmosphere as a result of chemical reactions involving primary pollutants. For example, ammonium sulphate is created when sulphur dioxide reacts with ammonia in the air. Secondary pollutants tend to be more harmful and persistent.
Secondary pollutants play a significant role in the formation of PM2.5, which refers to particulate matter that is less than 2.5 micrometers in diameter. These fine particles can penetrate deep into the lungs, posing serious health risks.
According to the Centre for Research on Energy and Clean Air (CREA), ammonium sulphate accounts for approximately 34% of PM2.5 levels across India.
Ammonium sulphate is formed when sulphur dioxide (SO₂), primarily emitted from coal-fired thermal power plants, reacts with ammonia (NH₃) in the atmosphere. This reaction is most intense near emission sources.
Research shows that ammonium sulphate concentrations are 2.5 times higher within 10 km of coal-fired power plants compared to areas beyond this distance, indicating the localized impact of thermal power emissions.
Another significant secondary pollutant is ammonium nitrate. Together, these pollutants can account for up to 50% of PM2.5 mass in certain regions.
Near coal-based power plants, up to 36% of PM2.5 is associated with their operations. Additionally, even in regions farther away, 23% of PM2.5 can still be traced back to sulphur dioxide and ammonia emissions due to atmospheric transport.
To mitigate the effects of secondary pollutants, CREA suggests implementing flue-gas desulphurisation (FGD) units in coal-fired plants and improving agricultural practices to manage ammonia emissions. Rigorous enforcement of emission standards is essential.
In 130 cities under the National Clean Air Programme (NCAP), ammonium sulphate constitutes over 30% of PM2.5 levels in 114 cities, significantly affecting clean air goals set for 2025-26.
Secondary pollutants can travel long distances, impacting cities that are not close to major emission sources. This highlights the need for comprehensive pollution management strategies.
Q1. What are secondary pollutants and why are they important in PM2.5 formation?
Answer: Secondary pollutants form through chemical reactions between primary emissions like sulphur dioxide and ammonia. They significantly contribute to PM2.5, which poses major health risks due to its ability to penetrate deep into the lungs.
Q2. What proportion of India’s PM2.5 pollution comes from ammonium sulphate?
Answer: Ammonium sulphate accounts for approximately 34% of PM2.5 levels nationwide, highlighting its significant contribution to air quality issues in India.
Q3. Where are ammonium sulphate concentrations highest in India?
Answer: Concentrations are notably higher near coal-fired power plants, where levels are 2.5 times greater within 10 km compared to areas further away, indicating localized pollution impacts.
Q4. What are the recommendations for controlling secondary pollutants?
Answer: Recommendations include deploying flue-gas desulphurisation units in thermal power plants and improving fertilizer practices to manage ammonia emissions effectively.
Q5. How are India’s clean air targets affected by these pollutants?
Answer: Ammonium sulphate significantly influences PM2.5 levels in numerous cities, directly impacting India's clean air targets set for 2025-26 under the National Clean Air Programme.
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