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31 Dec 2025·Source: The Indian Express
2 min
Environment & EcologyScience & TechnologyEXPLAINED

Delhi's PM2.5 Pollution: Secondary Sources Like Ammonium Sulfate are Key

Delhi's PM2.5 pollution significantly driven by secondary aerosols like ammonium sulfate, not just direct emissions.

Delhi's PM2.5 Pollution: Secondary Sources Like Ammonium Sulfate are Key

Photo by Max Chen

पृष्ठभूमि संदर्भ

Delhi consistently ranks among the world's most polluted cities. Previous understanding often focused on primary sources like vehicular emissions, industrial pollution, and stubble burning. This study introduces a deeper chemical understanding of PM2.5 formation.

वर्तमान प्रासंगिकता

This understanding is critical for designing more effective air pollution control strategies in Delhi and other highly polluted urban centers, moving beyond just direct emission controls to managing precursor gases.

मुख्य बातें

  • •PM2.5 is not just directly emitted; a substantial portion forms in the atmosphere.
  • •Ammonium sulfate is a major secondary PM2.5 component in Delhi.
  • •Precursor gases like ammonia and sulfur dioxide are crucial to control.
  • •Effective air pollution mitigation requires a multi-faceted approach targeting both primary and secondary sources.

विभिन्न दृष्टिकोण

  • •Traditional view: Focus on direct emission sources (vehicles, industry, dust).
  • •New perspective: Emphasize the role of atmospheric chemistry and precursor gases in forming secondary pollutants.

संबंधित अवधारणाएं

Primary PollutantsSecondary PollutantsAir Quality Index (AQI)National Clean Air Programme (NCAP)Stubble BurningVehicular PollutionIndustrial EmissionsAtmospheric Chemistry

A recent study by TERI and the Energy Policy Institute at the University of Chicago reveals that a significant portion of Delhi's PM2.5 pollution comes from secondary sources, particularly ammonium sulfate. This means that instead of being directly emitted, these fine particulate matters form in the atmosphere from chemical reactions between primary gaseous emissions like ammonia and sulfur dioxide.

This finding is crucial because it shifts the focus from just controlling direct emissions to also managing precursor gases, highlighting the complex chemistry of air pollution and the urgent need for a multi-pronged strategy to combat Delhi's severe air quality crisis. Understanding this mechanism is vital for effective policy interventions.

मुख्य तथ्य

1.

Study by TERI and Energy Policy Institute at the University of Chicago.

2.

Secondary sources contribute significantly to Delhi's PM2.5 pollution.

3.

Ammonium sulfate is identified as a key secondary pollutant.

4.

Formed from precursor gases like ammonia and sulfur dioxide.

UPSC परीक्षा के दृष्टिकोण

1.

Distinction between primary and secondary air pollutants and their formation mechanisms.

2.

Sources and environmental impacts of precursor gases like ammonia and sulfur dioxide.

3.

Policy implications for air quality management, including the need for multi-sectoral strategies and precursor gas control.

4.

Role of scientific research and institutions (TERI, international collaborations) in informing environmental policy.

5.

Challenges in implementing comprehensive air pollution control measures in urban areas.

दृश्य सामग्री

Formation of Secondary PM2.5 (Ammonium Sulfate)

This flowchart illustrates the chemical process by which primary gaseous emissions transform into secondary particulate matter like ammonium sulfate, a key contributor to Delhi's PM2.5 pollution.

  1. 1.Primary Gaseous Emissions (e.g., Sulfur Dioxide (SO2), Ammonia (NH3) from agriculture, industry, vehicles)
  2. 2.Atmospheric Conditions (Sunlight, Water Vapor, Oxidants like OH radicals)
  3. 3.Chemical Reactions (Oxidation of SO2 to Sulfuric Acid, Reaction with NH3)
  4. 4.Formation of Secondary Particulate Matter (Ammonium Sulfate (NH4)2SO4)

बहुविकल्पीय प्रश्न (MCQ)

1. Consider the following statements regarding air pollution in the context of Delhi's PM2.5 problem: 1. Secondary particulate matter, such as ammonium sulfate, forms directly from combustion processes. 2. Ammonia (NH3) and Sulfur Dioxide (SO2) are primary gaseous emissions that act as precursors for secondary PM2.5 formation. 3. Exposure to PM2.5 is primarily linked to respiratory illnesses but has no significant impact on cardiovascular health. Which of the statements given above is/are correct?

उत्तर देखें

सही उत्तर: B

Statement 1 is incorrect. Secondary particulate matter forms in the atmosphere from chemical reactions of primary gaseous emissions, not directly from combustion. Statement 2 is correct, as highlighted by the study, ammonium sulfate forms from chemical reactions between primary gaseous emissions like ammonia and sulfur dioxide. Statement 3 is incorrect. PM2.5 is known to significantly impact both respiratory and cardiovascular health, and can even cross into the bloodstream, affecting multiple organ systems.

2. With reference to air quality management in India, consider the following statements: 1. The National Clean Air Programme (NCAP) aims to achieve a 20% to 30% reduction in PM2.5 and PM10 concentrations by 2024, with 2017 as the base year. 2. Ammonia emissions in India primarily originate from agricultural activities, particularly the use of nitrogenous fertilizers. 3. The Commission for Air Quality Management (CAQM) in NCR and adjoining areas is a statutory body established under the Environment (Protection) Act, 1986. Which of the statements given above are correct?

उत्तर देखें

सही उत्तर: D

Statement 1 is correct. NCAP's initial target was indeed a 20-30% reduction in PM2.5 and PM10 by 2024, using 2017 as the base year (though the target was later revised to 40% by 2026). Statement 2 is correct. Agriculture, including fertilizer application and livestock, is a major source of ammonia emissions. Statement 3 is correct. CAQM was established by an ordinance in 2020 and subsequently replaced by an Act in 2021, making it a statutory body with powers under the Environment (Protection) Act, 1986.

3. Assertion (A): A significant portion of Delhi's PM2.5 pollution is attributed to secondary sources like ammonium sulfate. Reason (R): These secondary particulates are formed in the atmosphere through chemical reactions between primary gaseous emissions such as ammonia and sulfur dioxide. In the context of the above two statements, which one of the following is correct?

उत्तर देखें

सही उत्तर: A

Both the assertion and the reason are true, and the reason correctly explains the mechanism by which secondary particulates like ammonium sulfate contribute to PM2.5 pollution, as highlighted by the recent study. The formation of ammonium sulfate from precursor gases is the core finding.

4. Which of the following statements about the National Air Quality Index (AQI) in India is NOT correct?

उत्तर देखें

सही उत्तर: C

Statement A is correct. This is a primary objective of the AQI to simplify air quality information. Statement B is correct. These are indeed the eight pollutants monitored under India's AQI. Statement C is incorrect. An AQI value between 0-50 is categorized as 'Good', not 'Satisfactory'. 'Satisfactory' is for values between 51-100. The 'Severe' category (401-500) is correct. Statement D is correct. The AQI is determined by the pollutant with the highest sub-index value, reflecting the most harmful pollutant at that specific time, not an average.

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