Context:

  • A recent study by the Centre for Research on Energy and Clean Air (CREA) reveals that secondary pollutants, particularly ammonium sulphate, are responsible for nearly one-third of India’s fine particulate matter (PM2.5) pollution.
  • This highlights a critical, often overlooked, aspect of India's air pollution crisis.

PRIMARY V/S SECONDARY POLLUTANTS

  • Primary Pollutants: These are air pollutants emitted directly into the atmosphere from a source. Examples include:
    • Sulphur dioxide (SO2) from coal burning.
    • Nitrogen oxides (NOx) from vehicle exhaust and industries.
    • Carbon monoxide (CO) and particulate matter (soot, dust) from burning fossil fuels and organic matter.
    • Ammonia (NH3) from agriculture.
  • Secondary Pollutants: These are not directly emitted from a source.
    • Instead, they form in the atmosphere when primary pollutants react with each other or with other atmospheric components like gases, water vapor, and sunlight.
    • These reactions create more complex and often more harmful particles.
  • Examples include:
    • Ammonium Sulphate: Formed from the reaction of Sulphur dioxide (SO2) and Ammonia (NH3).
    • Ammonium Nitrate: Formed from the reaction of Nitrogen oxides (NOx) and Ammonia (NH3).
    • Ozone (O3) at ground level.
    • Sulphuric acid and nitric acid (components of acid rain).

KEY FINDINGS OF THE CREA STUDY

  • Significant Contribution: Secondary pollutants, specifically ammonium sulphate, account for approximately 34% of India’s overall PM2.5 mass, representing nearly one-third of the country's fine particulate pollution.
  • When other secondary pollutants like ammonium nitrate are included, secondary particles can contribute up to 50% of the PM2.5 mass in some areas.
  • Methodology: The CREA findings are based on a combination of satellite-data imagery and advanced atmospheric modeling analysis.
  • Widespread Presence & Transboundary Nature:
    • The nationwide average concentration of ammonium sulphate in India is 11.9 µg/m³.
    • Its widespread presence has been observed across a large number of cities, irrespective of their proximity to major emission sources.
    • This is due to the trans-boundary nature of pollution, meaning these secondary particles can travel long distances, affecting areas far from their original sources.
  • Coal Power Plants as Major Source:
    • The main driver of ammonium sulphate formation is sulphur dioxide (SO2) emissions.
    • More than 60% of SO2 emissions in India originate from coal-fired thermal power plants.
    • This makes these power plants a critical target for reducing secondary PM2.5 pollution.
  • Local and Regional Impact:
    • Ammonium sulphate concentrations are significantly higher closer to coal plants: 2.5 times higher within 10 km of coal-fired power plants (15 µg/m³) compared to areas beyond 10 km (6 µg/m³).
    • While its contribution to PM2.5 is 36% near coal-fired thermal power plants, it remains substantial at 23% in other areas, clearly indicating both local and trans-boundary impacts of SO2 emissions.

IMPLICATIONS FOR AIR POLLUTION CONTROL

  • Flue Gas Desulphurisation (FGD) Systems: The report highlights that implementing Flue Gas Desulphurisation (FGD) systems in coal-fired thermal power plants is crucial for reducing SO2 emissions and, consequently, secondary PM2.5 pollution.
    • FGD technology removes sulphur compounds (like SO2) from exhaust gases before they are released into the atmosphere.
    • Though it is mandatory for Indian coal thermal plants to install such units, the study notes that only about 8% have actually installed them.
    • There are also concerns that the government is considering doing away with this requirement, which could severely undermine efforts to control air pollution.
  • National Clean Air Programme (NCAP) Cities:
    • Across the 130 cities designated under the National Clean Air Programme (NCAP), concentrations of ammonium sulphate ranged from 3.9 to 22.5 µg/m³.
    • The share of PM2.5 from ammonium sulphate in these cities varied between 20% and 43%.
    • Notably, ammonium sulphate constituted more than 30% of the total PM2.5 levels in 114 out of 130 NCAP cities.
    • NCAP aims to reduce PM2.5 and PM10 concentrations by a certain percentage by 2025-26 (original target was 20-30% by 2024, now updated to 40% reduction by 2026, using 2017 as base year).
  • Recommendations for Mitigation: An analyst at CREA, emphasizes the need for:
    • Targeted strategies: Such as deploying FGD in coal-fired thermal power plants to reduce sulphur dioxide and promoting efficient fertilizer management to reduce ammonia emissions (a source of NH3).
    • Full compliance: Ensuring all polluting sectors fully comply with existing emission norms.
    • Source-specific actions and regulatory compliance: Are critical for reducing precursor gas emissions and effectively tackling the growing burden of secondary PM2.5 pollution in India.

BROADER SIGNIFICANCE

  • This study underscores that India's air pollution challenge is complex and goes beyond directly emitted pollutants. The chemical transformations in the atmosphere play a substantial role.
  • Understanding the role of secondary pollutants is essential for designing effective, holistic air quality management strategies that target both primary emission sources and their atmospheric transformations.
  • Without addressing precursor gases, significant improvements in PM2.5 levels may remain elusive.