Secondary Pollutants

Recent analysis shows that secondary aerosols account for at least one-third of Delhi’s annual PM2.5 pollution, helping explain severe smog episodes.

Primary vs Secondary Pollutants

• Primary pollutants are emitted directly from sources like Vehicular emissions, coal-fired power plants, biomass burning and Industrial processes.
  • Examples: Carbon monoxide , Sulfur dioxide, Ammonia,  Nitrogen oxides, Hydrocarbons)
• Secondary Pollutants form when these primary emissions react with other elements in the atmosphere (e.g., humidity, temperature, and sunlight) and include harmful compounds such as ammonium sulphate and ammonium nitrate.
  • These pollutants significantly increase PM2.5 levels, especially during prolonged smog spells, even when local sources seem under control.
  • Examples: Particulate Matter (PM), Tropospheric Ozone (O₃), Sulfuric acid and Nitric acid, Peroxyacetyl Nitrate (PAN), Photochemical Smog.

Health Impacts of Secondary Pollutants

• Respiratory Health Impact: Largely present as PM2.5, it penetrate deep into the lungs, aggravating asthma, bronchitis, and COPD, reducing lung function, and increasing respiratory infections, especially among children and the elderly.
• Cardiovascular Effects: Fine secondary aerosols enter the bloodstream, triggering systemic inflammation and increasing the risk of heart attacks, strokes, hypertension, and arrhythmias.
• Impact on Children and Maternal Health: Exposure impairs lung development in children and is associated with low birth weight, preterm births, and stillbirths, as children inhale more air per unit body weight.
• Neurological and Cognitive Effects: Ultrafine secondary particles can cross the blood–brain barrier, causing neuroinflammation, cognitive decline, and increasing the risk of neurodegenerative disorders.
• Long-term and Systemic Health Risks: Prolonged exposure weakens immunity, raises cancer risk (PM2.5 classified as carcinogenic by WHO), and contributes to metabolic disorders, leading to higher public health and economic burden.

Ammonium Sulfate

• Ammonium Sulfate: Ammonium sulfate is the most dominant secondary inorganic aerosol in Delhi, accounting for nearly one-third of the city’s annual PM2.5 load, particularly during the post-monsoon winter months when pollution is at its peak.
• Formation Process: Ammonium sulfate is formed when sulfur dioxide (SO₂), reacts with ammonia in the presence of moisture and suitable temperature conditions.

Source of Precursor Pollutants

• Sulfur Dioxide (SO₂): SO₂ is released by coal-based power plants, oil refineries, brick kilns, shipping, and heavy industries, contributing significantly to secondary aerosol formation.
• Ammonia Sources: Ammonia is released from agriculture (fertilizers, livestock waste), sewage systems, landfills, biomass burning, and certain industrial processes.

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Seasonal Variation in Pollution Levels

• Role of Meteorology: Humidity, low winter temperatures, and stagnant air during the winter months accelerate the transformation of gases into particles, exacerbating pollution during these periods.
• Contribution in Different Seasons:
• • Post-Monsoon and Winter: Ammonium sulfate contributes 49% of PM2.5 in the post-monsoon period and 41% in winter.
  • Summer and Monsoon: The contribution drops to 21% during these seasons due to lower humidity and rain.
• Nationwide Impact: States like Chhattisgarh, Odisha, Jharkhand, and Telangana have shown high levels of ammonium sulfate due to coal-based power generation, contributing to transboundary pollution in regions like Delhi-NCR.

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