Secondary Pollutants
Kartavya Desk Staff
Source: IE
Subject: Environment
Context: Recent analysis by the Centre for Research on Energy and Clean Air (CREA) shows that secondary pollutants now account for nearly one-third of Delhi’s annual PM2.5 load.
About Secondary Pollutants:
What are secondary pollutants?
• Secondary pollutants are not emitted directly from a source; instead, they are formed in the atmosphere when primary pollutants (gases) undergo chemical reactions influenced by sunlight, humidity, temperature and stagnation.
• Unlike visible local emissions, they often build up downwind and over time, making control more complex.
Major secondary pollutants:
• Secondary particulate matter (PM2.5): Ammonium sulfate, ammonium nitrate.
• Ozone (O₃): Formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs) under sunlight.
• Acids: Sulfuric acid and nitric acid (contributors to acid rain).
• Photochemical smog components: Peroxyacetyl nitrates (PANs), nitrogen dioxide (NO₂).
How are secondary pollutants formed?
• Emission of precursor gases: SO₂ (coal-fired power plants, refineries), NOx (vehicles, power plants), ammonia (fertiliser use, livestock, sewage).
• Atmospheric transformation: SO₂ oxidises to sulfate → reacts with ammonia → ammonium sulfate. NOx oxidises to nitric acid → combines with ammonia → ammonium nitrate.
• SO₂ oxidises to sulfate → reacts with ammonia → ammonium sulfate.
• NOx oxidises to nitric acid → combines with ammonia → ammonium nitrate.
• Favourable weather conditions: High humidity, fog, low temperatures and low wind speeds accelerate these reactions, especially in winter, allowing particles to form within hours and remain airborne for days.
Implications:
• Regional and transboundary impact: Secondary aerosols can travel hundreds of kilometres, meaning Delhi’s air quality is affected by emissions from coal-dominated states beyond NCR.
• Severe winter smog: Moist, stagnant winter conditions sharply increase secondary PM2.5, explaining sudden pollution spikes even when local sources are restricted.
• Policy blind spots: Focus on visible PM10 or local sources alone is insufficient; control of precursor gases (SO₂, NOx, NH₃) is crucial.
• Health risks: Fine secondary particles penetrate deep into lungs, increasing risks of respiratory and cardiovascular diseases.