Why in news: Recent analysis by the Centre for Research on Energy and Clean Air (CREA) shows that secondary pollutants contribute nearly one-third of Delhi’s annual PM2.5 load, highlighting their growing role in urban air pollution.
What Are Secondary Pollutants?
Definition: Pollutants not emitted directly from a source but formed in the atmosphere through chemical reactions of primary pollutants under the influence of sunlight, humidity, temperature, and stagnation.
Unlike primary pollutants, they accumulate over time and often affect regions downwind from the emission source, making their control more challenging.
Major Secondary Pollutants
1Secondary Particulate Matter (PM2.5): Ammonium sulfate, ammonium nitrate.
2Ozone (O₃): Formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs) under sunlight.
3Acids: Sulfuric acid (H₂SO₄) and nitric acid (HNO₃) – contributors to acid rain.
4Photochemical smog components: Peroxyacetyl nitrates (PANs), nitrogen dioxide (NO₂).
Formation Mechanism
1Emission of precursor gases:
oSO₂: Coal-fired power plants, refineries.
oNOx: Vehicles, power plants.
oAmmonia (NH₃): Fertiliser use, livestock, sewage.
2Atmospheric transformation:
oSO₂ oxidises → sulfate → reacts with NH₃ → ammonium sulfate.
oNOx oxidises → nitric acid → reacts with NH₃ → ammonium nitrate.
3Favourable meteorological conditions:
oHigh humidity, fog, low temperatures, low wind speeds (typical in winter) accelerate secondary particle formation, leading to sudden PM2.5 spikes.
Implications
Regional and Transboundary Impact: Secondary aerosols can travel hundreds of kilometres, so Delhi’s air quality is affected by emissions from coal-dominated states beyond NCR.
Winter Smog: Stagnant, moist winter conditions cause severe PM2.5 buildup even when local emissions are controlled.
Policy Blind Spots: Controlling visible PM10 or local sources alone is insufficient; precursor gases (SO₂, NOx, NH₃) must also be targeted.
Health Risks: Fine secondary particles penetrate deep into lungs, increasing respiratory, cardiovascular, and chronic health risks.
IAS-2026 - OPTIONAL / GEOGRAPHY / PUBLIC ADMINISTRATION / SOCIOLOGY / ANTHROPOLOGY / ORIENTATION ON 03 & 04-10-2025 
