Why in the News?

A recent analysis by the Centre for Research on Energy and Clean Air has revealed that secondary pollutants now contribute nearly one third of Delhi’s annual PM2.5 load, highlighting a major shift in the nature of urban air pollution.

What are Secondary Pollutants?

• Secondary pollutants are not emitted directly from pollution sources.
• They are formed in the atmosphere when primary pollutants such as SO₂, NOx and VOCs undergo chemical reactions.
• These reactions are influenced by sunlight, temperature, humidity and stagnant air conditions.
• They often accumulate downwind and over time, making monitoring and control more complex than primary pollutants.

Major Secondary Pollutants

• Secondary PM2.5: Ammonium sulfate and Ammonium nitrate
• Ozone (O₃): Formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight
• Acids: Sulfuric acid and Nitric acid (contributors to acid rain)
• Photochemical smog components: Peroxyacetyl nitrates (PANs) and Nitrogen dioxide (NO₂)

Implications

Regional and transboundary impact

• Secondary aerosols can travel hundreds of kilometres
• Delhi’s air quality is influenced by emissions from coal intensive states beyond the NCR

Winter smog intensification

• Moist and stagnant winter air sharply increases secondary PM2.5

Health risks

• Fine secondary particles penetrate deep into the lungs
• Increase risks of respiratory and cardiovascular diseases

Prelims Pointers

• Secondary pollutants are formed in the atmosphere, not emitted directly
• Ammonia plays a key role in secondary PM2.5 formation
• Winter meteorology is crucial for secondary aerosol build up
• Air pollution control requires regional coordination, not only city level measures

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