0.0 Context: Why This Study Matters

0.0.1 A large part of PM2.5 that Delhi residents breathe does not come directly from vehicles, factories, or burning activities.
0.0.2 Instead, this pollution is chemically produced in the atmosphere after certain gases are released.
0.0.3 This explains why Delhi’s air quality worsens sharply during long smog episodes, even when visible emissions seem stable.

0.1 What the Study Highlights

0.1.1 The study shows that a substantial share of Delhi’s PM2.5 is secondary in nature, not directly emitted.
0.1.2 These particles are created when gases interact chemically in the air over time.
0.1.3 Such secondary aerosols now account for at least one-third of Delhi’s annual PM2.5 load.

0.2 Why Air Quality Worsens Despite Control Measures

0.2.1 During stagnant winter conditions, secondary aerosol formation accelerates even if local emissions are reduced.
0.2.2 This causes sudden air quality deterioration without any obvious spike in pollution sources.
0.2.3 Since formation occurs after emissions, traditional source-based controls show limited immediate impact.

0.3 Difference Between Primary and Secondary PM2.5

0.3.1 Primary PM2.5 is released directly from road dust, construction, vehicles, industries, and open burning.
0.3.2 Secondary PM2.5 forms later when gases like SO₂ and ammonia chemically transform into particles.
0.3.3 Weather factors such as humidity, temperature, and sunlight strongly influence this transformation.

0.4 How Secondary Particles Affect Health

0.4.1 Secondary particles are extremely fine, allowing them to penetrate deep into lung tissue.
0.4.2 Their ability to remain airborne for several days increases exposure duration.
0.4.3 Long atmospheric life enables cross-state and regional pollution transport.

0.5 Dominant Secondary Pollutant in Delhi

0.5.1 Ammonium sulfate is identified as the most significant secondary inorganic aerosol in Delhi.
0.5.2 It forms a major component of PM2.5 during severe pollution episodes.
0.5.3 This conclusion is based on analysis by the Centre for Research on Energy and Clean Air.

0.6 Contribution of Ammonium Sulfate to PM2.5

0.6.1 Ammonium sulfate alone contributes nearly one-third of Delhi’s total annual PM2.5.
0.6.2 Its concentration increases sharply in post-monsoon and winter months.
0.6.3 These months coincide with the worst air pollution episodes in Delhi.

0.7 How Ammonium Sulfate Is Formed

0.7.1 Formation starts with emissions of sulphur dioxide (SO₂) into the atmosphere.
0.7.2 SO₂ undergoes oxidation, converting into sulfate particles.
0.7.3 These sulfates combine with ammonia, producing ammonium sulfate aerosols.

0.8 Sources of Sulphur Dioxide (SO₂)

0.8.1 Coal-fired thermal power plants are the dominant source of SO₂ emissions.
0.8.2 Additional sources include oil refineries, heavy industries, brick kilns, diesel combustion, and shipping.
0.8.3 Due to coal dependence, India is the largest global emitter of SO₂.

0.9 Sources of Ammonia

0.9.1 Agricultural fertiliser use is the biggest contributor to ammonia emissions.
0.9.2 Livestock waste, sewage systems, and biomass burning add to ammonia levels.
0.9.3 Diesel vehicles with catalytic converters also release ammonia as a by-product.

1.0 Behaviour of Ammonium Sulfate in the Atmosphere

1.0.1 Once formed, ammonium sulfate remains suspended as fine particulate matter.
1.0.2 It can stay airborne for several days, increasing exposure.
1.0.3 During this period, it spreads across large geographic regions.

1.1 Why This Is a Major Concern for India

1.1.1 India’s high SO₂ emissions make secondary aerosol formation a national issue.
1.1.2 In July 2025, 78% of coal-based power plants were exempted from installing FGD systems.
1.1.3 This reduced effective source-level control of sulphur emissions.

1.2 Findings from Satellite-Based Assessment

1.2.1 CREA’s 2024 satellite-based study mapped ammonium sulfate contributions across states.
1.2.2 PM2.5 shares were highest in Chhattisgarh (42%), Odisha (41%), Jharkhand and Telangana (40%).
1.2.3 Elevated levels were also recorded in Bihar, Uttar Pradesh, Maharashtra, Andhra Pradesh and West Bengal.

1.3 Implications for Delhi-NCR

1.3.1 Secondary aerosols formed hundreds of kilometres away reach Delhi through atmospheric transport.
1.3.2 Precursor gases, not local dust alone, significantly raise Delhi’s PM2.5 levels.
1.3.3 This explains Delhi’s persistently extreme pollution rankings.

1.4 Delhi’s Air Quality Status

1.4.1 The 2024 World Air Quality Report by IQAir ranked Delhi as the most polluted capital city.
1.4.2 Delhi recorded an annual PM2.5 average of 91.6 μg/m³.
1.4.3 This reflects combined effects of local emissions and regional secondary pollution.

1.5 Policy Implication under NCAP

1.5.1 The National Clean Air Programme is currently under revision.
1.5.2 Experts argue that policy focus must include secondary aerosol chemistry, not just PM10 control.
1.5.3 Reducing SO₂ and ammonia is essential for effective PM2.5 mitigation.

1.6 Key Drivers of Ammonium Sulfate Formation

1.6.1 High humidity, fog, and low winter temperatures accelerate chemical reactions.
1.6.2 These conditions allow gases to convert into particles within hours.
1.6.3 This explains severe winter smog even without sudden emission increases.

1.7 Seasonal Contribution Pattern

1.7.1 Ammonium sulfate contributes around 49% of PM2.5 during the post-monsoon period.
1.7.2 Contribution remains high at 41% in winter.
1.7.3 It drops to about 21% during summer and monsoon due to better dispersion.