Photochemical smog is a type of air pollution that forms when sunlight reacts with nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere. Unlike classical London smog (sulfur-based), photochemical smog is often called Los Angeles smog due to its first major identification in the 1940s. It is now a common urban problem, especially in megacities with high vehicular density.

Formation of Photochemical Smog

Schematic diagram of Fog formation

1. Primary Emissions – Automobiles and industries release NOx and VOCs.

2. Sunlight-Driven Reactions – Under strong sunlight, NO₂ undergoes photolysis to form Ozone (O₃).

   • NO₂ + sunlight → NO + O
   • O + O₂ → O₃.

3. Secondary Pollutants – Further reactions with VOCs produce Peroxyacyl Nitrates (PANs), Aldehydes, and other Oxidants.

4. Climatic Conditions – High temperature, stagnant air, and thermal inversion (common in Delhi winters, Los Angeles summers) worsen smog.

Effects of Photochemical Smog

1. Human Health

   • Respiratory issues: aggravates asthma, chronic obstructive pulmonary disease (COPD), lung cancer risk.

   • Cardiovascular problems: long-term exposure linked to hypertension, heart attacks, strokes (WHO study, 2018).

   • Neurological effects: emerging research links ozone exposure to cognitive decline and dementia risk.

   • Public health burden: A Lancet study (2019) estimated ~1.2 million premature deaths in India annually due to outdoor air pollution, much of it linked to ozone and PM.

2. Environment

   • Agriculture: Ozone causes “leaf bronzing” and reduced photosynthesis, lowering yields of sensitive crops (wheat, soybean, cotton).

   • Forests & Biodiversity: Long-term ozone exposure reduces growth of sensitive trees (e.g., pinus species), altering ecosystems.

   • Soil & Water: Deposition of nitrates from smog increases eutrophication in water bodies.

3. Infrastructure & Economy

   • Material degradation: Corrosion of metals, paints, plastics, and cultural heritage (Taj Mahal, Charminar).

   • Energy costs: Higher smog leads to reduced solar power generation efficiency due to haze blocking sunlight.

   • Economic losses: World Bank (2016) estimated India loses nearly 8.5% of its GDP annually due to air pollution-related health and productivity losses.

4. Climate Linkages

   • Ground-level ozone is a short-lived climate pollutant (SLCP), contributing to global warming.

   • Black carbon and ozone together amplify the melting of Himalayan glaciers.

Mitigation Strategies for Photochemical Smog

1. Regulatory and Policy Measures

   • Emission Standards: Implementation of Bharat Stage (BS-VI) norms (2020) reducing NOx and SO₂ emissions from vehicles.

   • National Clean Air Programme (NCAP): Target to reduce particulate matter by 20–30% by 2024, indirectly curbing smog precursors.

   • Odd-Even Scheme (Delhi): Temporary reduction in vehicular traffic to cut NOx emissions.

   • Strict industrial zoning: Moving polluting industries outside city limits (e.g., Delhi shifting tanneries).

2. Technological Interventions

   • Cleaner fuels: Promotion of Compressed Natural Gas (CNG), ethanol-blended petrol, and electric vehicles.

   • Scrubbers & Catalytic Converters: Industrial and vehicular emission control devices reduce precursors of smog.

   • Green Infrastructure: Urban forestry and green belts absorb pollutants.

3. Public Transport & Urban Planning

   • Mass Rapid Transit Systems (MRTS): Delhi Metro reduces ~6.3 lakh tonnes of CO₂ annually.

   • Congestion pricing: London’s “congestion charge” zone cut NO₂ levels by ~13%.

   • Car-free zones: Example – Copenhagen promotes cycling as a mainstream transport mode.

4. Awareness & Behavioral Change

   • Promoting carpooling, public transport use, eco-friendly lifestyle.

   • Awareness campaigns on stubble burning alternatives (Happy Seeder, bio-decomposers).

5. International Best Practices

   • California Clean Air Act (1967): Strictest vehicle emission norms, ozone reduced by ~50% since 1980s.

   • China’s “War on Smog” (2013 onwards): Reduced PM2.5 levels by ~35% in major cities by shifting from coal to natural gas.

   • EU Gothenburg Protocol (1999): Binding targets for reducing SO₂, NOx, VOCs, and ammonia.

6. Scientific & Technological Innovations

   • Smog Towers: Piloted in Delhi to absorb pollutants, though limited impact.

   • Artificial rain (cloud seeding): Tested in China to reduce smog episodes.

   • Remote sensing & satellite monitoring: Real-time tracking of ozone & precursors.

1999 Gothenburg Protocol

1. Background:

   • Adopted under the UNECE (United Nations Economic Commission for Europe) Convention on Long-Range Transboundary Air Pollution (CLRTAP).
   • Entered into force in 2005, amended in 2012 to include tighter emission targets.
   • First international treaty to set legally binding national emission ceilings for multiple pollutants contributing to photochemical smog, acid rain, and eutrophication.

2. Key Commitments:

   • Targeted reduction of four major pollutants:

     • Sulphur dioxide (SO₂) – precursor of acid rain.
     • Nitrogen oxides (NOx) – contributes to ozone formation & eutrophication.
     • Volatile Organic Compounds (VOCs) – form ground-level ozone (photochemical smog).
     • Ammonia (NH₃) – contributes to particulate matter and eutrophication.

   • Introduced the concept of multi-effect and multi-pollutant approach, rather than tackling pollutants in isolation.

   • Stressed best available techniques (BATs) for industry, transport, and agriculture.

3. Implementation Mechanisms:

   • Member countries submit annual emission inventories.

   • Aimed to cut European SO₂ emissions by 63%, NOx by 41%, VOCs by 40%, and NH₃ by 17% from 1990 levels by 2010.

4. Achievements/Outcomes:

   • By 2010, most targets were met:

     • SO₂ emissions dropped by ~70%.
     • NOx emissions fell by ~40%.
     • Significant reduction in ozone-related mortality and crop damage across Europe.

   • Helped establish a model framework for cooperative air pollution control, influencing EU’s National Emissions Ceilings (NEC) Directive.

5. Significance for India:

   • Though India is not a signatory (since CLRTAP is European), it provides a global best practice for addressing transboundary and urban air pollution.

   • India could adopt similar multi-pollutant national emission ceiling frameworks to deal with stubble burning, vehicular emissions, and industrial NOx/SO₂ pollution.

Photochemical smog exemplifies the growing urban environmental health challenge in a developing economy like India. While local mitigation (clean fuels, public transport, BS-VI norms) is essential, global learning from agreements like the Gothenburg Protocol underscores the need for regional cooperation, science-based emission targets, and integrated policy frameworks