Discuss in detail photochemical smog, emphasizing its formation, effects, and mitigation. Explain the 1999 Gothenburg Protocol.

Introduction: Photochemical smog is a brownish, oxidising haze that forms when sunlight drives reactions among nitrogen oxides (NOx) and volatile organic compounds (VOCs). Its signature pollutant is ground-level ozone, accompanied by peroxyacetyl nitrate (PAN) and aldehydes. It is a secondary pollutant — created in the air, not emitted directly from any chimney or tailpipe.

Formation: Vehicle exhaust and industry release NOx and VOCs. Ultraviolet radiation splits nitrogen dioxide into nitric oxide and atomic oxygen; that free oxygen joins molecular O2 to make ozone. VOCs feed and sustain the cycle, so ozone builds up faster than it breaks down. So smog peaks on hot, sunny, traffic-heavy afternoons in cities such as Delhi and Los Angeles.

Effects:

• Health — eye and throat irritation, aggravated asthma and bronchitis, and reduced lung function over time.
• Vegetation — ozone scorches leaves, slows photosynthesis and cuts crop yields.
• Materials — it degrades rubber, paint and textiles, corrodes surfaces and sharply reduces visibility.

Mitigation: The only durable fix is cutting precursors — tighter vehicle emission norms (BS-VI), catalytic converters, cleaner fuels, a shift to public transport and electric vehicles, VOC controls on refineries and paints, and real-time air-quality monitoring.

Gothenburg Protocol, 1999: Adopted under the UNECE Convention on Long-range Transboundary Air Pollution and in force since 2005, it was the first agreement to tackle four pollutants at once — sulphur, NOx, VOCs and ammonia — through binding national emission ceilings. Its 2012 revision added fine particulate matter (PM2.5) and black carbon, applying a multi-pollutant, multi-effect approach.

Conclusion: Smog yields only when precursor emissions are attacked at source; Gothenburg shows how shared, science-based ceilings can deliver cleaner air across borders.