Revision summary
Photochemical smog forms when sunlight acts on NOx and VOCs, producing ground-level ozone and PAN. It peaks downwind in the afternoon under stagnant, sunny conditions. Effects are respiratory harm, crop damage and material cracking; this ozone is not the stratospheric shield. Mitigation is BS emission norms, VOC controls, public transport and regional air-shed policy. The Gothenburg Protocol 1999 under CLRTAP sets national ceilings on sulphur, NOx, ammonia and VOCs to cut acidification, eutrophication and ozone.
Model answer
Introduction
Photochemical smog is a brown, oxidising haze of ozone and peroxyacyl nitrates formed when sunlight cooks nitrogen oxides and volatile organic compounds. It is the Los Angeles-type smog, not the classic London sulphur fog. The Gothenburg Protocol (1999) is a UNECE treaty to cut the gases that make this smog and acid rain in Europe and North America.
Body
Formation
- Primary emissions: NO and NO2 (NOx) from vehicles and combustion; VOCs from petrol, solvents, and some industry and vegetation.
- In sunlight, NO2 photolyses; the oxygen atom makes O3 with O2. VOCs feed peroxy radicals that convert NO back to NO2 without destroying ozone, so ozone accumulates.
- PAN (peroxyacyl nitrates) and aldehydes form in the same radical soup. Peak is typically afternoon, downwind of the city.
- Stagnant high pressure, a basin, and a temperature inversion trap the mix. Indian examples include summer-ozone episodes in some metros, distinct from winter PM2.5 smog.
Effects
- Human health: ozone inflames airways, triggers asthma, and reduces lung function; PAN is a lachrymator.
- Plants: ozone flecks leaves, cuts photosynthesis and crop yield.
- Materials: rubber and some paints crack; visibility falls.
- Ozone at the ground is a pollutant; ozone in the stratosphere is a shield. Do not confuse the two.
Mitigation
- Cut NOx: Bharat Stage emission norms, catalytic converters, inspection of in-use vehicles, power-plant controls.
- Cut VOCs: vapour recovery at petrol pumps, solvent standards, leak control.
- Shift to public transport, electrified fleets, and less congestion; ozone is a regional pollutant, so one city’s fleet is not enough.
- Alert systems and reducing outdoor work on peak ozone days protect health while emissions fall.
Gothenburg Protocol, 1999
- Protocol to the 1979 Convention on Long-Range Transboundary Air Pollution (CLRTAP), adopted at Gothenburg in 1999.
- It sets national emission ceilings for sulphur, NOx, ammonia and VOCs—the precursors of acidification, eutrophication and ground-level ozone.
- Later amendments added fine particulate matter and tightened ceilings. Parties are mainly UNECE (Europe, plus the United States, Canada).
- India is not a party in the European sense; the Protocol is still the model of multi-pollutant, multi-effect ceilings that Indian NOx and VOC policy can learn from.
Flow diagram
flowchart TD N[NOx] --> S[Sunlight] V[VOCs] --> S S --> O[Ground ozone PAN] O --> H[Health crops materials] M[BS norms VOC control] --> N M --> V G[Gothenburg 1999] --> C[Ceilings S NOx NH3 VOC]
Conclusion
Photochemical smog is sunlight plus NOx and VOCs, yielding ozone and PAN that harm lungs and crops. Mitigation is cleaner vehicles, VOC control and regional air-shed action. The 1999 Gothenburg Protocol is the UNECE bargain to cap those same precursors across borders.
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Is Delhi winter smog photochemical smog?
Much of it is particulate (PM2.5) from combustion, dust and stubble, trapped by inversion. Photochemical ozone is a sunnier, NOx-VOC problem.
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Does Gothenburg bind India?
No. It binds UNECE parties. It is still the reference for multi-pollutant emission ceilings.
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