Discuss in detail the photochemical smog emphasizing its formation, effects and mitigation. Explain the 1999 Gothenburg Protocol. (UPSC 2022)

Photochemical smog is a brownish, oxidising haze seen in warm, sunny, dry cities with heavy traffic and industry.

• Formation: NOx and VOCs from vehicles and industries react under sunlight; NO2 photolyses to form atomic oxygen, which produces ground-level ozone. VOCs generate secondary pollutants like PAN and aldehydes and prevent ozone removal.
• Effects and mitigation: It irritates eyes, triggers cough, asthma and reduced lung function, harms crops, cracks rubber, corrodes materials and lowers visibility; control needs cleaner fuels, BS-VI norms, catalytic converters, vapour recovery, public transport, electric mobility, traffic management and strict NOx/VOC regulation.

The 1999 Gothenburg Protocol under UNECE-LRTAP fixed binding emission ceilings for SO2, NOx, VOCs and NH3 to curb acidification, eutrophication and ozone pollution, and was later strengthened to include PM2.5.