Environmental Science
The Chemistry of Smog Formation in Urban Areas
Quick fact
The term 'smog' was coined in 1905 from 'smoke' and 'fog,' but today's urban smog is primarily photochemical, driven by sunlight reacting with car exhaust—not coal smoke.
Why this is interesting
You step outside on a sunny afternoon and see a brownish haze hanging over the city. Ever wonder how that smog actually forms?
Read the full explanation
Understanding The Chemistry of Smog Formation in Urban Areas
Smog is a chemical soup, not just visible pollution. Two key ingredients come from vehicles and factories: nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the morning, these pollutants build up. Then, as sunlight intensifies, ultraviolet light triggers a chain of reactions: nitrogen dioxide (NO₂) splits apart, releasing oxygen atoms that combine with O₂ to form ground-level ozone. Ozone then reacts with VOCs, creating fine particles and more oxidants. This entire process is called photochemical smog. You can picture it as a photochemical 'cooker' that uses sunlight to turn invisible gases into a brown, irritating haze.
A deeper explanation
The core mechanism starts with NO₂ photolysis: NO₂ + UV light → NO + O. The free O atom quickly reacts with O₂ to form ozone (O₃). But ozone itself is reactive and attacks VOCs, breaking them into radicals that propagate more reactions, including the formation of secondary organic aerosols (tiny particles). This cycle amplifies ozone and particulate levels. Temperature inversions (a layer of warm air trapping cooler air near the ground) prevent vertical mixing, concentrating pollutants. The result is a thick, toxic blanket that can linger for days. Understanding this chemistry is crucial because it shows why controlling NOx and VOC emissions reduces smog, and why sunny, calm days are the worst for air quality.