Photochemical Smog
Introduced| English | Meaning |
|---|---|
| photochemical smog/ˌfəʊtəʊˈkemɪkl smɒɡ/ | Air pollution formed when sunlight drives reactions involving nitrogen oxides and volatile organic compounds, producing pollutants such as ozone. |
| sunlight/ˈsʌnlaɪt/ | sunlight |
| nitrogen oxides/ˈnaɪtrədʒn ˈɒksaɪdz/ | Compounds of nitrogen and oxygen; in air-pollution discussions, NOx mainly refers to nitric oxide and nitrogen dioxide. |
| vapours/ˈveɪpəz/ | vapours |
| ozone/ˈəʊzəʊn/ | The molecule O3, made of three oxygen atoms; it absorbs ultraviolet radiation in the stratosphere but is a pollutant near the ground. |
Smog you can see
- On hot city days, a brown haze can hang in the air.
- This is photochemical smog 光化学烟雾.
- "Photochemical" means it is made by sunlight 阳光.
- It stings the eyes and makes it hard to breathe.
It starts with traffic
- Cars and trucks pour out nitrogen oxides 氮氧化物 (NOx).
- They also release vapours 蒸气 from fuel, called VOCs.
- On their own these are primary pollutants.
- But sunlight turns them into something worse.
Photochemical smog over Los Angeles: on a sunny day, car exhaust reacts in the light to make a brown haze that hides the skyline
Photochemical smog needs which one to form?
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"Photochemical" means light-driven — strong sunlight powers the reactions that make smog.
Sunlight builds ozone
- Strong sunlight drives NOx and vapours to react.
- The reaction makes ground-level ozone 臭氧.
- Ozone is a secondary pollutant — no engine emits it directly.
- It is the main harmful gas in smog.
How photochemical smog forms
Follow car exhaust and sunlight combining into the ozone smog over a city.
The pollutants that start photochemical smog come mostly from…
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Vehicle exhaust supplies the nitrogen oxides and vapours that sunlight turns into smog.
The main harmful gas in photochemical smog is ground-level ____.
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Ground-level ozone is the key ingredient of smog and irritates the lungs.
Why it is worst when it is
- Smog needs sunlight, so it peaks on hot, sunny afternoons.
- It needs traffic, so it is worst in big, crowded cities.
- Still air lets it build up instead of blowing away.
- Hot + sunny + traffic + calm = a bad smog day.
Smog is usually worst on cold, cloudy days with little traffic.
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False. Photochemical smog needs sunlight and heat acting on vehicle exhaust, so it is worst on hot, sunny days in cities with heavy traffic.
Select all ingredients photochemical smog needs.
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Smog forms from sunlight + nitrogen oxides + vapours. Rain actually clears the air.
Ground-level ozone is the villain here — and it's a secondary pollutant. No tailpipe emits ozone; sunlight builds it in the air from exhaust. Don't confuse it with the good ozone high up in the stratosphere, which shields us from UV. Ozone is helpful up high and harmful down low — same molecule, opposite effect.
A hot city afternoon:
- Morning traffic has filled the air with nitrogen oxides and fuel vapours.
- The midday sun beats down, driving those pollutants to react and build up ozone.
- With no wind to clear it, a brown smog settles over the skyline — and hospitals see more people with breathing trouble.
Photochemical smog forms when sunlight drives nitrogen oxides and fuel vapours (from traffic) to react, building up ground-level ozone — a secondary pollutant that irritates the lungs. It is worst on hot, sunny, calm days in traffic-heavy cities.