This is a well-mixed box model of the four reactions that
drive Los-Angeles-type photochemical smog in the lowest layer
of the troposphere — a different process from the stratospheric
ozone layer 15–35 km up, which protects us from UV
instead of harming lungs at ground level.
(1) NO2 + hv (sunlight) -> NO + O photolysis, daylight only
(2) O + O2 -> O3 fast, forms ozone
(3) O3 + NO -> NO2 + O2 titration, consumes ozone (dominant at night)
(4) VOC + OH -> RO2 -> (+NO) -> NO2 + ... regenerates NO2 WITHOUT consuming O3
-> lets O3 accumulate through the afternoon
- NOx emissions — NO2/NO released by traffic exhaust; the raw material for reactions 1 and 3.
- VOC emissions — fuel vapour/solvent hydrocarbons; feed reaction 4, which is the reason ozone doesn't just cap out at a fixed level.
- Time of day — sets sunlight intensity (peaks near noon) and OH-radical production; also nudges temperature, since hot sunny days speed every reaction here and worsen smog.
Run the clock forward: NOx peaks with the morning commute,
then sunlight kicks in and photolyzes NO2 (step 1–2), O3
starts rising. Left alone, O3 would just react back with NO
(step 3) and plateau — but VOC + OH (step 4) keeps converting
NO back to NO2 without touching O3, so ozone keeps climbing all
afternoon and peaks several hours after the traffic and sun
both peaked. After sunset photolysis stops and only step 3
runs, so O3 falls back down overnight.