Cars and the factory stack release soot/PM2.5 plus NOx, SO2
and VOC (grey-brown haze). Wind disperses and dilutes the
plume; without enough wind it piles up over the city as smog.
In sunlight, NOx and VOC react to form ground-level ozone
(the haze shifts toward hazy amber) — this is the same
photochemistry behind a summer smog alert. Switching on
exhaust/flue filters (catalytic converters, scrubbers, EVs)
cuts emissions at the source instead of after the fact.
NOx + VOC + sunlight (UV) → O3 + secondary aerosol
PM2.5(t+dt) = PM2.5 + emission·dt − k_wind·wind·PM2.5·dt − k_settle·PM2.5·dt
AQI ≈ max( f_PM(PM2.5), f_O3(O3) ) (US EPA-style breakpoints, simplified)
visibility ∝ 1 / (1 + PM2.5/20)
- Traffic density — exhaust PM2.5/NOx/VOC released along the road per second.
- Industrial output — PM2.5/SO2/NOx released from the factory stack per second.
- Wind speed — how fast the city's air is replaced with clean air; low wind traps pollutants (a "stagnation" episode).
- Filters — catalytic converters, flue-gas scrubbers and cleaner engines cut emissions at the source by roughly 65%.
Real cities see exactly this pattern: rush-hour traffic and
industry raise PM2.5 and NOx, calm hot days turn that NOx into
ozone smog, and emission-control regulations (catalytic
converters since the 1970s–90s, low-emission zones today) are
what actually brought AQI down in most Western cities despite
more cars on the road.