Vehicles and coal/industrial combustion release fine particulate matter — PM2.5, particles under 2.5 micrometres across, small enough to pass deep into the lungs and into the bloodstream. Each haze particle here represents a parcel of PM2.5 released at its source, then advected by wind and turbulence.
Ground concentration (sampled in a monitoring box):
C = (N_zone / V_zone) · k [μg/m³, k = calibration constant]
US EPA Air Quality Index — piecewise-linear breakpoints on C:
0–12.0 μg/m³ → AQI 0–50 Good
12.1–35.4 → AQI 51–100 Moderate
35.5–55.4 → AQI 101–150 Unhealthy for sensitive groups
55.5–150.4 → AQI 151–200 Unhealthy
150.5–250.4 → AQI 201–300 Very unhealthy
250.5–500.4 → AQI 301–500 Hazardous
Temperature inversion: a warm-air layer aloft stops the usual upward
mixing, capping the polluted layer near the ground — the mechanism behind
Delhi's and other basin cities' worst smog days.
- Traffic / industrial emissions — set how fast new PM2.5 particles spawn at the road and the smokestack.
- Wind speed — blows particles downwind and out of the monitored zone faster, diluting concentration.
- Temperature inversion — On caps particles under an invisible lid near the ground so they accumulate; Off lets them rise and disperse into the upper atmosphere.
Real-world reference: WHO estimates ~7 million deaths a year linked to air pollution, and 99% of people breathe air exceeding WHO guidelines. Delhi's annual PM2.5 average of ~110 μg/m³ is about 22× the WHO guideline used above.