Air Pollution: Why the Worst Spot Isn't Under the Chimney

Where the pollution actually comes from

Air pollution sources split into two broad categories. Primary sources release pollutants directly: vehicle exhaust (nitrogen oxides, particulate matter, carbon monoxide, VOCs), industrial and power-plant emissions (sulfur dioxide, NOx, particulates, heavy metals), agriculture (ammonia from manure and fertilizer), and residential wood burning. Secondary sources form in the atmosphere itself — ground-level ozone forms when NOx and volatile organic compounds react in sunlight, and acid rain forms when SO2 and NOx react with water vapor.

The standard yardstick for how bad any given day is happens to be the Air Quality Index (AQI), running 0-500 and built from six core pollutants — PM2.5 and PM10 (fine and coarse particles), ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. PM2.5 draws particular concern because particles under 2.5 micrometers are small enough to penetrate deep into the lungs and even enter the bloodstream.

The Gaussian plume model

To predict how a pollutant released from a single point — a chimney, stack, or vent — spreads downwind, atmospheric scientists commonly use the Gaussian plume model, which estimates ground-level concentration as a function of downwind distance x:

C(x) ≈ Q / (u · σy · σz · 2π) × exp(−h² / (2σz²))

Q = emission rate,  u = wind speed,  h = source height
σy, σz = horizontal and vertical plume spread (grow with distance x)

Wind speed u sits in the denominator for a simple physical reason: faster wind carries the same emitted mass further in the same time, diluting it across a longer stretch of air — so a windier day, all else equal, means lower ground-level concentration near any given source. The exponential term captures the vertical geometry: for a stack releasing pollution well above ground level, that height h suppresses ground concentration until the plume has spread down far enough to reach it.

Why the worst concentration isn't right under the stack

A counterintuitive result of the model: for an elevated source, ground-level concentration is actually low immediately beneath the stack, because the plume's vertical spread σz hasn't grown large enough yet for much of it to have reached the ground. As distance increases, σz grows, more of the plume reaches ground level, and concentration rises — until the ever-widening σy·σz spreads the same total pollutant mass over such a large area that concentration falls again. The result is a peak some distance downwind, not directly under the source — exactly the shape shown in the simulation on this page.

Stability classes: the hidden multiplier

How fast σy and σz grow with distance depends heavily on atmospheric stability, commonly classified from A (very unstable) to F (stable) in the Pasquill-Gifford scheme. Unstable conditions — typically strong sunshine and light wind, with warm air actively rising and mixing — disperse pollutants quickly, diluting them fast. Stable conditions — classically a calm, clear night with a temperature inversion (a layer of warm air trapping cooler air, and pollution, near the ground) — barely mix at all, letting pollutants accumulate close to the source at far higher concentrations for the same emission rate. Many of history's worst smog episodes occurred precisely because a persistent inversion sat over a city for days, suppressing the vertical mixing that would otherwise have cleared the air.

What actually reduces pollution

Since the model shows concentration scaling directly with emission rate Q, the most direct lever is simply emitting less: transitioning to renewable energy, electrifying transport, installing industrial pollution controls (scrubbers, filters), and stricter emission regulations. Reforestation and urban green space provide a smaller but real assist, as vegetation absorbs some pollutants directly. None of these change the physics of dispersion — they change Q, the one variable everything else in the plume equation multiplies.

Frequently asked questions

Try it live

Everything above runs in your browser — open Air Pollution Dispersion and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.