Dispersion: What Happens After an Emission Leaves the Source?

An emissions measurement describes a release at its source.

Environmental exposure may occur near the source or after pollutants have travelled.[R05]

Between those two points lies dispersion.

Release conditions

The initial behaviour of an emission can depend on release height, gas temperature, exhaust velocity, volume flow, particle size, surrounding buildings and terrain.

A warm, rapidly moving plume from a high stack behaves differently from a low, cool release near ground level.

Meteorology

Once in the atmosphere, pollutants are transported by air movement.

Important variables include wind speed, wind direction, atmospheric stability, temperature, turbulence, mixing height and precipitation.

The same source can therefore produce different downwind concentration patterns under different weather conditions.

Dilution

As an emission mixes with surrounding air, its concentration generally changes.

This is why a stack concentration cannot simply be interpreted as the concentration experienced by a person several kilometres away.

The atmospheric pathway must be considered.

Terrain

Mountains, valleys, coastlines and urban structures can influence airflow.

Buildings may create local turbulence and alter plume behaviour.

Valleys can experience atmospheric conditions very different from those on exposed plains.

Dispersion is therefore partly geographical.

Particle behaviour

Particles do not behave exactly like gases.

Their size, mass, shape and density can affect transport and deposition.

Fine particles may remain suspended for longer than larger particles, while other substances may react chemically or partition between gas and particle phases.

Atmospheric models

Dispersion models combine source characteristics, emissions data, meteorology, terrain and physical and chemical assumptions to estimate concentrations away from a source.

Models are essential analytical tools.

They are not direct measurements.

Their usefulness depends on the quality of their inputs and assumptions.

Multiple sources

Ambient air frequently contains emissions from many sources.

These may include traffic, domestic heating, industry, shipping, agriculture, fires and regional transport.

The concentration measured at one receptor may therefore reflect a mixture.

Source attribution requires more evidence than detecting the pollutant itself.

The governing distinction

Emission concentration is a source measurement.

Ambient concentration is an environmental condition.

Dispersion connects the two.

Understanding that connection is essential before moving from an emissions result to a claim about environmental exposure.