Atmospheric Boundary Layer

The atmospheric boundary layer is the lowest part of the atmosphere, directly influenced by exchanges of heat, moisture and momentum with Earth's surface. Its turbulence mixes pollutants vertically, while winds transport them horizontally. The depth and stability of this layer therefore strongly influence near-surface pollutant concentrations, alongside emission rates.

How the boundary layer controls dispersion

Pollutants released near the ground are diluted as they mix with surrounding air. Boundary-layer conditions determine how effectively this dilution occurs.

• A greater mixing height, the vertical extent through which pollutants are effectively mixed, generally provides more air for dilution under otherwise comparable conditions.
• Turbulence consists of irregular air motions that redistribute pollutants; it is generated by surface heating and by wind shear, meaning changes in wind speed or direction with height.
• Stronger winds generally transport pollutants away more rapidly, whereas weak winds favour local accumulation.
• Vertical mixing and horizontal transport disperse pollution, but do not themselves destroy pollutants; transported pollution can affect locations downwind.

Why dispersion changes between day and night

The boundary layer changes as the ground heats and cools. This daily cycle can produce substantial changes in pollution concentrations even without an equivalent change in emissions.

• During sunny daytime conditions, surface heating generates rising thermals and a deeper convective boundary layer, usually improving vertical dilution.
• At night, radiative cooling of the ground can cool the air immediately above it, producing a shallow stable boundary layer with weaker vertical mixing.
• A temperature inversion, in which temperature increases with height, inhibits vertical mixing and can restrict the upward dispersion of pollutants.
• The cycle is not automatic: wind-generated turbulence can sustain mixing at night, while cloud cover modifies surface heating and cooling.

What this means for air pollution

High concentrations are especially likely when continuing emissions coincide with a shallow mixing layer and weak winds. Conversely, deeper mixing and stronger ventilation generally reduce local concentrations for comparable emissions.

• The ventilation coefficient, commonly expressed as mixing height multiplied by mean wind speed within the mixing layer, indicates the atmosphere's capacity to disperse pollutants.
• Poor dispersion amplifies the effects of emissions; it does not create the emitted pollutants.
• Boundary-layer conditions alone cannot fully explain air quality because emission strength, regional transport, chemical reactions and pollutant removal also matter.