The planetary boundary layer (PBL) is the layer of the atmosphere that interacts with the surface on a timescale of a day or less (Fig 21). It typically extends up to 1–3 km above the surface. The air above the PBL is called the free troposphere. The free troposphere has a general slow sinking motion, balancing the few locations where deep convection or frontal lifting injects PBL air to high altitudes. The compressional heating from this sinking air produces a semi-permanent subsidence inversion (Section 6) that caps the PBL and sharply restricts mixing between the PBL and the free troposphere.
Fig 21 Diurnal evolution of the planetary boundary layer (PBL) over land (a) and implications for chemical concentrations in surface air (b).
PBL dynamics plays an important role in determining the fate of chemicals emitted at the surface and the resulting concentrations in surface air. Vertical mixing driven by solar heating of the surface can drive large diurnal cycles of concentrations within the PBL. Venting of the PBL to the free troposphere is critical for global dispersal of chemicals.
Vertical mixing within the PBL is driven by turbulent eddies. These eddies are generated at the surface by the action of the wind on rough surface elements (mechanical turbulence) and by buoyancy (buoyant turbulence). Over land, sensible heating of the surface during the day generates buoyant plumes that may rise up to the base of the subsidence inversion. Conversely, nighttime cooling of the land surface produces stable conditions that dampen the mechanical turbulence. Over the oceans, the large heat capacity of the ocean minimizes this diurnal cycle of heating and cooling and the PBL remains neutral throughout the day.
Figure 21 shows the diurnal evolution of the PBL structure over land. At night, mechanical turbulence usually maintains a shallow, well-mixed layer typically 10–100 m deep called the surface layer. Above that altitude, the atmosphere is stable because of surface cooling; this is the residual layer. After sunrise, surface heating erodes the stable residual layer from below, producing an unstable mixed layer that grows over the morning hours to eventually reach the full depth of the PBL. Clouds may develop in the upper part; these are the familiar fair-weather cumuli and the corresponding layer is called the convective cloud layer (CCL). The CCL tends to have moderate stability due to the latent heat release from cloud condensation, resulting in some separation from the mixed layer. The depth of the mixed layer (excluding any CCL) is called the mixing depth. Suppression of surface heating at sunset causes rapid collapse of the mixed layer and the nighttime conditions return.
The diurnal variation of PBL structure has important implications for the diurnal evolution of chemical concentrations in surface air, as shown in Figure 21. An inert chemical continuously emitted at the surface will accumulate in surface air over the course of the night, leading to high concentrations. During morning the concentration will decrease as growth of the mixed layer causes dilution. By contrast, a chemical originating in the free troposphere and removed by deposition to the surface will be depleted in surface air over the course of the night, and replenished during morning by entrainment from aloft as the mixed layer grows.
Over the ocean there is no diurnal cycle of surface heating and cooling, and neutral conditions prevail where vertical mixing is driven by mechanical turbulence. The mixed layer is called the marine boundary layer (MBL) and typically extends to about 1 km altitude with no diurnal variation. It is often capped by a shallow cloud layer, either cumulus clouds or stratus, capped in turn by the subsidence inversion.
Entrainment of air from the free troposphere into the PBL and ventilation of PBL air to the free troposphere are important processes for atmospheric chemistry, connecting the surface to the global atmosphere. Ventilation generally takes place by weather events, such as frontal systems or deep convective updrafts that force boundary layer air to the free troposphere. Entrainment, by contrast, generally takes place as a slow, steady process involving the large-scale sinking of the atmosphere to compensate for the convective updrafts. Typical downward entrainment velocities at the top of the PBL are of the order of 0.1–1 cm s–1, and this replaces the PBL air on a timescale of days to a week.