Structure and Composition of the Present-Day Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 50

Free troposphere

Free troposphere

Entrainment zone /
Inversion layer

Entrainment zone /
Inversion layer

Cloud layer

Subcloud layer Residual layer

Convective mixed layer

Nocturnal
boundary
layer

Surface layer

Surface layer
Nighttime temperature Daytime temperature

(a) (b)

Figure 3.4. Variation of temperature with height during the (a) day and (b) night in the atmospheric boundary
layer over land under a high-pressure system (Chapter 6). Adapted from Stull (1988).

with increasing height. The inversion inhibits the rise of
thermals originating from the surface layer or the mixed
layer. Some mixing (entrainment) between the inversion
and mixed layer does occur; thus, the inversion layer is
also called an entrainment zone . Pollutants are gen-
erally trapped beneath or within an inversion; thus, the
closer the inversion is to the ground, the higher pollutant
concentrations become.
Other features of the daytime boundary layer are the
cloud and subcloud layers. A region in which clouds
appear in the boundary layer is the cloud layer , and the
region underneath is the subcloud layer .
During the night (Figure 3.4b), the ground cools
radiatively, causing air temperatures to increase with
increasing height from the ground, creating a surface
inversion. Once the nighttime surface inversion forms,
pollutants, when emitted, are confined to the surface
layer.
Cooling at the top of the surface layer at night cools
the bottom of the mixed layer, reducing the buoyancy
and associated mixing at the base of the mixed layer.
The portion of the daytime mixed layer that loses its
buoyancy at night is the nocturnal boundary layer .
The remaining portion of the mixed layer is the resid-
ual layer .Because thermals do not form at night, the
residual layer does not undergo much change at night,
except at its base. At night, the nocturnal boundary

layer thickens, eroding the residual layer base. Above
the residual layer, the inversion remains.

3.3.1.2. Background Troposphere
The background troposphere lies between the bound-
ary layer and the tropopause. It is a region in which,
on average, the temperature decreases with increasing
altitude. The average rate of temperature decrease in the
background troposphere is about 6.5 K km − 1 .Thetem-
perature decreases with increasing altitude in the back-
ground troposphere for the following reason: the ground
surface receives solar energy from the sun daily, heat-
ing the ground. The ground converts that solar energy to
thermal infrared (heat) radiation, which is emitted back
to the atmosphere. Heat from the ground is also trans-
ferred vertically by conduction and convection. Green-
house gases, aerosol particles, and hydrometeor parti-
cles in each layer of the atmosphere absorb the heat
and reemit some of it back down toward the surface
and some upward in the form of thermal-IR radiation.
Thus, each successively higher layer in the atmosphere
receives less heat than did the layer below. In sum,
the troposphere, itself, has relatively little capacity to
absorb solar energy; thus, each layer of air in the tro-
posphere relies on heat transfer from the layer of air
below it for its energy, so temperature in the tropo-
sphere decreases with increasing height.