Page 341 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

11.2 General Gaussian Dispersion Model 319

The next region is stratosphere, and it is below 45–55 km altitude above the
ground; temperature increases with altitude and vertical mixing of air is slow.
Mesosphere extends to 80–90 km altitude where temperature decreases again
with altitude to the coldest point in the atmosphere; Vertical mixing becomes rapid
again.
Thermosphere is the region above the mesosphere characterized by high tem-
peratures and rapid vertical mixing. The next layer, ionosphere, is characterized by
ions that are produced by photoionization. The outermost region of the atmosphere,
Exosphere is >500 km altitude.
The boundary between troposphere and stratosphere is called tropopause. Its
height changes with location. For example, the average heights of the tropopause
over the equator and over the poles are about 8 and 18 km, respectively.
The atmosphere is a sink. It is an imperfect sink with limited ability to carry
away (transport), dilute (dispersion) and remove (deposition) pollutants. Air
motions carry pollutants from one region of the atmosphere to another. On the way
to its destination, air and pollutants are dispersed by—mixing of pollutants with air.
Air dispersion takes place primarily in the lower layers of the atmosphere which
interacts with the surface of the Earth. Sometimes referred to as ground boundary
layer, the planetary boundary layer (PBL) is the lowest layer of the troposphere
where wind is influenced by friction. The thickness (depth) of the PBL is not
constant and it is dependent on many factors. At night and in the cool season the
PBL tends to be lower in thickness while during the day and in the warm season it
tends to have a greater thickness. This is because the wind speed and air density
change with temperature. Stronger wind speeds enable more convective mixing,
which cause the PBL to expand. At night, the PBL contracts due to a reduction of
rising air from the surface. Cold air is denser than warm air; therefore, the PBL
tends to be thin in the cool season.
Other conditions include, solar heating and cooling, temperature, pressure of the
air, and the wind speed and direction. They affect the result of most commonly used
air dispersion models, because these parameters contribute to the vertical motion of
air pollutant in the atmosphere. They affect the atmosphere stability, which will be
introduced soon.
Most of the motions of the atmosphere are actually horizontal as a result of
uneven heating of the Earth’s surface (most to the equator and least to the poles),
the Earth’s rotation (Carioles force) and the influence of the ground and the sea. The
surface of the land and the oceans is a well-defined lower boundary for dispersion
modeling in atmosphere. Major mountain ranges like the Himalayas, Rockies, Alps,
and Andes are major barriers to horizontal winds. Even smaller mountains and
valleys can strongly influence wind direction but on a smaller scale. The surface of
the ground and seas also changes the temperature at these boundaries depending on
the surface properties.