Page 342 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

320 11 Air Dispersion

11.2.2 Atmospheric Motion and Properties

The density of any part of the atmosphere can be determined by the ideal gas law
described in Eq. (11.6).
MP
q ¼ ð11:6Þ
RT

where the ideal gas constant is R ¼ 8:314 J=ðmol:KÞ; M is the molar weight of the
air in this case. Using this equation, the density of air is determined by M and T at
one particular altitude, if P is fixed. Notice that M changes with the water vapor
content; increased amount of water vapor content decrease the value of M.
When P is a variable, the air density at any point in the atmosphere can be
calculated by

1 dP
q ¼ : ð11:7Þ
g dz
Combining these two equations leads to

dP gMP
¼ : ð11:8Þ
dz RT
When temperature and molar weight of air are both constant, integration of this
equation from P 0 at z 0 to P at z leads to

gM
PðzÞ¼ P 0 exp z : ð11:9Þ
RT

11.2.3 Air Parcel

We need to put forward an important term that is used in air dispersion analysis,
which is air parcel. An air parcel is an imaginary body of air to which may be
assigned any or all of the basic dynamic and thermodynamic properties of atmo-
spheric air. The air parcel is large enough to contain a great number of molecules,
but its volume is small enough to be assigned with a uniform property. In air
dispersion modelling, it is most likely part of the “air” from the emission source.
There are many factors affecting the motion of an air parcel and its dispersion
after being emitted from a certain source. One of them is atmosphere environment,
which contributes to the initial and boundary conditions and many other input
parameters.
Air dispersion is greatly affected by the interaction between atmosphere and the
pollutant containing air parcel. Most commonly considered factors are wind,