Page 345 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

11.2 General Gaussian Dispersion Model 323

move upward or remain still with respect to the surrounding air. The atmosphere is
called stable, unstable and neutral atmosphere in terms of stability. They are
depicted in Fig. 11.4.

• Stable atmosphere: Consider an air parcel in an atmosphere with the same
temperature at its initial position. When the air parcel temperature elapse along
elevation is greater than that of the surrounding air, the air parcel is colder than
the surrounding air when it moves up or hotter while it goes down. As a result,
the surrounding air exerts a total force to move the air parcel back to its original
position. This total force is a result of the combination of buoyancy, friction, and
gravity.
• Unstable atmosphere: When the air parcel temperature elapse along elevation is
weaker than that of the surrounding air, the air parcel is colder than the sur-
rounding air when it moves down and hotter when it moves up. As a result, the
surrounding air exerts a total force to drive the air parcel away from its original
position and convection is produced.
• Neutral atmosphere: When the air parcel temperature elapse along elevation is
the same as that of the surrounding air, the air parcel will remain still with
respect to the surrounding air. There will be no relative motion between the air
parcel and the surrounding air in the atmosphere.
Atmosphere stability is affected significantly by so called temperature inversion,
when atmosphere temperature increases with elevation. Temperature inversion
leads to extremely stable atmosphere and sinking air emission parcel. As a result,
poor air dispersion causes accumulation of pollutants at the ground level.
The stratification of air temperature in any control volume leads to the air parcel
movement vertically. In the same place, atmosphere could be stable, neutral, or
unstable depending on the time of the day and weather condition. For example, the
ground surface and the air above it are cooled overnight. At dawn, the temperature
increases with height below 300 m or so, and the atmosphere is stable; any vertical
disturbances are strongly damped out.
Ground level stability is also affected by the heat transfer between the air and the
Earth surface. The direction of net heat transfer depends on the temperature dif-
ference between the air and the surroundings usually from high temperature to low
temperature. At the ground level, when the Earth surface temperature is higher than
the nearby air, the heat transfer from the Earth surface to the air leads to unstable
conditions and promotes air convection. Vice versa, air is cooled by a cooler Earth
surface results in a stable atmosphere. When air and Earth surface temperatures are
the same, there is no heat transfer between them and it is likely a neutral condition.
This ground-level stability changes over the hours in a day. It is a cyclic
behavior, and this cycle is qualitatively depicted in Fig. 11.5 for guidance only. The
rising sun in the morning heats the Earth surface and the air above it. The warmed
air near the ground rises to a certain elevation until it reaches the cold air at higher
elevations. Over time, this rising air gradually changes the temperature profile in the
near-ground atmosphere. This heat transfer from ground level to atmosphere con-
tinues during the day until it reaches mid afternoon. At this moment, the heated air