Understanding atmospheric stability and plume behavior is crucial for air pollution, particularly in air quality management and pollution control. Atmospheric stability influences how air pollutants disperse, affecting local and regional air quality. Plume behavior, which describes the movement and dispersion of emissions from sources like factories and vehicles, depends significantly on these stability conditions. This article explores the fundamental concepts of atmospheric stability and examines how they impact plume dispersion patterns, providing insights into effective environmental management practices.
Adiabatic Lapse Rate:
- the air is stable, that is, air at a given altitude has physical forces acting on it that make it want to remain at that elevation. Stable air discourages the dispersion and dilution of pollutants.
- the air is unstable. In this case, rapid vertical mixing takes place that encourages pollutant dispersal and increases air quality.
- If the ambient lapse rate is equal to the adiabatic lapse rate, moving the parcel upward or downward results in its temperature changing by the same amount as its surroundings. In any new position, it experiences no forces that either make it continue its motion or make it want to return to its original elevation. The parcel likes where it was, and it likes its new position too. Such an atmospheric is said to be neutrally stable.
- If the ambient lapse rate shows cooling at a faster rate than the dry adiabatic lapse rate, the atmosphere is absolutely unstable.
- The air will always want to move to some new altitude so vertical dispersion of pollution is enhanced. For ambient temperatures that cool less rapidly than the saturated adiabatic lapse rate, the atmosphere is absolutely stable.
- Relationship between atmospheric stability and temperature. It is useful to imagine a “parcel” of air being made up of a number of air molecules with an imaginary boundary around them. If this parcel of air moves upward in the atmosphere, it will experience less pressure, causing it to expand and cool. On the other hand, if it moves downward, more pressure will compress the air, and its temperature will increase.
- The adiabatic lapse rate is the rate at which the temperature of a parcel of air decreases as it is lifted in the atmosphere. The air parcel cannot exchange heat with its environment.
Dry Adiabatic Lapse Rate (DALR):
The dry adiabatic lapse rate is the rate at which the temperature of a dry air parcel changes as it ascends or descends without exchanging heat with its surroundings. The average value of the dry adiabatic lapse rate is approximately 9.8°C per kilometer (or 5.5°F per 1,000 feet).
Explanation: As an air parcel rises, it expands due to decreasing atmospheric pressure. This expansion leads to a decrease in temperature without the addition or removal of heat. Conversely, as an air parcel descends, it compresses due to increasing atmospheric pressure, causing an increase in temperature without the addition or removal of heat.
Moist Adiabatic Lapse Rate (MALR):
The moist adiabatic lapse rate is the rate at which the temperature of a moist air parcel changes as it ascends or descends without exchanging heat with its surroundings. The moist adiabatic lapse rate is variable and depends on the amount of water vapor present in the air parcel. The average value of the moist adiabatic lapse rate is approximately 5.5 °C per kilometer
Explanation: Unlike the dry adiabatic process, the moist adiabatic process involves the condensation or evaporation of water vapor as the air parcel rises or descends. This latent heat release or absorption affects the temperature change. The moist adiabatic lapse rate is generally less than the dry adiabatic lapse rate and varies with atmospheric conditions.
Environmental Lapse Rate (ELR):
The environmental lapse rate is the actual rate at which temperature changes with altitude in the surrounding atmosphere. It may vary from the dry adiabatic lapse rate and the moist adiabatic lapse rate depending on factors such as humidity, cloud cover, and atmospheric stability. 6.5°C per kilometer
2 Conditions
Super Adiabatic:
Ambient lapse rate > adiabatic
indicates unstable atmosphere. Vertical motion and mixing processes are enhanced. Dispersion of pollution plume is enhanced.
Sub Adiabatic: Ambient lapse rate < adiabatic.
It indicates stable atmosphere, vertical motion, and mixing are suppressed.
Dispersion is suppressed, and contamination is trapped.
Inversion:
An extreme case of sub adiabatic, where temperature actually increases with altitude near the ground before it begins to decrease with altitude. This results in warm, low-density air riding on top of cool high-density air; a very stable air column that traps pollution near the ground.
2 types of Inversion
- Radiation inversions are caused by nocturnal cooling of the Earth’s surface, especially on clear winter nights.
The surface of the Earth cools down at night by radiating energy toward space. On a cloudy night, the Earth’s radiation tends to be absorbed by water vapor, which in turn reradiates some of that energy back to the ground.
On a clear night, however, the surface more readily radiates energy to space, and thus ground cooling occurs much more rapidly. As the ground cools, the temperature of the air in contact with the ground also drops. As is often the case on clear winter nights, the temperature of this air just above the ground becomes colder than the air above it, creating an inversion. Radiation inversions begin to form at about dusk.
As the evening progresses, the inversion extends to a higher and higher elevation, reaching perhaps a few hundred meters before the morning sun warms the ground again, breaking up the inversion.
2. Subsidence inversions are the result of the compressive heating of descending air masses in high pressure zones.
While radiation inversions are mostly a short-lived, ground-level, wintertime phenomenon, the other important cause of inversions, subsidence, creates quite the opposite characteristics.
Subsidence inversions may last for months on end, occur at higher elevations, and are more common in summer than winter. Subsidence inversions are associated with high-pressure weather systems, known as anticyclones.
Air in the middle of a high pressure zone is descending, while on the edges, it is rising. Air near the ground moves outward from the center, while air aloft moves toward the center from the edges. The result is a massive vertical circulation system.
- There are other, less important, causes of inversions such as frontal inversions. A frontal inversion is created when a cold air mass passes under a warm air mass, but these are short lived and tend to be accompanied by precipitation that cleanses the air.
Plume behaviour in Air Pollution:
- Plume behaviour is the pattern of how gaseous pollutants disperse in the atmosphere.
- Typically, in lower atmosphere up to 300m from ground surface.
Looping Plume:
- Super Adiabatic Lapse Rate prevail in Atmosphere, (ELR>ALR), resulting in a very unstable atmosphere due to rapid mixing.
- Light to Moderate wind speed, Hot summer afternoon
- Wavy Plume
- High turbulence so rapid dispersion
- High concentration touch at ground level
Neutral Plume:
- In neutral atmospheric circumstances (ELR=ALR), a neutral plume form. A neutral plume rises vertically in an upward direction.
- The plume will continue to rise until it reaches a height where the density and temperature of the surrounding air are equal.
Coning Plume:
- Formed when horizontal wind velocity exceeds 32 km/h and cloud blocks solar radiation during the day and terrestrial radiation during the night.
- There is little vertical mixing.
- The environment is slightly stable under sub-adiabatic conditions (ELR<ALR).
- The plume shape is vertically symmetrical about the plume line.
Fanning Plume:
- Formed at extreme inversion conditions owing to a negative lapse rate.
- When the environment is under conditions of inversion, a stable environment occurs just above the stack, and the plume moves horizontally rather than upwards.
- Occurs more frequently when there is less turbulence.
- For high stack, fanning is considered a favourable meteorological condition as it doesn’t cause ground pollution.
Lofting Plume:
- Lofting plume is produced by a strong super adiabatic lapse rate immediately above the stack and a negative lapse rate (inversion) immediately below the stack opening.
- The downward movement is stopped by inversion.
- This results in a very rapid and turbulent upward mixing of the plume. But the downward mixing is less.
- As a result, the dispersion of pollutants becomes quick, and pollutants cannot come down to the ground.
- Such a plume is good for dispersing air contaminants and providing significant protection to living beings.
Fumigating Plume:
- The fumigant plume is the exact opposite of the lofting plume.
- Formed when there is a negative lapse rate (inversion) just above the stack and a strong super adiabatic lapse rate below the stack.
- Pollutants cannot escape above the stack under these conditions; thus, they settle towards the ground due to turbulence and mixing.
- As a result, the dispersion of contaminants in a fumigant plume is exceedingly poor.
Trapping Plume:
- When an inversion layer exists above and below the stack, the plume does not rise or fall.
- Rather, it is constrained or trapped between the two inversion levels, resulting in a trapping plume.
- This plume isn’t optimal for pollution dispersion since it can’t go past a particular height.
