Study of plume behaviour

behaviour of air pollutants, plume types
lapse rate
inversion
looping plume
neutral plume
coning plume
fanning plume
lofting plume
fumigating plume
trapping plume

1. STUDY OF PLUME BEHAVIOUR RAVI KUMAR GARRE

2. Contents covered Lapse rates Inversions Winds Moisture Plume behaviour

3. Lapse rate The rate of decrease of atmospheric temperature with altitude is known as lapse rate . In a well mixed air which is dry, for every 300m increase in altitude, the temperature decreases by 1.8˚C. This vertical temperature gradient is called as lapse rate. Wherever temperature is high, the concentration of pollutants will be low. Because gases increase their volume at high temperatures and get diluted in atmosphere.

4. Negative lapse rate If the temperature of atmosphere increases with altitude, the rate of decrease of temperature with altitude is called as negative lapse rate . When negative lapse rate occurs, a dense cold stratum of air at ground level gets covered by warmer air at higher level. during negative lapse rate, the concentration of pollutants increases near their source of emission.

5. Adiabatic lapse rate The lapse rate of an air parcel as it moves upwards in atmosphere, and expands slowly to a lower environmental pressure without exchange of heat, is known as adiabatic lapse rate . Normal adiabatic lapse rate is 0.9 to 1˚C per 100m altitude. The smoke or any gaseous pollutants mass, as a parcel move upwards, by virtue of lower density and higher temperature, reach atmosphere density equal to air density.

6. Atmospheric stability Super adiabatic lapse rate – When adiabatic lapse rate is less than environmental lapse rate, then that adiabatic lapse rate is called as super adiabatic lapse rate . In that case atmosphere is said to be super adiabatic condition and unstable atmosphere . This unstable atmosphere enhance the vertical movement or air pollutants. So during super adiabatic condition, the dispersion of air pollutants will be more.

7. Atmospheric stability Sub adiabatic lapse rate – When adiabatic lapse rate is greater than environmental lapse rate, then that adiabatic lapse rate is called as sub adiabatic lapse rate . In that case atmosphere is said to be sub adiabatic condition and stable atmosphere . This stable atmosphere stops the vertical movement of air pollutants. So, during sub adiabatic condition and when the atmosphere is stable, the dispersion of air pollutants will be too low. And in stable atmosphere the pollutants tends to concentrate near their sources of emission and increase the severity of air pollutants near their sources.

8. Inversions Some times a dense cold stratum of air at ground level gets covered by light warmer air at higher level. This phenomenon is called as inversion . During inversions, Negative lapse rate occurs Atmosphere is stable Sub adiabatic condition takes place Air pollutants do not disperse Vertical movement of air pollutants will be low/ blocked Air pollutants accumulated below warmer air, i.e. with in the cold stratum of air

9. Inversions Radiation inversion – Radiation inversion usually occurs during nights. When the earth loses heat by radiation and cools the air contact with surface. If the air is moist and its temperature is below the dew point, fog will form. The cold air stratum is covered by warmer air, and the vertical movement is stopped until the Sun warms the lower air next morning. This type of inversion is more common in winter because of longer nights.

10. Inversions Subsidence inversion – It occurs at modest altitudes an often remains for several days. It is caused by anti cyclones (i.e. high pressure areas surrounded by low pressure areas). At the air sinks, it compressed and gets heated to form a warm dense layer. This act as a lid to prevent the upward movement of contaminants. When drops to less than 200m, extreme pollution occurs.

11. Winds The moving air is known as wind. In friction layer at the earth’s surface, winds are generally gusty and changeable, due to mechanical and thermal turbulences. The direction and speed of surface winds primarily govern the drift and diffusion of pollutant emissions near ground levels. Gustiness determines the extent to which the pollutants are mixed and diluted with atmosphere. The higher the wind speed near the source of emission, the more rapidly the pollutants would be carried away from the source.

12. Moisture Water vapour present in the air can block and obstruct the solar radiation reaching the ground and also the heat radiation reflected from the sun. Humidity also leads to formation of fogs and makes atmosphere stable. Such a stability stops the wind speed and make the pollutants to be concentrated at an area. Excessive moisture in the atmosphere leads to rains, which are help in improving the quality of ambient air.

13. Plume behaviour Plume – the emitted pollutants either gases/ smoke from a source Stack – source of origin of pollutants The diffusion of pollutants in atmosphere considered as plume behaviour . It is governed by environmental lapse rate and adiabatic lapse rate.

14. Plume behaviour

15. Plume behaviour Types of plume – the following are different types of plumes characterised by different relative positions of environmental lapse rate and adiabatic lapse rate: Looping plume Neutral plume Coning plume Fanning plume Lofting plume Fumigating plume Trapping plume

16. Looping plume Looping plume has a wavy character Occurs under super adiabatic conditions, when atmosphere is highly stable and under strong lapse rates When ELR>ALR

17. Looping plume Unstable atmosphere allows rapid mixing of pollutants with high degree of turbulence The dispersion of plume would be rapid.

18. Neutral plume Neutral plume is upward vertical rise of the plume from stack. This occurs when ELR is equal to ALR. The upward lifting of plume will continue till it reaches an air of density similar to that of the plume itself.

19. Neutral plume

20. Coning plume Neutral plume tends to cone when wind velocity is greater than 32 Kmph And when clouds block the solar radiation during day and earths radiation during nights. This plume dispersion is called as a cone because it makes a cone like shape about the plume line.

21. Coning plume Coning plume occurs under sub adiabatic conditions (i.e. when ELR<ALR). Under such conditions, atmosphere is slightly stable, and there is limited vertical mixing.

22. Fanning plume Fanning plume, under extreme inversion conditions, caused by negative environmental lapse rate. Extending even above the top of the stack, the emission will spread horizontally. It cannot lift upward due to stable environment.

23. Fanning plume In this case there will be no vertical mixing, and the plume will extend horizontally. Such plume pattern is called as fanning plume. In areas where such conditions are caused by radiation inversions, high-rise stacks, rising higher than the usual inversion layer may be adopted.

24. Lofting plume When there exists a strong super adiabatic lapse rate above a surface inversion, then the plume is said to be lofting. Such a plume have minimum down wind mixing, as its downward motion is prevented by inversion, but the upward mixing is quite turbulent and rapid.

25. Lofting plume The dispersion of pollutants be rapid No concentration of pollutants will touch the ground This would be most ideal case for dispersion of pollutants.

26. Fumigating plume When an inversion layer occurs at a short distance above the top of the stack, and super adiabatic conditions prevail below the stack, the plume is said to be fumigating plume.

27. Fumigating plume The pollutants cannot escape above the top of the stack because of inversion layer, and they will be brought down near the ground and below the inversion caused by strong lapse rate. Quite bad case of atmospheric conditions for dispersion.

28. Trapping plume When inversion layers occurs both above and below the stack opening, trapping plume results. The emitted plume will neither go up and nor move down. Would remain confined between two inversions. This is a bad condition of dispersion as the dispersion cannot go above a certain height.

29. Plume types

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