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conditions, Mathematical models of dispersion of air pollutants, Plume behaviour in valley and terrains. Plume behaviour under different meteorological conditions, Concept of isoplates

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- 1. Transport of Pollutionin AtmospherePrepared byBibhabasu MohantyDept. of Civil EngineeringSALITER, AhmedabadMODULE- II
- 2. Contents…Transport of Pollution in Atmosphere: Plumebehaviour under different atmosphericconditions, Mathematical models of dispersionof air pollutants, Plume behaviour in valleyand terrains. Plume behaviour under differentmeteorological conditions, Concept ofisopleths
- 3. General Characteristics of StackPlumes• Dispersion of pollutants– Wind – carries pollution downstream from source– Atmospheric turbulence - causes pollutants tofluctuate from mainstream in vertical and cross-winddirections• Mechanical & atmospheric heating both presentat same time but in varying ratios• Affect plume dispersion differently
- 4. Six Classes of PlumeBehaviorLooping:• Plume has wavy character.• Occurs in highly unstable conditions becauseof rapid mixing.• High turbulence helps dispersing plume rapidly.• High conc. may occur close to stack if plumetouches ground.
- 5. Coning:• Plume shaped like a cone• Takes place in neutral atmosphere, when windvelocity > 32 km/h.• Plume reaches ground at greater distance thanlooping.
- 6. Fanning:• Plume emitted under extreme inversionconditions.• Plume spread horizontally.• Prediction of ground level conc. is difficult.• Light wind very little turbulence.
- 7. Fumigation:• Fan or cone with well defined cone.• Pollutants are loft in air are brought rapidly toground level when air destabilizes.• Little turbulence in upper layer.• Large probability of ground contact.
- 8. Lofting:• Loops or cone with well defined bottom.• Occurs when strong lapse rate above surface inversion.• Moderate winds.• Ground contact small.• Best condition for pollutant dispersion.
- 9. Trapping:• Inversion above and below stack• Diffusion of pollutants is limited to layerbetween inversions• Very critical from point of ground levelpollutant.
- 10. Atmospheric dispersion modeling• mathematical simulation of how airpollutants disperse in the ambient atmosphere.• performed with computer programs that solvethe mathematical equations and algorithmswhich simulate the pollutant dispersion.• dispersion models are used to estimate or topredict the downwind concentration of airpollutants or toxins emitted from sources suchas industrial plants, vehicular traffic or accidentalchemical releases.
- 11. • models are important to governmental agenciestasked with protecting and managing theambient air quality.• models are typically employed to determinewhether existing or proposed new industrialfacilities are or will be in compliance withthe National Ambient Air QualityStandards (NAAQS)• also serve to assist in the design of effectivecontrol strategies to reduce emissions of harmfulair pollutants.
- 12. • dispersion models vary depending on themathematics used to develop the model, but allrequire the input of data that may include:– Meteorological conditions such as wind speed and direction– amount of atmospheric turbulence (as characterized by whatis called the "stability class"),– ambient air temperature,– height to the bottom of any inversion aloft that may bepresent,– cloud cover and– solar radiation.
- 13. • Emissions or release parameters such as source locationand height, type of source (i.e., fire, pool or ventstack)and exit velocity, exit temperature and mass flowrate or release rate.• Terrain elevations at the source location and at thereceptor location(s), such as nearby homes, schools,businesses and hospitals.• The location, height and width of any obstructions(such as buildings or other structures) in the path of theemitted gaseous plume, surface roughness or the use ofa more generic parameter “rural” or “city” terrain.
- 14. Gaussian air pollutant dispersionequation• Gaussian model which incorporates theGaussian distribution equation is the mostcommonly used.• Gaussian distribution equation uses relativelysimple calculations requiring only two dispersionparameters (i.e. σy and σz) to identify thevariation of pollutant concentrations away fromthe centre of the plume.
- 15. • This distribution equation determines groundlevel pollutant concentrations based on time-averaged atmospheric variables (e.g. temperature,wind speed).• Time averages of ten minutes to one hour areused to calculate the time-averaged atmosphericvariables in Gaussian distribution equation.
- 16. • The Gaussian distribution determines the size ofthe plume downwind from the source.• The plume size is dependent on the stability ofthe atmosphere and the dispersion of the plumein the horizontal and vertical directions.
- 17. • These horizontal and vertical dispersioncoefficients (σy and σz respectively) are merelythe standard deviation from normal on theGaussian distribution curve in the y and zdirections.• These dispersion coefficients, σy and σz, arefunctions of wind speed, cloud cover, andsurface heating by the sun.
- 18. • In order for a plume to be modelled using theGaussian distribution the following assumptionmust be made:– The plume spread has a normal distribution– The emission rate (Q) is constant and continuous– Wind speed and direction is uniform– Total reflection of the plume takes place at thesurface
- 19. Briggs plume rise equations• The most common plume rise formulas arethose developed by Gary A. Briggs. One ofthese that applies to buoyancy-dominatedplumes is included.• Plume rise formulas are to be used on plumeswith temperatures greater than the ambient airtemperature.• The Briggs’ plume rise formula is as follows:
- 20. Source Effects on Plume Rise• Due to the configuration of the stack oradjacent buildings, the plume may not rise freelyinto the atmosphere.• Some aerodynamic effects due to the way thewind moves around adjacent buildings and thestack can force the plume toward the groundinstead of allowing it to rise in the atmosphere.
- 21. • Stack tip downwash can occur where the ratio ofthe stack exit velocity to wind speed is small.• In this case, low pressure in the wake of thestack may cause the plume to be drawndownward behind the stack.• Pollutant dispersion is reduced when this occursand can lead to elevated pollutantconcentrations immediately downwind of thesource.
- 22. • As air moves over and around buildings andother structures, turbulent wakes are formed.• Depending upon the release height of a plume(stack height) it may be possible for the plumeto be pulled down into this wake area.• This is referred to as aerodynamic or buildingdownwash of the plume and can lead to elevatedpollutant concentrations immediately downwindof the source.
- 23. Concepts of isopleths• In geography, the word isopleths is used forcontour lines that depict a variable which cannotbe measured at a point, but which instead mustbe calculated from data collected over an area.• An example is population density, which can becalculated by dividing the population of a censusdistrict by the surface area of that district.
- 24. • In meteorology, the word isopleths is used forany type of contour line.• Meteorological contour lines are basedon generalization from the point data receivedfrom weather stations.• Weather stations are seldom exactly positionedat a contour line.• Instead, lines are drawn to best approximate thelocations of exact values, based on the scatteredinformation points available.
- 25. • Meteorological contour maps may presentcollected data such as actual air pressure at agiven time, or generalized data such as averagepressure over a period of time, or forecast datasuch as predicted air pressure at some point inthe future.
- 26. A two dimensional contour graph
- 27. A three dimensional contour graph
- 28. • In discussing pollution, density maps can bevery useful in indicating sources and areas ofgreatest contamination.• Contour maps are especially useful for diffuseforms or scales of pollution.• Acid precipitation is indicated on mapswith isoplats.
- 29. • Some of the most widespread applications ofenvironmental science contour maps involvemapping of environmental noise (where lines ofequal sound pressure level are denoted isobels), airpollution, soil contamination, thermalpollution and groundwatercontamination.• By contour planting and contour ploughing, therate of water runoff and thus soil erosion can besubstantially reduced.
- 30. Technical construction factors• Line weight is simply the darkness or thickness of theline used.• If there is little or no content on the base map, thecontour lines may be drawn with relatively heavythickness.• Also, for many forms of contours such as topographicmaps, it is common to vary the line weight and/orcolor, so that a different line characteristic occurs forcertain numerical values.
- 31. • Line color is the choice of any number of pigments thatsuit the display.• Sometimes a sheen or gloss is used as well as color to setthe contour lines apart from the base map.• Line colour can be varied to show other information.
- 32. • Line type refers to whether the basic contourline is solid, dashed, dotted or broken in someother pattern to create the desired effect.• Dotted or dashed lines are often used when theunderlying base map conveys very important (ordifficult to read) information.• Broken line types are used when the location ofthe contour line is inferred.
- 33. • Numerical marking is the manner of denotingthe arithmetical values of contour lines.• This can be done by placing numbers alongsome of the contour lines, typicallyusing interpolation for intervening lines.• Alternatively a map key can be producedassociating the contours with their values.

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