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

1. Transport of Pollution
in Atmosphere
Prepared by
Bibhabasu Mohanty
Dept. of Civil Engineering
SALITER, Ahmedabad
MODULE- II

2. Contents…
Transport of Pollution in Atmosphere: Plume
behaviour under different atmospheric
conditions, Mathematical models of dispersion
of air pollutants, Plume behaviour in valley
and terrains. Plume behaviour under different
meteorological conditions, Concept of
isopleths

3. General Characteristics of Stack
Plumes
• Dispersion of pollutants
– Wind – carries pollution downstream from source
– Atmospheric turbulence - causes pollutants to
fluctuate from mainstream in vertical and cross-wind
directions
• Mechanical & atmospheric heating both present
at same time but in varying ratios
• Affect plume dispersion differently

4. Six Classes of Plume
Behavior
Looping:
• Plume has wavy character.
• Occurs in highly unstable conditions because
of rapid mixing.
• High turbulence helps dispersing plume rapidly.
• High conc. may occur close to stack if plume
touches ground.

6. Coning:
• Plume shaped like a cone
• Takes place in neutral atmosphere, when wind
velocity > 32 km/h.
• Plume reaches ground at greater distance than
looping.

8. Fanning:
• Plume emitted under extreme inversion
conditions.
• Plume spread horizontally.
• Prediction of ground level conc. is difficult.
• Light wind very little turbulence.

10. Fumigation:
• Fan or cone with well defined cone.
• Pollutants are loft in air are brought rapidly to
ground level when air destabilizes.
• Little turbulence in upper layer.
• Large probability of ground contact.

12. 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.

14. Trapping:
• Inversion above and below stack
• Diffusion of pollutants is limited to layer
between inversions
• Very critical from point of ground level
pollutant.

16. Atmospheric dispersion modeling
• mathematical simulation of how air
pollutants disperse in the ambient atmosphere.
• performed with computer programs that solve
the mathematical equations and algorithms
which simulate the pollutant dispersion.
• dispersion models are used to estimate or to
predict the downwind concentration of air
pollutants or toxins emitted from sources such
as industrial plants, vehicular traffic or accidental
chemical releases.

17. • models are important to governmental agencies
tasked with protecting and managing the
ambient air quality.
• models are typically employed to determine
whether existing or proposed new industrial
facilities are or will be in compliance with
the National Ambient Air Quality
Standards (NAAQS)
• also serve to assist in the design of effective
control strategies to reduce emissions of harmful
air pollutants.

18. • dispersion models vary depending on the
mathematics used to develop the model, but all
require the input of data that may include:
– Meteorological conditions such as wind speed and direction
– amount of atmospheric turbulence (as characterized by what
is called the "stability class"),
– ambient air temperature,
– height to the bottom of any inversion aloft that may be
present,
– cloud cover and
– solar radiation.

19. • Emissions or release parameters such as source location
and height, type of source (i.e., fire, pool or vent
stack)and exit velocity, exit temperature and mass flow
rate or release rate.
• Terrain elevations at the source location and at the
receptor 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 the
emitted gaseous plume, surface roughness or the use of
a more generic parameter “rural” or “city” terrain.

20. Gaussian air pollutant dispersion
equation
• Gaussian model which incorporates the
Gaussian distribution equation is the most
commonly used.
• Gaussian distribution equation uses relatively
simple calculations requiring only two dispersion
parameters (i.e. σy and σz) to identify the
variation of pollutant concentrations away from
the centre of the plume.

21. • This distribution equation determines ground
level pollutant concentrations based on time-
averaged atmospheric variables (e.g. temperature,
wind speed).
• Time averages of ten minutes to one hour are
used to calculate the time-averaged atmospheric
variables in Gaussian distribution equation.

25. • The Gaussian distribution determines the size of
the plume downwind from the source.
• The plume size is dependent on the stability of
the atmosphere and the dispersion of the plume
in the horizontal and vertical directions.

26. • These horizontal and vertical dispersion
coefficients (σy and σz respectively) are merely
the standard deviation from normal on the
Gaussian distribution curve in the y and z
directions.
• These dispersion coefficients, σy and σz, are
functions of wind speed, cloud cover, and
surface heating by the sun.

27. • In order for a plume to be modelled using the
Gaussian distribution the following assumption
must 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 the
surface

28. Briggs plume rise equations
• The most common plume rise formulas are
those developed by Gary A. Briggs. One of
these that applies to buoyancy-dominated
plumes is included.
• Plume rise formulas are to be used on plumes
with temperatures greater than the ambient air
temperature.
• The Briggs’ plume rise formula is as follows:

30. Source Effects on Plume Rise
• Due to the configuration of the stack or
adjacent buildings, the plume may not rise freely
into the atmosphere.
• Some aerodynamic effects due to the way the
wind moves around adjacent buildings and the
stack can force the plume toward the ground
instead of allowing it to rise in the atmosphere.

31. • Stack tip downwash can occur where the ratio of
the stack exit velocity to wind speed is small.
• In this case, low pressure in the wake of the
stack may cause the plume to be drawn
downward behind the stack.
• Pollutant dispersion is reduced when this occurs
and can lead to elevated pollutant
concentrations immediately downwind of the
source.

32. • As air moves over and around buildings and
other structures, turbulent wakes are formed.
• Depending upon the release height of a plume
(stack height) it may be possible for the plume
to be pulled down into this wake area.
• This is referred to as aerodynamic or building
downwash of the plume and can lead to elevated
pollutant concentrations immediately downwind
of the source.

34. Concepts of isopleths
• In geography, the word isopleths is used for
contour lines that depict a variable which cannot
be measured at a point, but which instead must
be calculated from data collected over an area.
• An example is population density, which can be
calculated by dividing the population of a census
district by the surface area of that district.

35. • In meteorology, the word isopleths is used for
any type of contour line.
• Meteorological contour lines are based
on generalization from the point data received
from weather stations.
• Weather stations are seldom exactly positioned
at a contour line.
• Instead, lines are drawn to best approximate the
locations of exact values, based on the scattered
information points available.

36. • Meteorological contour maps may present
collected data such as actual air pressure at a
given time, or generalized data such as average
pressure over a period of time, or forecast data
such as predicted air pressure at some point in
the future.

37. A two dimensional contour graph

38. A three dimensional contour graph

39. • In discussing pollution, density maps can be
very useful in indicating sources and areas of
greatest contamination.
• Contour maps are especially useful for diffuse
forms or scales of pollution.
• Acid precipitation is indicated on maps
with isoplats.

40. • Some of the most widespread applications of
environmental science contour maps involve
mapping of environmental noise (where lines of
equal sound pressure level are denoted isobels), air
pollution, soil contamination, thermal
pollution and groundwater
contamination.
• By contour planting and contour ploughing, the
rate of water runoff and thus soil erosion can be
substantially reduced.

41. Technical construction factors
• Line weight is simply the darkness or thickness of the
line used.
• If there is little or no content on the base map, the
contour lines may be drawn with relatively heavy
thickness.
• Also, for many forms of contours such as topographic
maps, it is common to vary the line weight and/or
color, so that a different line characteristic occurs for
certain numerical values.

42. • Line color is the choice of any number of pigments that
suit the display.
• Sometimes a sheen or gloss is used as well as color to set
the contour lines apart from the base map.
• Line colour can be varied to show other information.

43. • Line type refers to whether the basic contour
line is solid, dashed, dotted or broken in some
other pattern to create the desired effect.
• Dotted or dashed lines are often used when the
underlying base map conveys very important (or
difficult to read) information.
• Broken line types are used when the location of
the contour line is inferred.

44. • Numerical marking is the manner of denoting
the arithmetical values of contour lines.
• This can be done by placing numbers along
some of the contour lines, typically
using interpolation for intervening lines.
• Alternatively a map key can be produced
associating the contours with their values.