Transport of Pollution in Atmosphere

Prepared by Bibhabasu Mohanty

Dept. of Civil EngineeringSALITER, Ahmedabad

MODULE- II

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

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

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.

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.

Fanning:• Plume emitted under extreme

inversion conditions.• Plume spread horizontally. • Prediction of ground level conc. is

difficult.• Light wind very little turbulence.

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.

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.

Trapping: • Inversion above and below stack• Diffusion of pollutants is limited to

layer between inversions• Very critical from point of ground

level pollutant.

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.

• 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.

• 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.

• 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.

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.

• 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.

• 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.

• 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.

• 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

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:

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.

• 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.

• 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.

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.

• 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.

• 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.

A two dimensional contour graph

A three dimensional contour graph

• 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.

• 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.

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.

• 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.

• 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.

• 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.

Thanks to all…