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Air pollution dispersion modelAir quality dispersion modelAir quality models are used to predict ground level concentrations down point of sources. The object of a model is to relate mathematically the effects of source emissions on ground level concentrations, and to establish that permissible levels are, or are not, being exceeded. Models have been developed to meet these objectives for a variety of pollutants and time circumstances.

Models may be described according to the chemical reactions involved. So-called nonreactive models are applied to pollutants such as CO and SO2 because of the simple manner in which their chemical reactions can be represented. Reactive models address complex multiple-species chemical mechanism common to atmospheric photochemistry and apply to pollutants such as NO, NO2, and O3.

Models can be described as simple or advanced based on the assumptions used and the degree of sophisticated with which the important variables are treated. Advanced models have been developed for such problems as photochemical pollution, dispersion in complex terrain, long-range transport, and point sources over flat terrain. The most widely used models for predicting the impact of relative unreactive gases, such as SO2, released from smokestacks are based on Gaussian diffusion.

In Gaussian models, the spread of a plume in vertical horizontal directions is assumed to occur by simple diffusion along the direction of the mean wind. The maximum ground level concentration is calculated by means of the following Equation.

Table 1 : Key to stability classesWind speed 10m Day Night(m/sec) Incoming solar radiation Thinly Overcast

Strong Moderate Slight >4/8 Cloud <3/8Cloud<2 A A-B B E F2 - 3 A-B B C D E3 - 5 B B-C C D D

<6 C D D D D

Where Cx = ground level concentration at some distance x downwind (g/m3)

Q = average emission rate (g/sec)u = mean wind speed (m/sec)H = effective stack height (m)

σy = standard deviation of wind direction in the horizontal (m)

σz = standard deviation of wind direction in the vertical (m)

y = off-centerline distance (m)e = natural log equal to 2.71828

The parameters σy and σz describe horizontal and vertical dispersion characteristics of a plume at various distances downwind of a source as function of different atmospheric stability conditions. Values are determined from the graphs found n the figure.

The effective stack height H is equal to the physical stack height (h) plus the height of the plume (plume rises, Δh) determined from where the plume bends over. Plume rises must be calculated from model equations before the effective stack height can be calculated.

For purposes of illustration, let us determine the ground level concentration (Cx) at some downwind distance (x). For the following conditions let us calculate the ground level concentrations at 10 km directly downwind.

A power plant burning 9 tons of 2.5% sulfur coal/hr emits SO2 at a rate of 113 g/sec. The effective stack height is 100 m, and the wind speed is 3 m/sec. It is 1 hour before sunrise, and the sky is clear. Since the off centerline distance (Y) in this case is equal to O, the following equation reduces to:

From table 1, the atmospheric stability classes for the condition described is F. It represent a nighttime condition with <37.5% cloud cover. The horizontal dispersion coefficient σy for a downtime distance of 5 km for atmospheric stability class F is approximately 90 m (figure 1); the vertical dispersion coefficient σz is approximately 20 m (figure 2)

Therefore :

The ground level concentration of SO2 from this source would be approximately 44 μg/m3 under the conditions given.

Although the use of air quality models is the subject of considerable controversy, there's a general agreement that there a few alternatives to the use of models, particulately to make decisions on an action which is know in advance to pose potential environmental problem. The debate arises as to which models should be used, and the interpretation of models results. The underlying question such in debates is how well, or how accurately, does the model predict concentrations under the specific circumstances, since model accuracy may vary from 30% to a factor of 2 or more? If a model is conservative , i.e., it over-predicts ground level concentrations, a source may be required to install costly control equipment unnecessarily. Less conservative models may under-predict concentrations and thus violations of air quality standards may occur. The uncertainty associated with input variables, such as wind data, and source emission data. Such data are usually estimated and not well documented.

Source: Air Quality 2nd edition, Thad Goddish

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Plume Types

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-Delhi/Environmental%20Air%20Pollution/air%20pollution%20%28Civil%29/Module-4/1.htm

MODULE IV

Learning ObjectivesTo make the student aware of dispersion phenomenon of air pollutants covering diffusion and

advection, meteorological components, stability of atmosphere and corresponding plume shapes.

THE ECOLOGICAL CRISIS - A Philosophical Perspective

Transport and diffusion from source to receptor

Air Pollutant Cycle

Dispersion

General mean air motion Turbulent velocity fluctuationsTurbulent velocity fluctuations Diffusion due to concentration gradients – from plumes Aerodynamic characteristics of pollution Particles

- Size- Shape- Weight

Not always completely understood Two types: Atmospheric heating

- Causes natural convection currents --- discussed - Thermal eddies

Mechanical turbulence

- Results from shear wind effects

- Result from air movement over the earth�s surface, influenced by location of buildings and relative roughness of terrain.

Lapse Rate

Important characteristic of atmosphere is ability to resist vertical motion: stability Affects ability to disperse pollutants When small volume of air is displaced upward

- Encounters lower pressure - Expands to lower temperature- Assume no heat transfers to surrounding atmosphere - Called adiabatic expansion

Adiabatic Expansion

To determine the change in temp. w/ elevation due to adiabatic expansion

.- Atmosphere considered a stationary column of air in a gravitational field - Gas is a dry ideal gas - Ignoring friction and inertial effects

( dT/dz)adiabatic perfect gas = - (g M/ Cp)

T = temperature z = vertical distance g = acceleration due to gravity M = molecular weight of air Cp = heat capacity of the gas at constant pressure

Adiabatic Expansion

( dT/dz)adiabatic perfect gas = -0.0098°C/m or( dT/dz)adiabatic perfect gas = -5.4°F/ft

Change in Temp. with change in height

Lapse rate

Lapse rate is the negative of temperature gradient Dry adiabatic lapse rate =

Metric:Metric:G = - 1°C/100m orSI:G = - 5.4°F/1000ft

Important is ability to resist vertical motion: stability. Comparison of G to actual environment lapse rate indicates stability of atmosphere. Degree of stability is a measure of the ability of the atmosphere to disperse pollutants.

Atmospheric Stability

Affects dispersion of pollutants Temperature/elevation relationship principal determinant of atmospheric stability Stable

- Little vertical mixing- Pollutants emitted near surface tend to stay there- Environmental lapse rate is same as the dry adiabatic lapse rate

4 common scenarios

Stability Classes

Developed for use in dispersion models Developed for use in dispersion models Stability classified into 6 classes (A – F)

A: strongly unstableB: moderately unstableC: slightly unstableD: neutralE: slightly stableF: moderately stable

Vertical Temperature Profiles

Environmental lapse rate (ELR)Dry adiabatic lapse rate (DALR)

If,

ELR > DALR =sub adiabatic condition, atmosphere is stable.

ELR >> DALR= Inversion conditions. Very stable atmosphere.

ELR= DALR= atmosphere is neutral.

ELR< DALR = super adiabatic condition, atmosphere is unstable.

Shapes of plumes depends upon atmospheric stability conditions.

Mixing Height of atmosphere

The height of the base of the inversion layer from ground surface.

MORNING AND AFTERNOON MIXING DEPTH CALCULATIONS

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 crosswind directions Mechanical & atmospheric heating both present at same time but in varying ratios Affect plume dispersion differently

Plume Types

Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.

Looping Plume

High degree of convective turbulence

Superadiabatic lapse rate -- strong instabilities

Associated with clear daytime conditions accompanied by strong solar heating & light winds

High probability of high concentrations sporadically at ground level close to stack.

Occurs in unstable atmospheric conditions.

Coning Plume

Stable with small-scale turbulence

Associated with overcast moderate to strong winds

Roughly 10° cone

Pollutants travel fairly long distances before reaching ground level in significant amounts

Occurs in neutral atmospheric conditions

Fanning Plume

Occurs under large negative lapse rate

Strong inversion at a considerable distance above the stack

Extremely stable atmosphere

Little turbulence

If plume density is similar to air, travels downwind at approximately same elevation

Lofting Plume

Favorable in the sense that fewer impacts at ground level.

Pollutants go up into environment.

They are created when atmospheric conditions are unstable above the plume and stable below.

Fumigation

Most dangerous plume: contaminants are all coming down to ground level.

They are created when atmospheric conditions are stable above the plume and unstable below.

This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.

References USEPA, 2007. Online literature from www.epa.gov Meteorology and Air Quality Modeling Support for Measurement Projects

http://files.harc.edu/Sites/TERC/About/Events/ Other200503/MeteorologyAndAirQuality.pdf

Rao, M.N. and Rao, H. V. N., 1993. Air Pollution, Tata Mc-Graw Hill, New Delhi.

Murty, B. P., 2004. Environmental Meteorology, I.K. International Pvt.

Ltd., New Delhi. Nevers, N.D. 2000. Air Pollution Control Engineering, Second Edition,

Pub., McGraw Hill, New York. Cheremisinoff, N.P., 2002. Handbook of Air Pollution Prevention and

Control, Pub., Butterworth-Heinemann, Elsevier Science, USA.

http://en.wikipedia.org/wiki/Air_pollution_dispersion_terminology

The types of air pollutant emission sources are commonly characterized as either point, line, area or volume sources:

Point source — A point source is a single, identifiable source of air pollutant emissions (for example, the emissions from a combustion furnace flue gas stack). Point sources are also characterized as being either elevated or at ground-level. A point source has no geometric dimensions.

Line sources — A line source is one-dimensional source of air pollutant emissions (for example, the emissions from the vehicular traffic on a roadway).

Area source — An area source is a two-dimensional source of diffuse air pollutant emissions (for example, the emissions from a forest fire, a landfill or the evaporated vapors from a large spill of volatile liquid).

Volume source — A volume source is a three-dimensional source of diffuse air pollutant emissions. Essentially, it is an area source with a third (height) dimension (for example, the fugitive gaseous emissions from piping flanges, valves and other equipment at various heights within industrial facilities such as oil refineries and petrochemical plants). Another example would be the emissions from an automobile paint shop with multiple roof vents or multiple open windows.

Other air pollutant emission source characterizations are:

Sources may be characterized as either stationary or mobile. Flue gas stacks are examples of stationary sources and busses are examples of mobile sources.

Sources may be characterized as either urban or rural because urban areas constitute a so-called heat island and the heat rising from an urban area causes the atmosphere above an urban area to be more turbulent than the atmosphere above a rural area.

Sources may be characterized by their elevation relative to the ground as either surface or ground-level, near surface or elevated sources.

Sources may also be characterized by their time duration: o puff or intermittent: short term sources (for example, many accidental emission

releases are short term puffs)o continuous: a long term source (for example, most flue gas stack emissions are

continuous)

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