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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 or SI: 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 unstable B: moderately unstable C: slightly unstable D: neutral E: slightly stable F: 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.
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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.
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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
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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
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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.
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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.
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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.
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