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.

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.