study guide
EHST 3600: Air Pollution 1
LAST SESSION REVIEW
• Wind velocity profile
• Planetary boundary layer
• Gradient wind
• Maximum mixing depth
• Rawinsonde measurements
• Horizontal dispersion of pollutants
• Wind speed and direction
• Wind rose
• Atmospheric turbulence
• Thermal
• Mechanical
• Stack plume characteristics
• Looping
• Coning
• Fanning
• Fumigation
• Lofting
• Trapping
• Plume behavior
• Terrain
• Buildings
LAST SESSION REVIEW
• Urban heat island effect
• Human impact on earth-atmosphere system
• Depletion of the ozone layer
• Health and ecological effects
• Rising levels of atmospheric CO2
DISPERSION OF POLLUTANTS IN THE ATMOSPHERE
EHST 3600: Air Pollution
SESSION OBJECTIVES
• To know the dispersion of pollutants
• To know the factors affecting dispersion of pollutants
• To know micro- and macro-scale dispersion of pollutants
• To know how to calculate effective stack height
SESSION OUTLINE
• Settling velocity
• The eddy diffusion model
• The Gaussian dispersion model
• Calculation of the effective stack height
ATMOSPHERIC DISPERSION OF EFFLUENTS FROM SOURCES
• Physical and chemical nature of effluents
• Meteorological characteristics of the environment
• Location of the stack with relation to obstructions to air motion
• Nature of the terrain downwind from the stack
EHST 3600: Air Pollution 2
SETTLING VELOCITY • Low settling velocity
• Gases
• Fine particles (20 um or smaller)
• Larger settling velocity
• Large particles • Results in a higher
ground-level concentration of
the solid pollutant closer to the stack than is the case for gases.
• No wind speed • Low-density plumes tend to
reach high elevations; ground
concentrations are low • Large particles and dense gas
plumes fall to the ground in the vicinity of the stack
• High wind speed
• Increases the diluting action of the atmosphere
• Gives rise to lower ground- level concentrations downwind from the stack
𝑉𝑠 𝑢 ≥ 1.5
PREVENTION OF DOWNWASH
To achieve maximum
dispersion, the
effluents should leave the stack with
sufficient momentum and
buoyancy that they
continue to rise from the stack exit.
𝑉𝑠 𝑢 ≥ 1.5
• Stack gas velocity (Vs) = 6.8 mph
• Wind velocity on top of stack (u) = 6.0 mph
• Possibility of downwash?
• If <1.5, calculate the reduced stack height:
ℎ𝑠 ′ = ℎ𝑠 + 2𝑑𝑠
𝑉𝑠 𝑢𝑠 − 1.5
hs
THE EDDY DIFFUSION MODEL
• Any diffusion process by which
substances are mixed in the
atmosphere or in any fluid system due
to eddy motion
• Involves the use of the “mixing length”
concept
• Basis of the most comprehensive approach to transport theory
• Usual starting point in the development of a dispersion model for the atmosphere
GAUSSIAN DISPERSION MODEL
Fig. 4-2
H = effective stack height
hs = stack height
h = plume rise
• Stack characteristics
• Meteorological
conditions
• Physical and chemical
nature of the effluent
H = hs + h
CALCULATION OF THE EFFECTIVE STACK HEIGHT
• Determine if there is downwash of plume due to low exit velocity
• 𝑉𝑠
𝑢 ≥ 1.5 downwash is negligible ℎ𝑠
′ = ℎ𝑠
• 𝑉𝑠
𝑢 < 1.5 downwash is likely calculate the reduced stack height, ℎ𝑠
′ :
• ℎ𝑠 ′ = ℎ𝑠 + 2𝑑𝑠
𝑉𝑠
𝑢𝑠 − 1.5
𝑉𝑠 - stack velocity (m/s) 𝑢 – wind speed measured at the height, ℎ𝑠 ℎ𝑠 - physical stack height (m) 𝑑𝑠 - stack exit diameter (m)
CALCULATION OF THE EFFECTIVE STACK HEIGHT
• Determine stability category:
• Unstable or neutral condition
• Stable condition
• Calculate the buoyancy flux, 𝐹𝑏
• 𝐹𝑏 = 𝑔𝑉𝑠𝑑𝑠 2 𝑇𝑠−𝑇𝑎
4𝑇𝑠 < or 55 m4/s3 where g = 9.8 m/s2
• Calculate the crossover temperature difference, (T)c
• Calculate the difference between the stack gas and ambient temp, T
• ∆𝑇 = 𝑇𝑠 − 𝑇𝑎
EHST 3600: Air Pollution 3
CALCULATION OF THE EFFECTIVE STACK HEIGHT
• Compare (T)c and T to determine where plume rise is dominated by momentum or buoyancy
H = hs + h
PROBLEM SOLVING
Example 4-6
• Wind speed at top of stack = 3 m/s
• Stack gas velocity = 6 m/s
• Stack diameter = 2 m
• Physical stack height = 40 m
• Atmospheric stability = neutral
• Air temp = 300 K
• Stack temp = 440 K
SUMMARY
• Settling velocity
• The eddy diffusion model
• The Gaussian dispersion model
• Calculation of the effective stack height
• Plume downwash
• Atmosphere stability conditions
• Buoyancy flux, Fb
• Crossover temperature difference, (T)c
• Difference between the stack gas and ambient temp, T
• Plume rise, h