Unformatted text preview: Air Pollution
-- Air pollution means the presence of contaminants within the
atmosphere at concentration detrimental to health or of a nuisance
nature (such as SO2 and radon)
-- It can occur either naturally or as a result of anthropogenic
-- Historically, urbanization/industrialization had been the main
cause of air pollution episodes.
Increase in population and housing density
Increased heating/power need
Greater amount of fossil fuel consumption (coal,
oil for heating, transportation, electricity)
Increase in emission from fossil fuel combustion
resulting in high atmospheric concentration.
1 Air Pollution Episodes - Historical Context -- 1
-- Pollution episodes started getting more severe than previous
ages (rapid industrialization and particularly urbanization in
-- Magnitude for each episode increases exponentially
-- Pollutant episodes become quite severe [due to urban population
concentration] resulting in widespread illness and death
Meuse Valley, Belgium, 1930 - 63 deaths and 8000 illness
(due to intense smog)
Donora, PA, 1948 - 17 deaths and 6000 illness (due to SO2
with particulates from steel and zinc plants)
London, England, 1952 - 4,000 deaths (smog in form of
SO2 with particulates from household coal burning)
Mid-Sixties - New York city, particularly bad episodes (again,
due to SO2 with particulates from fossil fuel)
2 Air Pollution Episodes - Historical Context -- 2
-- Life threatening episodes have decreased due to improvement in
emissions control and preventive measures
-- Cases of dispersive air pollution [from spatially distributed
sources/nonpoint-source such as automobile exhausts] far
exceed traditional point-source [spatially static source such
as factory stacks] air pollution 3 Major Air Pollutants -- 1
1) Particulates (TSP = Total Suspended Particulate)
-- Small particles (less than 2 µm) that penetrate both URT
(Upper Respiratory Tract) and LRT (Lower
Respiratory Tract), settle slowly to bronchi/alveoli
and damage lungs. 4 Major Air Pollutants -- 2
2) Hydrocarbon (HC)
-- Associated with fossil fuels, petroleum products such as
gasoline, composed of H and C atoms.
-- PAHs (polynuclear aromatic HC), PCBs (polychlorinated
biphenyls), Pesticide, dioxins -- all categorized as
3) Carbon Monoxide (CO)
-- Associated with incomplete oxidation [=combustion] of
organic carbon, largely from automobile.
-- Form Carboxylhemoglobin (COHb) in blood in high
concentration which will deprive the blood of
oxygen. (lethal in a few minutes at concentration
exceeding 5,000 ppm -- carbon monoxide
5 Major Air Pollutants -- 3
4) Ozone (O3)
-- Formed by reaction of hydrocarbons and nitrous oxides
(NOx). Respiratory irritant.
5) Nitrogen Oxides (NOx)
-- Formed at high temperatures
from transportation, mainly
from automobile engine
combustion, and stationary
sources such as coal-fired power
plants, factory furnace, and home
(Common brown NOx pollution on
the horizon on a late afternoon)
6 Major Air Pollutants -- 4
6) Sulfur Oxides (SOx)
-- Consist of Sulfur dioxide (SO2), Sulfur trioxide (SO3),
their acids, and salts of their acids.
-- Particularly, SO2 can be absorbed in mucous membranes
of the URT -- can easily cause chronic bronchitis.
-- Largely due to combustion of high sulfur coal from coalfired power plants and factories.
7) Lead (Pb)
-- High concentration causes a brain and nerve damages.
-- Plant/Vegetation damage can result from high
concentrations of lead particulate in the air (i.e.,
chronically sick/lethargic trees in the urban areas).
7 Major Air Pollutants -- 5
8) Acid Deposition (Acid Rain)
-- Largely the result of SOx (Sulfur dioxide, SO2 + Sulfur trioxide,
SO3) releases to the atmosphere.
a) During combustion process S + O 2 → SO 2 b) Sulfur dioxide reacts in the air
with oxygen to form Sulfur trioxide SO 2 + c) Formation of sulfurous acid and
sulfuric acid with available
atmospheric moisture SO 2 + H 2 O → H 2 SO 3 1
O 2 → SO 3
2 SO 3 + H 2 O → H 2 SO 4 d) These acids are swept from the air by precipitation or
deposited in particulate form.
e) If the amount of acids exceeds natural carbonate
buffer capacity, water becomes acidic – aquatic
ecosystem will suffer and start dying off.
8 Major Air Pollutants -- 5
9) Nitrous Oxides (NOx) and Ozone
-- Largest source NOx is the natural decay of organic matter
-- Largest anthropogenic sources of NOx are transportation and fuel
combustion from stationary sources.
-- Elevated levels of NOx in urban areas are primarily due to
-- For example, EDF (Environmental Defense Fund) report contends
upto 50% of eutrophication-causing nitrogen influx to the
Chesapeake Bay originated from transportation/automobile
exhaust from surrounding high-density, metropolitan urban areas
(including the Hampton Roads, of course). 9 Major Air Pollutants -- 6
9) Particulates (TSP = Total Suspended Particulate)
Carbon Monoxide (CO)
Nitrous Oxides (NOx)
Sulfur Oxides (SOx)
Acid Deposition (Acid Rain)
Nitrous Oxides (NOx) and Ozone 10 Photochemical Reactions (Smog) -- 1
a) Small amount of atmospheric NO2 can trigger subsequent reactions
called NO2 photolytic cycle. NO 2 + photon energy absorbed → NO + O
b) Atomic oxygen liberated from NO2 photolytic cycle reacts with
atmospheric molecular O2, produce ozone O + O2 + M → O3 + M
where M is a third molecule (usually O2 or N2 since they’re
abundant in the atmosphere) that absorbs the excess energy
from the reaction.
Without M, O3 will possess too much energy to stay stable, and
will simply dissociate back to O and O2.
11 Photochemical Reactions (Smog) -- 2
c) Ozone subsequently reacts with atmospheric NO to regenerate NO2
and molecular O2, thus completing the cycle. Thus NO 2 + O 2 ⇔ NO + O 3
Similarly for NO3 and HC, NO 3 + O 2 ⇔ NO 2 + O 3
NO x + hydrocarbon → O 3 -- NO2 is a lung irritant (LRT, especially in alveolar sac and bronchiole).
Long term exposure might cause increased respiratory illness in a
-- O3 and PAN (peroxyacetyl nitrate) formed by reaction with
hydrocarbons can cause lung distress and impair vegetation
growth -- "visible air pollution phenomena from public's point of
12 Units of Measure (= Concentration) -- Normally µg per cubic meter (µg/m3) at 0°C and 101.325 kPa
-- parts per million (ppm) is also commonly used
-- Conversion from µg/m3 to ppm Mass of pollutant( μg)
ppm = MW Temp (K)
Measure pressure in kPa
1 m 3 × 1000 3
m × 22.414× 13 National Ambient Air Quality Standard
(NAAQS) - 1970
-- National Ambient Air Quality Standard (NAAQS) was
proposed by EPA on July, 1997. NAAQS was based on the Clean
Air Act (CAA) of 1970.
-- Primary Standard was established to protect human health
with an adequate margin of safety. 14 Greenhouse Effect
-- Short wavelength radiation (less than 0.32 µm) from the sun
is absorbed as heat energy at earth ground surface.
-- Some portion of heat energy is emitted back from earth surface
to space as a long wavelength radiation (0.33 and 0.4 µm).
-- Water vapor, CO2, other gases such as methane and NOx (Nitrous
oxide) , CFC-11/12 (Chlorofluorocarbon) in atmosphere will trap
and strongly absorb this earth surface-reflected long wavelength
-- In >> Out, getting
Greenhouse. Short wavelength
emitted to earth reflected Sun absorbed atmosphere
Earth Particles, gases, moisture
can reflect radiation
radiated back to
space by earth Short wavelength
surface 15 Composition of Atmosphere
- adjacent to earth surface, where weather occurs, most
pollutants accumulate. Troposphere starts at the Earth's
surface and extends 8 to 14.5 kilometers high (5 to 9 miles).
- Stratosphere starts just above the troposphere and extends to
50 kilometers (31 miles) high.
- The ozone layer, which absorbs and scatters the solar UV
radiation, is in this layer. 16 Atmospheric Stability Related to Heating
-- Atmospheric stability is about the movement of warm air parcels
(since warm air is less dense than surrounding colder air, i.e.,
lighter than surrounding air).
-- A specific parcel of air with a temperature higher than surrounding
ambient air will keep on rising until it reaches a height at which
specific parcel of air's temperature and density equal those of
-- As the air parcel rises, it expands due to lower pressure.
Subsequently its temperature decreases and becomes lighter and
cooler due to expansion.
-- Significance of this "air parcel" movement is that "air parcel" will
carry and deposit air pollutant(s). In other words, "air
parcel" is a vehicle for possible pollutant transports, and is the
key to understand air pollution phenomena. 17 Ambient Lapse Rate
-- Lapse Rate represents the rate of the temperature changes
vertically (with height) in the atmosphere.
-- The rate can be determined for a particular place at a particular
time by sending up a balloon equipped with thermometer. The
balloon moves through the air and the temperature gradient of
ambient air, which the rising balloon measures, is called
ambient lapse rate (also known as environmental lapse rate or
prevailing lapse rate).
-- Dry adiabatic lapse rate, Γ (=gamma) represents the rate of
decrease in temperature in the air parcel, i.e., max. rate of
cooling only due to expansion of air parcel.
rate Temperature 18 Stability vs. Lapse Rate Relationship -- 1
-- The tendency of the atmosphere to resist or enhance vertical
motion is called as stability. It is related to both wind speed
and the change of air temperature with height (= lapse rate).
-- For example, if atmosphere is stable, then not much air
movement, and subsequently less pollutant transport (i.e.,
being dispersed to larger area and contaminate more) would
Stable High air pollution potential Unstable Low air pollution potential 19 Stability vs. Lapse Rate Relationship -- 2
1) Unstable Atmosphere Unstable Atmosphere Lifted air
(Superadiabatic) Temperature -- If the ambient lapse rate < dry
adiabatic lapse rate (i.e., less
temperature loss vertically than the
temperature loss due to expansion of
the air parcel), the “still-warm” parcel
of air lifted into atmosphere will
continue to rise through the
-- Unstable atmosphere (i.e., stability of
atmosphere keeps changing due to
movement of air parcels) is favorable
to dispersing of pollutants. (i.e., more
mixing, more dispersion and more
less likely causes acute air
pollution problems) 20 Stability vs. Lapse Rate Relationship -- 3 2) Stable Atmosphere
Stable Atmosphere Height
(Subadiabatic) Lifted air parcel
(adiabatic cooling) Temperature -- Ambient lapse rate is greater than dry
adiabatic lapse rate.
-- The lifted air parcel is more dense, much
cooler than surrounding atmosphere. It
results in sinking back.
-- Stable atmosphere keeps pollutants closer to
surface. (i.e., pollutants are stuck near the
ground level, and don't go anywhere)
more likely causes air pollution
problems 21 Quick Recap 1) National Ambient Air Quality Standard (NAAQS)
2) Greenhouse Effect – Short wavelength radiations are in,
but less amount of Long wavelength radiations are
3) Ambient Lapse Rate represents the rate of the
temperature changes vertically (with height) in the
4) Dry Adiabatic Lapse Rate represents the rate of decrease
in temperature, i.e., max. rate of cooling due to
expansion of air parcel.
5) Stable atmosphere is more likely have air pollution
22 Effects of Pressure on Contaminant Dispersion
-- Ambient Lapse Rate represents the rate of the temperature
changes with height in the atmosphere.
-- Dry Adiabatic Lapse Rate (Γ) represents the rate of decrease in
temperature, i.e., max. rate of cooling due to expansion of air
-- By comparing ambient lapse rate to the adiabatic lapse rate, it is
possible to predict what will happen to gases emitted from a stack.
-- Shape of smoke trail or plume from a tall stack located on flat
terrain will depend on the stability of the atmosphere.
-- Depending on stability of the atmosphere, several types of plume
• Looping plume
Trapping plume • Neutral plume
• Fanning plume
• Fumigation plume
23 Shape of Plume as F(Stability) -- 1
1) Looping plume (Superadiabatic condition)
-- Highly unstable atmosphere (= strong lapse condition,
superadiabatic), and pollutants in warmer air parcel will
rise and disperse very easily.
-- High mixing rate, i.e., wind may carry the plume down to to
the ground before dispersion is complete.
-- Occurs in clear days with light wind. 24 Shape of Plume as F(Stability) -- 2
2) Neutral plume
-- Ambient lapse rate ≈ dry adiabatic lapse rate (=Neutral
-- The plume rises directly to the atmosphere until it reaches to air
layer with a similar density to that of plume itself. (=Neutral)
-- Occurs during calm days 25 Shape of Plume as F(Stability) -- 3
3) Coning plume (Subadiabatic condition)
-- Ambient lapse rate >> Dry adiabatic lapse rate (=Subdiabatic
-- Plume cools down quicker than surrounding air during
transit/rise (=Subdiabatic), and the plume cannot rise,
i.e., “sinking back.” (downward buoyancy force pushes
the displaced plume earthward) Resulting in plumes that are
-- Limited vertical mixing, and
of air pollution in the
-- This plume tends to cone when wind velocity greater than 20
mile/hr and when cloud cover blocks solar radiation by day
and terrestrial radiation by night. (= at night and/or when
it’s cloudy) Shape of Plume as F(Stability) -- 4
4) Fanning plume
-- Ambient lapse rate is negative (=inversion or temperature
increase with elevation), the dispersion of stack gas is
minimal due to lack of turbulence.
-- Plume spreads horizontally with little vertical mixing due to
-- Visible from miles away in flat terrain. 27 Shape of Plume as F(Stability) -- 5
5) Lofting plume
-- When inversion exists below the source/plume and unstable
layer lies above the plume, the plume will disperse aloft
but not below.
-- This is a desirable situation since the pollutants are dispersed
without contributing any significant ground-level conc.
-- Lofting conditions prevail in the lake afternoon and early evening
under clear skies. 28 Shape of Plume as F(Stability) -- 6
6) Fumigation plume
-- Inverse of lofting condition
-- Occurs when a stable layer of air lies a short distance above the
release point of the plume and an unstable air layer lies
below the plume.
-- Inversion layer occurs at some point above stack height, and
literally blocking any upward vertical movement of air
-- Can cause high ground concentration of air contaminants 29 Shape of Plume as F(Stability) -- 7
7) Trapping plume
-- Occurs when an inversion exists both below and above the stack
-- Results in a coning of plume between inversion layer boundaries.
-- Trapping plume only lasts relatively short time period, and
hardly persisting. 30 Dispersion -- Terrain Effects -- 1
-- Results from “mass” materials such as concrete structures,
asphalt, etc. that absorbs and reradiates heat at a greater
rate than surrounding area.
-- Large industrial complexes, small to large cities, i.e., where
"concrete jungle" is. (=hotter in city core in summer)
-- Atmospheric stability will be lower over the area of heat
islands. (=better dispersion of pollutants) 31 Dispersion -- Terrain Effects -- 2
-- A strong local circulation pattern may develop across the
shoreline of large water bodies.
-- During the night, the land mass dissipates heat energy more
rapidly than the water body. Cooler air over the land mass
flows toward the water body due to density gradient (=Land
-- In the morning, the land mass absorbs heat energy more rapidly
than the water body. Cooler air over the water body flows
toward the land due to density gradient (=Sea breeze).
-- Plumes developed near the shoreline will form a fanning plume,
progresses into a fumigation plume several km in land. 32 Atmospheric Dispersion of Plumes
-- Plume expands and mixes with ambient atmosphere, and
contaminants eventually disperse toward the ground.
-- Plume rise affected by;
a) Upward inertia of gas stream (exit velocity of gas and
b) Buoyancy of gas stream
Plume rise (ΔH) + Physical stack height (h)
= Effective stack height (H)
-- Factors affecting dispersion of contaminants are;
a) Effective stack height
b) Downward distance (to a ground level receptor)
c) Wind speed and direction (affect rise and rate of mixing)
d) Stability of ambient atmosphere
33 Gaussian Dispersion Modeling Concept
-- Gaussian dispersion modeling technique is a mathematical
representation of meteorological transport and dispersion process
for given contaminant(s).
-- Numerical calculations yield estimates of concentration of the
particular pollutant for specific locations and times (spatially and
-- Based on Normal Probability Distribution concept
f (X ) =
PDF ⎡ − (x − μ )2 ⎤
⎣ 2σ 0.6826 0.95
0.997 -3σ -2σ -1σ μ 1σ 2σ Empirical Rule μ ± 1σ = 0.6826 ≈ 68%
μ ± 2σ = 0.9545 ≈ 95%
μ ± 3σ = 0.9973 ≈ 99.7% 3σ 34 Point Source (PS) Gaussian Dispersion Model -- 1
1) At discharged layer, atmospheric stability is uniform.
2) Random turbulent diffusion of contaminants in the gas stream;
-- both vertical and horizontal directions.
-- can be represented by a normal probability
approximation, i.e., the distribution of
pollutant concentration in the plume would
follow the standard normal probability
3) Gas stream is released into atmosphere
at a distance above ground level
that is equal to stack height (h) +
plume rise (ΔH) = Effective stack height
(H). ΔH 4) Degree of dilution of the plume is
inversely proportional to wind speed (u)
35 Point Source (PS) Gaussian Dispersion Model -- 2
-- Normally, drifting in the plume in x-direction is so much faster than
y-direction, so that the transport can be approximated solely by
-- Also, since plume propagation is very fast in a short duration, the
transport process can be considered steady-state.
-- With convective and steady-state approximation, downstream
average concentration of plume, cd can be expressed as
Ax x ⎛
c d = exp⎜ −
⎜ E x⎟
⎝ ≈ W
uA x Ax= Traverse cross-sectional area of the plume at downstream
distance x from the origin at x=0
Ax = a cross-sectional area of stack
Ax expand laterally due to dispersion
u= Wind speed
Exx= Dispersion coefficient applicable to downstream direction
W= Pollutant(s) loading from the stack
36 Point Source (PS) Gaussian Dispersion Model -- 3
-- Extent of Ax at distance x from the stack can be computed from the
variance of the distance, σ2 from the plume centerline, i.e., σ is
treated as a radius of Ax.
-- Now, using the Normal probability distribution, one standard
deviation distance from the sample mean (=the plume centerline)
equals to 68.26% of the area of the cross-sectional area.
Ax -- Also, σ2 represents the population's
variance. When used in a case-specific
computation, σ2 can be substituted with
s2, which is a sample's variance distance
from the plume centerline. Ax =
ΔH Ax π σ2
0.6826 = ⎛u
c d = exp⎜ −
⎝ π σ yσ z
0.6826 ≈ π s ysz
⎟ uA = uπ s s
37 Point Source (PS) Gaussian Dispersion Model -- 4
-- For turbulent flow in pipes which emulates a dispersive flow, the
ratio of the average velocity to the maximum velocity ranges from
0.5 to 0.82, and gives an average velocity ratio of 0.66.
-- Considering the plume as a turbulent pipe and approximating the
ratio of the average concentration of plume to the maximum
concentration at the center of plume as 0.66;
= 0.66 c d
uπ s y s z ⇒ cd = 1.03W
uπ s y s z -- With cd known, and assuming a steadystate transport process, the downwind
concentration at ground level, c at (x,y)
can be expressed by Turner’s Eq.;
2 ΔH Ax 2
⎟ exp⎛ − H ⎞
u π s ysz
2 sy ⎟
2 sz ⎟
⎟ exp⎜ − 0.5 ⎛ H ⎞ ⎟
exp⎜ − 0.5 ⎜
⎜s ⎟ ⎟
⎜ sy ⎟ ⎟
u π s ysz
⎝ z⎠ ⎠
⎝ 38 Point Source (PS) Gaussian Dispersion Model -- 5
IMPORTANT RATIONALE behind WHY THIS?
The idea of using the normal probability distribution for estimating
the extent of Ax [instead of using some empirical equations] is
a) Every case/stack situation is different, and estimating
case-specific empirical equation(s) for computing Ax is not
practical, at the same time doing such does not guarantee
Ax b) Normal probability distribution is based on a bell-shaped
curve of Cumulative probability that represents a general
trend observable, and is ideal to represent the likelihood
of natural random phenomena in a quantitative manner.
(such as turbulent mixing of pollutant in a hot/warm
gaseous plume!) x y
39 Point Source (PS) Gaussian Dispersion Model Calculation Procedure -- 1 1) Horizontal (sy) and vertical (sz) Plume standard deviations can be
(i) interpolated from charts (Figures 7-22 and 7-23,
page 592) if the atmospheric stability type and location x
at downwind direction from the stack are known
(ii) calculated using Martin's curve-fit equations s y (m ) = a[x (km )]0.894 s z (m ) = c[x (km )]d + f a, c, d and f are Martin's curve-fit coefficients.
(you will read them off from the table based on
Keep in mind that 'x' in Martin's equations above
are in km, and resulting syand sz are in m. 40 Point Source (PS) Gaussian Dispersion Model Calculation Procedure -- 2 Calculation Procedures for sy and sz in Martin's curve-fit
a) Find a atmospheric Stability Type based on the surface wind
speed at the stack, and a solar radiation condition at a given
time using following table. 41 Point Source (PS) Gaussian Dispersion Model Calculation Procedure -- 3 Calculation Procedures for sy and sz in Martin's curve-fit
equations – Cont.d
b) With the atmospheric stability type from step a), find
corresponding Martin's curve-fit coefficients and calculate sy and sz c) s y (m ) = a[x (km )]0.894 s z (m ) = c[x (km )]d + f Keep in mind that 'x' in Martin's equations above
are in in km, and resulting and sz sz are in . .
are km, and resulting sy syand are in m m
Be sure to use correct Units!!! 42 Point Source (PS) Gaussian Dispersion Model Calculation Procedure -- 4 2) Find Effective stack height, H = stack height (h) + plume
rise (ΔH). Normally, stack height is known, and ΔH can be
estimated using Holland's empirical equation ΔH = ⎛ Ts − Ta ⎞ ⎤
⎢1.5 + 2.68 × 10 × p⎜
⎜ T ⎟d⎥
vs = Stack exit velocity, m/sec
d = Diameter of the stack opening, m
u = Wind speed, m/sec
p = Atmospheric pressure, kPa
Ts = Stack temperature, K
Ta = Ambient air temperature, K
Be sure to use correct Units!!! 43 Point Source (PS) Gaussian Dispersion Model Calculation Procedure -- 5 3) Finally, the downwind concentration at ground level, c at (x,y) can
be calculated by using Turner’s Equation
u π s ysz
≈ Ax 2 ⎞
⎟ exp⎛ − H
⎠ 2 2
⎟ ⎟ exp⎜ − 0.5 ⎛ H ⎞ ⎟
exp⎜ − 0.5 ⎜
⎜s ⎟ ⎟
⎜ sy ⎟ ⎟
u π s ysz
⎝ z⎠ ⎠
Ax ΔH Ax c = Conc. at ground level at (x,y), g/m3
W = Emission/Loading rate of pollutant,
u = Wind speed, m/sec
sy and sz = Plume std. dev., m
y and H = Distance, m
Be sure to use correct Units!!! 44 Quick Recap
1) Shape of smoke trail or plume from a tall stack located on
flat terrain will depend on the Stability of the atmosphere.
2) Stable atmosphere is more likely to have air pollution
problems. (stuck and pollutants do not move/disperse
3) Stability of the atmosphere can be described by comparing
Ambient Lapse Rate and Dry Adiabatic Lapse Rate.
4) Coning and Fumigation plumes are the ones that would
cause air pollution episodes at ground levels!
5) Gaussian Dispersion Model is based on Normal
Probability Distribution, and is to estimate the likelihood
of downwind conc. of pollutants at the ground level at
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- Fall '10
- pH, ax, National Ambient Air Quality Standards, lapse rate