Download 6-7 AIR POLLUTION METEOROLOGY The Atmospheric Engine

AIRPOLLUTION 491
tration, the direct radiative effect of increasingCO2aloneis not sufficient to explain
currenttrends that showan increasein nighttime temperaturesbut not an increasein
daytime highs.45The input sourcesof greenhousegasesand their sinks are not yet
well described.The measurementof temperatureon a global basisis not sufficiently
uniform in technique and separatedfrom local influence to separatethe "noise" of
local variability from true trends. Natural changessuchas increasesin cloud cover
may not have been accuratelydepicted in existing modelsof climate change.
In general, projections of global warming have been based on assumptions
regardingthe growth of greenhousegases.If it is assumedthat they will continue
to grow exponentially, by the year 2040 the changein atmosphericconcentrationof
greenhousegaseswould be the equivalentof doubling of the CO2concentrationfrom
its preindustrial level. It is this doubling that leads the National ResearchCouncil
to estimatea temperaturerise of 10to 5°C.46In anotherprojection of emissionsby
the IntergovernmentalPanel on Climate Change,global temperaturesare expected
to rise between0.80 and 3.5°C by 2100.47Obviously, there is still considerabledisagreementabout the potential for global warming. On the other hand, the consequencesof ignoring thesetrendsare sufficiently dramatic that intensive researchwill
continuein the nextdecade.Even without the risks of climate change,improvements
in energy efficiency to reduce CO2 emissionsand to eliminate CFCs are justified.
The expectationof damagesfrom climate changeprovides a rationale for pursuing
theseprogramsvigorously.
6-7 AIR POLLUTION METEOROLOGY
The AtmosphericEngine
The atmosphereis somewhatlike an engine. It is continually expandingand compressinggases,exchangingheat, and generally raising chaos.The driving energy
for this unwieldy machinecomesfrom the sun.The difference in heatinput between
the equatorand the poles provides the initial overall circulation of the earth's atmosphere.The rotationof the earth coupled with the different heatconductivities of the
oceansand land produceweather.
Highs and lows. Becauseair hasmass,it alsoexertspressureonthings underit. Like
water, which we intuitively understandto exertgreaterpressuresat greaterdepths,the
atmosphereexertsmore pressureat the surfacethan it doesat higher elevations.The
highs and lows depicted on weathermapsare simply areasof greaterand lesserpressure.The elliptical lines shownon more detailed weathermapsare lines of constant
pressure,or isobars.A two-dimensionalplot of pressureanddistancethrougha highor low-pressuresystemwould appearas shownin Figure 6-11.
4SG.Kukla and T. R. Karl, "Nighttime Warming and Ihe GreenhouseEffect," Environmental Science
and Technology,27, pp. 1468-1474, 1993.
46L. B. Lave and H. Dow1atabadi,"Climate Change: The Effects of Personal Beliefs and Scientific
Uncertainty," Environmental Scienceand Technology,27, pp. 1962-1972, 1993.
47C&ENews. p. 20, August 28,1995.
492 INTRODUCTION
TOENVIRONMENTAL
ENGINEERING
I
Y
r
.-t
A
A
I
X
I
p
-""~=~~~"--O
2.8
102.4
102.0kPa
A
A
X
(a)
Y
("E~~
\,~~~~=~~~ t B
X
P
~
B
:::::~~~~7fO
1.2 k Pa
B
100.8
100.4
X
(b)
FIGURE
6.11
High and low pressuresystems.
The wind flows from the higher pressureareasto the lower pressureareas.On
a nonrotatingplanet, the wind direction would be perpendicularto the isobars(Figure 6-12a). However, since the earth rotates, an angular thrust called the Coriolis
effect is added to this motion. The resultant wind direction in the northern hemisphereis as shown in Figure 6-12b. The technical namesgiven to thesesystemsare
anticyclonesfor highs and cyclonesfor lows. Anticyclones are associatedwith good
weather.Cyclones are associatedwith foul weather.Tornadoesand hurricanes are
the foulest of the cyclones.
Wind speedis in part a function of the steepnessof the pressuresurface.When
the isobarsare closetogether,the pressuregradient (slope)is said to be steepand the
wind speedrelatively high. If the isobarsare well spreadout, the winds are light or
nonexistent.
AIRPOLLUTION
493
I
r
--<
~~~~~~~),,'
"
If
,.
t
\
.~
'"
(a) Anticyclonewithout Coriolis effect
---<:::::~~~~~~\.:,I
"'/.,1,,/ J
---"'.,1.,1
FIGURE 6-12
(b) Anticyclonewith Coriolis effect
Wind flow due to pressuregradient.
Thrbulence
Mechanical turbulence. In its simplest terms, we may considerturbulence to be
the addition of randomfluctuations of wind velocity (that is, speedand direction) to
the overall averagewind velocity. Thesefluctuations are caused,in part, by the fact
that the atmosphereis being sheared.The shearingresults from the fact that the wind
speedis zero at the groundsurfaceand rises with elevationto nearthe speedimposed
by the pressuregradient. The shearingresults in a tumbling, tearing motion as the
massjust abovethe surfacefalls overthe slower moving air atthe surface.The swirls
thus formed are called eddies.These small eddies feed larger ones.As you might
expect, the greater the mean wind speed,the greaterthe mechanical turbulence.
The more mechanicalturbulence,the easierit is to disperseand spreadatmospheric
pollutants.
Thermal turbulence. Like all otherthings in nature,the rather complex interaction
that producesmechanical turbulence is confoundedand further complicated by a
.
494
INTRODUCTIONTO ENVIRONMENTALENGINEERING
third party. Heating of the ground surfacecausesturbulence in the samefashion that
heatingthe bottom of a beakerfull of watercausesturbulence. At somepoint below
boiling, you can see density currents rising off the bottom. Likewise, if the earth's
surfaceis heated strongly and in turn heatsthe air aboveit, thermal turbulence will
be generated.Indeed, the "thermals" soughtby glider pilots and hot air balloonists
are thesethermal currents rising on what otherwisewould be a calm day.
The conversesituationcan arise during clear nights when the ground radiates
its heataway to the cold night sky. The cold ground, in turn, cools the air aboveit,
causinga sinking density current.
Stability
The tendencyof the atmosphereto resist or enhancevertical motion is termed stability. It is related to both wind speedand the changeof air temperaturewith height
(lapse rate). For our purpose,we may use the lapserate alone as an indicator of the
stability condition of the atmosphere.
There are three stability categories.When the atmosphereis classified as unstable,mechanicalturbulenceis enhancedby the thermal structure.A neutral atmosphereis one in which the thermal structure neitherenhancesnor resistsmechanical
turbulence. When the thermal structure inhibits mechanical turbulence, the atmosphereis said to be stable. Cyclones are associatedwith unstableair. Anticyclones
are associatedwith stableair.
Neutral stability. The lapse rate for a neutral atmosphereis defined by the rate of
temperatureincrease (or decrease)experiencedby a parcel of air that expands (or
contracts) adiabatically (without the addition or loss of heat) as it is raised through
the atmosphere.This rate of temperaturedecrease(dT/dz) is called the dry adiabatic lapse rate. It is designatedby the Greek letter gamma (f). It has a value of
approximately -1.00°C/100 m. (Note that this is not a slope in the normal sense,
that is, it is not dy/dx.) In Figure 6-13a, the dry adiabatic lapserate of a parcel of
air is shownas a dashedline and the temperatureof the atmosphere(ambientlapse
rate) is shown as a solid line. Since the ambient lapse rate is the same as f, the
atmosphereis said to have a neutral stability.
Unstable atmosphere. If the temperatureof the atmospherefalls at a rate greater
than f (for example, -1.01°C/100 m), the lapse rate is said to be superadiabatic,
and the atmosphereis unstable. Using Figure 6-13b, we can see that this is so. The
actual lapse rate is shown by the solid line. If we capturea balloon full of polluted
air at elevation A and adiabatically displace it 100 m vertically to elevationB, the
temperatureof the air inside the balloon will decreasefrom 21.15° to 20.15°C. At
a lapse rate of -1.25°C/100 m, the temperatureof the air outside the balloon will
decreasefrom 21.15° to 19.90°C. The air inside the balloon will be warmerthan the
air outside; this temperaturedifference gives the balloon buoyancy. It will behave
as a hot gas and continue to rise without any further mechanicaleffort. Thus, mechanical turbulenceis enhancedand the atmosphereis unstable.If we adiabatically
displace the balloon downward to elevation C, the temperatureinside the balloon
I
AIRPOLLUTION 495
Volume at Rest
T = 20.15
400
,/Dry
'"
e
.-mIen
~
~
Adiabat = r
"
300
ico
Same Temperature
as Surroundings
Displaced
100 m
~1;1t,
I
A
b.
Displaced
t
rd
200
LapseRate
100
1 00 C/l00
-AI~Z' = m
.
Ambient
T = 22.15
100
m
cccR)", Volume at Rest
'c"'Z~',. SameTemper.ature
,"cc" as Surroundmgs
~n
19
20
21
Temperature (OC)
22
(0)
500
400
:g
',#"
-B',
300
Dry Adiabat = r
'::
, ""
:L"""'~
:I:
200
C
Ambient
Lapse Rate
19
300
-', B'"
-A
-DAd'
~
:I:
200
,
ry
-C
, "
"
I
~
b t -r
la a-",
/
rt
-t';-Sin-k-
22
(b)
Volume Displaced
Upward 100 m
--v
Volume
v ":c"v
A-i~t.itSCcC
~
~'
A b'
" ccCC
Cc
"
"
"
,Ambient'"
~T = -0,5 "CIl00 m
-£2
19
Displaced Upward 100 m
Volume Displaced Downward 100 m
Ccvc",c"
Ambient clli.~lill
C
T = 22.cCCvvv
40 Cj'CCCCCCCCC!c!"C
"""-""",,~
Volume
Continues
Ambient
T = 20.65
m lent
Lapse Rate
100
'
.Cooler
-1.25 C/l00 m
\
"
"
",
20
21
Temperature (OC)
500
400
A_~l,tt...
'
'"
~T
-xz=
,i.~',
,<Olume
/
100
~
.E
""c"..,
~
-A
-§,
.-,
Q)
C .
0 ume onhnues
to Rise
V I
Warmer
./
Ambient i'}~'~c,,::i,'~
T = 19.90 "i:-.~'}'_B
20
21
Temperature (OC)
Cooler
Restored
to Original
Warmer
!!J!i"~;,,'-:--
L eve I an d
Temperature
C!;"C;CCC7~'!"'...
:ii:::i;c~:1t~;;}!
C
T = 21.65
I~. Jl
Volume Displaced
Downward 100 m
)~ (
22
Icl
FIGURE 6-13
Lapse rateand displacedair volume. (Source:Atomic Energy Commission, Meteorologyand Atomic Energy,Washington,
DC: U.S. GovernmentPrinting Office, 1955.)
AIRPOLLUTION 497
Plume types. The smoke trail or plume from a tall stacklocated on flat terrain has
beenfound to exhibit a characteristicshapethat is dependenton the stability of the
atmosphere.The six classicalplumes are shown in Figure 6-14, along with the correspondingtemperatureprofiles. In eachcase, r is given as a broken line to allow
t\
Z
~
r'
t;l
T
t
Z
T
Lapse Condition
-Weak
(Looping)
Lapse Condition
(Coning)
..,
..'. ".
[~~.""'._."'.,.
/
Condition
t ~ \ \.::"::..:.;.:.~;:::~:;;~::.'
Z~
T -Inversion
t ~,(
"
',..'
~
T
-Lapse
\
r'
T --Weak
.."...' ..., ..,..."
.."'"
(Fanning)
,'
'.'
Below, Lapse Aloft (Lofting)
"" .,.. .,. .'" .."..."
Z~
.
".
,.,.'
'.' ,..,' ,..,.,;' ".' ,:":,,, .,',",
.',:..: .
""',,',.,",,: ' ,.
.'
.
"'."":""::'::'::::..::"':'::"',:..:::,;:..':.::',:..:::::.~,..::.::':'.
.-
T -Inversion
t
Z
,..'
';:'::t,.i..;.'..'~.;.;~:.~i-.;
..:.,.i..':':..'::.':.;.:( ."..::::':.:::,.'
,:.,.,.."',.!
1:
,',;."".'.."(,.".:
..,'
".:"!'(~i;~~';~;.'~';;~i;:l:::'\::;:.',:::~::'~~..,,::.,'i'
--Strong
\
r'
t ~~\
Z
Wind
'.,..'
'...,
"".
..::'~.:.:':;~:'::.:...'.
Below, Inversion Aloft (Fumigation)
...'.' ...' '..'.,".'
".,.,:.;,;,.~.,.:".:'."".:"'.""...',;"
'.
.,., ...' ..'" "
, : ::..:: ::..;:.::..::::
Lapse Below, Inversion
Aloft (Trapping)
FIGURE 6-14
Six typesof plumebehavior.(Source:P.E. Church,"Dilution of WasteStackGasesin
the Atmosphere,"lndustrial
EngineeringChemistry,
vol. 41,pp. 3753-3756,1949.)
498
INTRODUCTIONTO ENVIRONMENTALENGINEERING
comparisonwith the actual lapse rate, which is given as a solid line. In the bottom
three cases,particular attentionshouldbe given to the location of the inflection point
with respectto the top of the stack.
Terrain Effects
Heat islands. A heat island results from a mass of material, either natural or anthropogenic, that absorbsand reradiatesheat at a greaterrate than the surrounding
area.This causesmoderateto strongvertical convectioncurrents abovethe heatisland. The effect is superimposedon the prevailing meteorologicalconditions. It is
nullified by strong winds. Large industrial complexesand small to large cities are
examplesof places that would have a heatisland.
Becauseof the heatisland effect, atmosphericstability will be less over a city
than it is over the surroundingcountryside.Dependinguponthe location of the pollutant sources,this can be either good news or bad news. First, the good news: For
ground level sourcessuch as automobiles,the bowl of unstable air that forms will
allow a greaterair volume for dilution of the pollutants. Now the bad news: Under
stableconditions,plumes from tall stackswould be carried out over the countryside
without increasing ground level pollutant concentrations.Unfortunately, the instability causedby the heatisland mixes theseplumes to the ground level.
Land/sea breezes. Under a stagnatinganticyclone,a stronglocal circulation pattern
may developacrossthe shoreline of large water bodies. During the night, the land
coolsmore rapidly than the water.The relatively coolerair overthe land flows toward
the water (a land breeze,Figure 6-15). During the morning the land heatsfasterthan
water. The air over the land becomesrelatively warm and begins to rise. The rising
air is replaced by air from over the water body (a sea or lake breeze,Figure 6-16).
FIGURE 6-15
Landbreezeduringthenight.
AIRPOLLUTION 499
~--,..
/
WamIAir overLand
Rises
" \
~
-A
\
'" 1/
-0/
I
) Air
'"
Lake
Breeze
WamI
~-,-'
~
'.":-'~
/'
--,(
--~
~- ~\
""
\."\\
~""'
FIGURE 6-16
Lake breezeduring the day.
The effect of the lake breezeon stability is to imposea surface-basedinversion
on the temperatureprofile. As the air moves from the water over the warm ground,
it is heated from below. Thus, for stack plumes originating near the shoreline,the
stablelapserate causesa fanning plume close to the stack(Figure 6-17). The lapse
condition grows to the height of the stack as the air moves inland. At some point
inland, a fumigation plume results.
Valleys. When the general circulation imposesmoderateto strong winds, valleys
that are oriented at an acuteangle to the wind direction channelthe wind. The valley
z~
zLL
T
T
-u
Several km
Fumigation
" :'.-".:';::':...,:..~t:;!.;:::;,;,:~,,"
;:: ,:,:,:;:::,.::..,.::::
:",.;.:,;".';-":'::-:"'.;;:.'Y;"
.""
,
,.'
:.::... ..~.'.::::~j,::,:.
FIGURE 6-17
Effectof lakebreezeonplumedispersion.
Fanmng
500
INTRODUCnON TO ENVIRONMENTALENGINEERING
effectively peelsoff part of the wind and forces it to follow the direction of the valley
floor.
Under a stagnatinganticyclone, the valley will set up its own circulation.
Warming of the valley walls will causethe valley air to be wanned. It will become
more buoyant and flow up the valley. At night the cooling processwill causethe
wind to flow down the valley.
Valleys oriented in the north-southdirection aremore susceptibleto inversions
than level terrain. The valley walls protect the floor from radiative heating by the
sun. Yet the walls and floor are free to radiate heat awayto the cold night sky. Thus,
underweak winds, the ground cannotheatthe air rapidly enoughduring the day to
dissipatethe inversion that formed during the night.
6-8 ATMOSPHERIC DISPERSION
Factors Affecting Dispersion of Air Pollutants
This discussionfollows the training documentsof the Texas Air Quality Control
Board.
The factors that affect the transport, dilution, and dispersionof air pollutants
can generallybe categorizedin terms of the emissionpoint characteristics,the nature
of the pollutantmaterial, meteorologicalconditions,andeffects of terrainand anthropogenicstructures.We havediscussedall of theseexceptthe sourceconditions.Now
we wish to integrate the first and third factors to describethe qualitative aspectsof
calculating pollutant concentrations.We shall follow this with a simple quantitative
model for a point source.More complex models for point sources(in rough terrain,
in industrial settings,or for long time periods),areasources,and mobile sourcesare
left for more advancedtexts.
Source characteristics. Most industrial effluents are dischargedvertically into the
open air through a stack or duct. As the contaminatedgas stream leaves the dischargepoint, the plume tendsto expandand mix with the ambientair. Horizontal air
movement will tend to bend the dischargeplume toward the downwind direction.
At some point between300 and 3,000 m downwind, the effluent plume will level
off. While the effluent plume is rising, bending, and beginningto move in a horizontal direction, the gaseouseffluents are being diluted by the ambientair surrounding
the plume. As the contaminatedgasesare diluted by larger and larger volumes of
ambientair, they are eventually dispersedtoward the ground.
The plume rise is affected by both the upward inertia of the dischargegas
streamand by its buoyancy.The vertical inertia is relatedto the exit gas velocity and
mass.The plume's buoyancyis related to the exit gasmassrelative to the surrounding air mass.Increasingthe exit velocity or the exit gas temperaturewill generally
increasethe plume rise. The plume rise, togetherwith the physical stackheight, is
called the effectivestack height.
The additional rise of the plume abovethe dischargepoint as the plume bends
and levels off is a factor in the resultant downwind ground level concentrations.
The higher the plume rises initially, the greater distance there is for diluting the
contaminatedgasesas they expand and mix downward.
AIRPOLLUTION 501
For a specific dischargeheight and a specific set of plume dilution conditions,
the ground level concentrationis proportional to the amount of contaminantmaterials dischargedfrom the stack outlet for a specific period of time. Thus, when all
otherconditionsare constant,an increasein the pollutant dischargerate will causea
proportional increasein the downwind ground level concentrations.
Downwind distance. The greaterthe distancebetweenthe point of dischargeand a
ground level receptordownwind, the greater will be the volume of air available for
diluting the contaminantdischargebefore it reachesthe receptor.
Wind speed and direction. The wind direction determinesthe direction in which
the contaminatedgas streamwill move acrosslocal terrain. Wind speedaffects the
plume rise and the rate of mixing or dilution of the contaminatedgasesas they leave
the dischargepoint. An increasein wind speedwill decreasethe plume rise by bending the plume over more rapidly. The decreasein plume rise tends to increasethe
pollutant's ground level concentration.On the other hand, an increasein wind speed
will increasethe rate of dilution of the effluent plume, tending to lower the downwind concentrations.Under different conditions, one or the other of the two wind
speedeffects becomesthe predominanteffect. Theseeffects, in turn, affect the distance downwind of the source at which the maximum ground level concentration
will occur.
Stability. The turbulence of the atmospherefollows no other factor in power of dilution. The more unstablethe atmosphere,the greaterthe diluting power.Inversions
that are not ground based,but begin at someheight abovethe stackexit, act as a lid
to restrict vertical dilution.
Dispersion Modeling
General considerations and use of models. A dispersionmodelis a mathematical
descriptionof the meteorologicaltransportand dispersionprocessthatis quantified in
terms of sourceand meteorologicparametersduring a particular time. The resultant
numerical calculations yield estimatesof concentrationsof the particular pollutant
for specific locations and times.
To verify the numerical results of such a model, actual measuredconcentrations of the particular atmosphericpollutant mustbe obtainedand comparedwith the
calculatedvalues by meansof statistical techniques.The meteorologicalparameters
required for use of the models include wind direction, wind speed,and atmospheric
stability. In somemodels,provisions may be made for including lapserate and vertical mixing height. Most models will require data aboutthe physical stackheight,
the diameterof the stack at the emissiondischargepoint, the exit gas temperature
and velocity, and the massrate of emissionof pollutants.
Models are usually classified as either short-term or climatological models.
Short-term models are generally used under the following circumstances:(1) to
estimate ambient concentrationswhere it is impractical to sample, such as over
rivers or lakes, or at great distancesabove the ground; (2) to estimatethe required
r:e.
502
emergency
'""000=0'
source
ro
reductions
BNvmONMBNTAL
associated
with
pollution
episode
high,
for
alert
short-term,
the
location
of
air
of time
day
for
aid
season
developing
models
in
Basic
point
equation
the
bulent
diffusion
stream
in
Gaussian
gas
or
stream
is equal
the
of
(u).
level
is
mirror
The
model
totally
at
an
angle.
suming
a virtual
level,
the
source
real
the
locations
of
over
at particular
the
effluent
a long
times
models
are
only
with
into
of the
used
as an
short-term
The
for
the
same
with
general
idea
equations,
is
such
can
as
level
that
the
wind
ground
striking
for
with
source
be used
limiting
light
-H
same
the
that
assumes
accounted
of
the
by
reaches
of
tur-
contaminated
to
that
a beam
at a distance
plume
described
the
model
material
into
that
contaminated
proportional
like
layer
ground
The
reflection
located
be
above
rise.
pollutant
the
that
inversely
ground
imaginary
modeled.
conditions
plume
atmosphere
this
of
can
at a distance
diffusion
the
assumes
dilution
assumes
is
Gaussian
model
direction
the
that
source
an
the
further
plume
the
basic
throughout
The
vertical
plus
assumes
back
The
hence
atmosphere
height
emitting
being
air
evaluation
concentrations
uniform
discharged.
model
stack
or imaginary
layer
and
Mathematically,
and
under
selection
be concerned
is
and
The
also
reflected
ground
boundary
of
probable
exist
model.
is
activity
into
dilution
most
Long-term
will
stability
horizontal
physical
mean
time.
We
stream
equation.
released
the
degree
the
stagnations
of a site
that
dispersion
a random
both
air
application.
gas
normal
is
to
speed
is
of
standards.
atmospheric
contaminated
the
as part
to estimate
period
Gaussian
that
to estimate
concentrations
a long
simple
source
used
mean
over
most
assumes
which
gas
are
emissions
their
(3)
of
equipment.
models
or to estimate
each
for
and
concentrations
monitoring
Climatological
period
conditions;
ground-level
periods
a
by
as-
respect
to
strength
as
to establish
horizontal
other
or vertical
mixing.
The
model.
Turner.48
nates
The
We
have
It gives
x and
standard
nated
by
ward
distance
the
selected
ground
y) downwind
deviation
Sy and
the
level
from
a stack
of the 'plume
Sz, respectively.
from
the
source
model
The
and
equation
concentration
with
in the
an
stability
of
form
pollutant
effective
horizontal
standard
the
in the
(x)
height
and
deviations
vertical
are
of the
presented
by
at a point
(H)
(Figure
directions
functions
atmosphere.
B.
6-18).
is desig-
of the
The
D.
(coordi-
down-
equation
is
as follows:
X(x,y,O,H)
=
[~][exp[
-~(*
)2]]
[exp
[ -~
(~)2]]
(6-19)
48D.Bruce Turner, Workbook ofAtmospheric Dispersion Estimates(U.S. Department of Health, Education and Welfare, Public Health Service, National Center for Air Pollution Control, Publication
No. 999-AP-28), Washington,DC: U.S. Government Printing Office, p. 6, 1967. (Note: Turner provides guidelines on the accuracy of this model. It is an estimating tool and not a definitive model to be
used indiscriminately.)
-
AIRPOLLUTION 503
where
X(x,y,O,H)
= downwindconcentration
atgroundlevel,g/m3
E = emissionrateof pollutant,glS
Sy'Sz= plumestandarddeviations,m
u = wind speed,m/s
x, y, z, andH = distances,m
exp = exponential
e suchthattermsin bracketsimmediatelyfollowingarepowersof e, thatis, e[] wheree = 2.7182
The valuefor the effectivestackheightis the sumof the physicalstackheight(h)
andtheplumerise 6.H:
H = h + 6.H
(6-20)
6.H maybe computedfromHolland'sformulaasfollows:49
6.H = ~
[1.5 + (2.68 X 10-2(P)(¥)d)]
(6-21)
z
x
(x.
-yo
z)
(x.
-y,
0)
FIGURE 6-18
Plume dispersioncoordinatesystem. [Source: D. Bruce Turner, Workbook ofAtmospheric Dispersion Estimates (U.S. Department of Health, Education and Welfare, Public Health Service,National Center for
Air Pollution Control, Publication No. 999-AP-26), Washington,DC: U.S. Government Printing Office,
1967.]
49J.Z. Holland, A Meteorological Survey ofthe Oak Ridge Area (U.S. Atomic Energy Commission
ReportNo. ORO-99), Washington,DC: U.S. GovernmentPrinting Office, p. 540,1953.
~
,
504
INTRODUCTIONTO ENVIRONMENTALENGINEERING
where Us=
d =
u =
P =
Ts =
Ta =
stackvelocity, m/s
stackdiameter, m
wind speed,m/s
pressure,kPa
stacktemperature,K
air temperature,K
The values of Syand Szdependupon the turbulent structure or stability of the atmosphere.Figures 6-19 and 6-20 provide graphical relationships betweenthe down10,0
5,00
2,00
1,000
5
~
20
e
"
-150
t/)
1
50
20
15
10
.-
5
4
-
3
2
3
4
Distance Downwind
5
20
100
[kIn)
FIGURE 6.19
Horizontal dispersion coefficient. [Source: Turner, Workbook ofAtmospheric Dispersion Estimates
(U,S. Departmentof Health, EducationandWelfare; Public Health Service,National Centerfor Air Pollution Control, PublicationNo. 999-AP-28), Washington,DC: U.S. GovernmentPrinting Office, 1967.J
~
,
AIRPOLLUTION 505
5,00
3,00
2,00
1,00
50
40
30
2
10
:g-
o
C/)
5
40
30
20
./
1
4
3
1,
..2.3
.4
.10
20
Distance Downwind
(km)
FIGURE 6-20
Vertical dispersioncoefficient. (Source: Turner, Workbook ofAtmospheric Dispersion Estimates.)
wind distance x in kilometers and values of sy and sz in meters.The curves on the
two figures are labeled "A" through "F." The label "A" refers to very unstableatmospheric conditions, "B" to unstableatmosphericconditions,"c" to slightly unstableC
conditions,"D" to stableoonwtions, "E" to stableatmosphericconditions,
506
INTRODUCTIONTO ENVIRONMENTALENGINEERING
TABLE 6-6
Key to stability categories
Day"
Incoming solar radiation
Night"
Surface wind
speed (at 10 m)
(m/s)
Strong
Moderate
Slight
Thinly overcast or
~ 4/8 Low cloud
~ 3/8 Cloud
<2
2-3
3-5
5-6
>6
A
A-B
B
C
C
A-B
B
B-C
C-D
D
B
C
C
D
D
E
D
D
D
F
E
D
D
a The neutral class, D, should be assumedfor overcastconditions during day or night. Note that "thinly overcast" is
not equivalent to "overcast."
Notes: Class A is the most unstable and class F is the most stable class consideredhere. Night refers to the period
from one hour before sunsetto one hour after sunrise. Note that the neutral class, D, can be assumedfor overcast
conditions during day or night, regardlessof wind speed.
"Strong" incoming solar radiation correspondsto a solar altitude greater than 6()° with clear skies; "slight" insolation correspondsto a solar altitude from 150to 350 with clear skies. Table 170, Solar Altitude and Azimuth, in
the Smithsonian Meteorological Tables,can be used in determining solarradiation. Incoming radiation that would
be strong with clear skies can be expectedto be reduced to moderatewith broken (5/8 to 7/8 cloud cover) middle
clouds and to slight with broken low clouds.
Source: D. Bruce Turner, Workbook ofAtmospheric Dispersion Estimates.
and "F' to very stable atmosphericconditions. Each of these stability parameters
representsan averagingtime of approximately3 to 15 min.
Other averagingtimes may be approximatedby multiplying by empirical constants, for example, 0.36 for 24 hours. Turner presenteda table and discussionthat
allows an estimate of stability basedon wind speedand the conditions of solar radiation. This is given in Table 6-6.
For computer solutions of the dispersionmodel, it is convenientto have an
algorithmto expressthe stability classlines in Figures6-19 and6-20. D. O. Martin50
TABLE 6-7
Values of a, c, d, and! for calculating Syand Sz
x~lkm
Stability
class
a
A
B
C
D
E
F
213
156
104
68
50.5
34
cd/
440.8
100.6
61
33.2
22.8
14.35
x~lkm
cd/
1.941
9.27
1.149
3.3
0.911
0
0.725 -1.7
0.678 -1.3
0.74
-0.35
459.7
108.2
61
44.5
55.4
62.6
2.094 -9.6
1.098
2
0.911
0
0.516 -13
0.305 -34
0.18
-48.6
Source:D. O. Martin.
soD.O. Martin, Comment on the Change of ConcentrationStandardDeviations with Distance,Journal
o/the Air PollutionControlAssociation,
26,pp. 145-146,1976.