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AtmosphericDispersion,Transport,and
Deposition
The atmospherehas servedas a sink for emissionsof volcanoesand a variety
of geological processes,forest and grasslandfires, and decompositionand other
biological processesfor hundredsof millions (if not billions) of years. It has also
servedas a sink for pollutantsgeneratedby humanactivities,proceedingfrom man's
first use of fire to the smelting of metal ores and use of fossil fuels such as coal,
oil, andnaturalgasto motor vehicle and other emissionsfrom our very industrialized
and technologicallyadvancedmoderntimes.
Despite its vastness,the atmosphere(at least in the short term) is not a perfect
sink. Its ability to carry away (transport),dilute (disperse),and ultimately remove
(deposition)wasteproductsreleasedto it is limited by various atmosphericmotion
phenomena.Pollutant concentrationsmay reach unacceptablelevels as a result of
local or regionaloverloadingof the near-surfaceatmosphere,topographicalbarriers,
and micro-, meso-,and macroscaleair motion phenomena.
The atmosphereserves as a medium for atmosphericchemical reactions that
ultimately serve to remove contaminants.Thesereactionsmay produce pollutants
that may themselvespose significant environmentalconcerns.Levels of long-lived
pollutantssuch as methane(CIt), nitrous oxide (NrO), and.carbondioxide (COr)
may increase,causingglobal warming and, in the caseofhalogelated hydrocarbons,
a
stratosphericozone (Or) depletion.
Becauseof technologicaland economiclimitations, we have little choice but to
usethe atmospherefor the disposalof airbome wastes.Like other natural resources,
our use of the atmospherehas to be a wise one,recognizingits limitations and using
it in a sustainablewav.
3.1 DISPERSIONAND TRANSPORT
Pollutants releasedfrom ground-leveland elevatedsources(smokestacks)are
immediatelysubject to atmosphericprocesses,with dispersionin ever-increasing
71
AIR QUALITY
volumes of air by both vertical and horizontal transport.Transportis the processby
which air motions carry gas- and particulate-phasespeciesfrom one region of the
atmosphereto another.Transportenhancesdispersionand providesan opportunity
for pollutants from different sourcesto interact.
Pollutant transportand dispersionare affectedby atmosphericdynamics, fluid
physical phenomenathat occur in the atmosphere,and physical laws that govern
them. Thesemay facilitate or constraintransportand dispersal.
Pollutantsareinitially releasedinto the planetaryboundarylayer (PBL), that portion
of the atmospheremost directly affectedby the Earth's surface.The PBL is subjectto
fluxes of heat and water (HtO) vapor from the surface,and other physical forces. Its
depth, on average,rangesfrom a few hundredsleters to I to 2 km. Above the PBL is
a relatively stablelayer of air that separatesit from the free troposphereabove.
Pollutant transport and dispersion are affected by different scalesof motion.
Theseare the microscale,mesoscale,synoptic scale,and macro-,or planetary,scale
(Table 3.1). Microscale refers to air motion in the near vicinity of a source and
includesphenomenathat affect plume behavior;mesoscale,atmosphericmotions on
the order of tens to hundredsof kilometers and suchphenomenaas fronts, airflows
in river valleys, and coastal airflows; synoptic scale,systemson the order of 106
km2 or more such as high- and low-pressuresystemsresponsiblefor day{o-day
weathervariations;and planetaryscale,atmosphericmotions on the order of continents or larger. These scalesof air motion are useful in describing atmospheric
phenomena.It is important to note that the atmosphereis one continuousflowing
fluid and all motions are a part of this larger flow.
In the context of a few months,the PBL is relatively well mixed. However,on
shortertimescalesand nearthe Earth's surface(wherepollutantsareemitted),transport and dispersionare often limited by atmosphericconditions.Someatmospheric
conditions result in elevatedgroundJevelpollutant concentrationsthat may potentially harm humansand our envitonment.Consequently,they are discussedin detail
below. Particular attention is given to horizontal wind (speedand direction), turbulence, topography,atmosphericstability, and inversions.
ATMOSPHERIC
DISPERSION,
IRANSPORT,
AND DEPOSITION
-74
3.1.1 Wind
Horizontal winds are characterizedby both velocity (wind speed)and direction.
As seenin Chapter1, wind speedis affectedby horizontalpressureand temperature
gradients(the higher the pressuregradient,the higher the wind speed)and friction,
which is proportionalto the roughnessof the Earth's surfaces(surfaceroughness).
Relationshipsbetweensurfaceroughnessand wind speedfor urban, suburban,and
rural areascan be seen in Figure 3.1. The maximum height of each wind profile
indicateswheresurfaceeffectsend and the gradientwind (wind affectedby pressure
differentialsand the coriolis effect) begins.For the urban areadepicted,this occurs
at -500 m (1650 ft); for the suburbanarea,-300 m (990 ft); and for the rural area,
-250 m (82s f0.
For continuouslyemitting stack sources,dilution begins at the point of release.
This plume dilution is inverselyproportionalto wind speed;i.e., by doubling wind
speed,pollutant concentrationis decreasedby 507oofits initial value.The effect of
wind speedis to increasethe volume of air availablefor pollutant dispersal.As seen
in Figure 3.1, urban areas are characteizedby relatively high surface roughness
and,as a consequence,
diminishedwind speeds.This is ironic in the sensethat urban
areas,becauseof their relatively high pollutant emissions,are in greater,not lesser,
needof being ventilatedby the wind.
winds havedirectionalaspects.Theseinclude the prevailing northeasterlyflows
in the subtropics,southwesterlyflows in the middle latitudes,and easterlyflows at
high latitudesin the northernhemisphere.They alsoinclude the cyclonic (clockwise)
and anticyclonic flows associatedwith migrating low- and high-pressuresystems.
Becauseflows are somewhatcircular, wind direction will dependon one's position
500
Table 3.1
Meteoroloqical Scales of Air Motion
400
Scale
Microscale
Geographical
Area (km'?)
2-15
Period
Minutes
Mesoscale
15-1 60+
Hourto days
Synopticscale
Planetaryscale
>106
Days
Weeks to months
Phenomena
Plumebehavior
Downwash
Sea, lake,and land
Dreezes
Mountainvalleywinds
Migratoryhigh-and lowpressuresystems
Cold and warm fronts
highSemipermanent
pressuresyslems
Hadleycell flows
Tropicalstorms
Jet streammeanders
Cold and warm fronts
c
;c
.q) 300
o
I
200
100
0
Figure3.1
Effectof surfaceroughnesson wind speed as a functionof height over urban,
suburban,and rural areas.(Adaptedfrom Turner,D.8., Workbookfor Atmospheric
DispersionEmissions,
EPAPublication
AP-26, EPA,Washington,D.C.,1969.)
AIR QUALITY
in the circulating pressurecell. It also dependson local topography.At night in river
valleys, airflows are downslope and downriver; they are upslope during daylight
hours.Along seaand lake coasts,winds during clear weatherflow inland during the
day and waterwardat night.
Wind direction is quite variable, with large changesoften occurring over relatively short periodsof time. A changein wind direction of 30' or more in I h is not
uncommon.Over a periodof 24hitmay shift by 180'. Seasonalfactorsmay cause
wind direction variationsof 360".
Wind direction and variability can have significant effects on air quality. Areas
downwind of point sourceswhere winds are relatively persistentmay experience
relatively high ground-levelconcentrationscomparedto other areasat similar distances.If the wind is more variable, pollutantswill be dispersedin a larger volume
of air and be more equally distributedaroundthe source;ground-levelconcentrations
are therefore likely to be lower.
Wind direction is particularly important in the transportand dispersionof polIutants over large geographicalareas.It is southwesterlyairflows that cany acid
Canada.
precriisorsfrom the U.S. Midwest to the northeasternstatesand southeastern
to the
of
southeastern
Asia
pollutants
from
countries
have
transported
Similar flows
WestCoastof the U.S.
3.1.2 Turbulence
Airflows within the PBL are influencedby prevailing high-altitude air motion,
frictional drag of the Earth's surface,and vertical airflows. Turbulenceis characterized by circular eddiesthat may be vertical, horizontal, and various other orientations. Theseeddiesrepresentair movementsover shortertimescalesthan thosethat
determine mean wind speeds.Turbulent eddies are produced by both mechanical
and thermal forces.
Mechanical turbulenceis induced by wind moving over and around structuresand
vegetation.It increaseswith wind speedand surfaceroughness.It is alsoproducedby
the shearingeffect of fast-movingair aloft as it flows over air slowedby friction.
Thermal turbulenceresults from the heating or cooling of air near the Earth's
surface.On clear days, solar heating of ground surfacestransfersheat to air above
it. Convectioncells of rising warm air and descendingcooler air develop.Under
intensivesurfaceheating,convectiveeddiesare generatedthat extendvertically on
the order of 1000to 1500m (-3600 to 5000 ft).
For the most part, mechanicaland thermal turbulenceare daytime phenomena.
Both aredampenedby nighttime radiativecooling of the groundandair adjacentto it.
The effect of both mechanicaland thermal turbulenceis to enhanceatmospheric
mixing and pollutant dispersion.As a consequence,pollutant concentrationsare
significantly decreased.An exception is downwashphenomenathat causeplumes
to be brought to the ground near smokestacks(by mechanicalturbulence).In most
cases,turbulencehas a positive effect on air quality.
Downwash resultsin high pollutant concentrationsin the turbulentwake downwind of a source.Downwashcan also occur as a consequenceof the shearingeffect
of high-velocity winds (>70 km/h, - 40 mi/ft).
ATMOSPHERIC
DISPERSION.
TRANSPORT.
AND DEPOSITION
75
3.1.3 Atmospheric Stability
The atmosphere,particularly in the PBL, is characterizedby highly variable
horizontaland vertical air movements.In turbulentflows (describedpreviously),the
atmosphereis unstableand pollutants are rapidly dispersed.Turbulent flows associatedwith heatingof the Earth's surfaceare dampenedon cloudy days. Under such
conditions,the atmosphereis more stableand pollutantsare less rapidly dispersed.
When an entity is undergoingrapid changeor has the immediatepotential to do
so, it is said to be unstable.When it is undergoinglittle or no change,and is even
resistantto change,it is said to be stable.Stability and instability representopposite
ends in a continuum of possibilities. This continuum is implicit in the phrase
atmosphericstability.
Vertical air motion is significantly affectedby temperaturegradients.The rate
of temperaturechange with height is described as the lapse rate. Tropospheric
temperatures,
on average,decreasewith height (Figure 1.7).This decrease,or normal
lapserate,is -6.5'C/km (18.9'F)or -0.65"C/100m.
The normal lapserate differs from what would be expectedif a parcel of warm,
dry air were releasedinto a dry atmosphere.In this theoreticalcase, the buoyant
parcelwould rise andexpandadiabatically(i.e., no energyis transferredto surrounding air). As it rises, its temperaturedecreasesat a constantrate. This theoretical
changeof temperaturewith height is called the adiabaticlapserate. It has valuesof
-10'C/km (-29"F) or -1"C/100 m.
Becauseparcels of air releasedinto the lower atmospherecontain HrO vapor,
the adiabatic lapse rate is used to describe how air cools when it rises in a dry
atmosphere.Under real-world conditions,air containsa significant amount of HrO
vaporthat cools asthe air parcelrises.When air reachesits saturationvapor pressure,
heat is releasedas HrO vapor condenses(heat of vaporizationis released).Air is
warmed,resulting in a somewhatsmaller decreaseof temperaturewith height than
that predictedfor adiabaticconditions.
The normal lapse rate representsa summation and averaging of many different
lapserate conditionsthat vary from more negative(than the adiabaticlapserate) to
positive values. Individual lapse rates (environmentallapse rates) are determined
from vertical temperatureprofile measurements.
Though environmentallapserates
arereportedas a single value, they representa summationand Averagingof temperaturevariationswith height.Becausethey representtemperaturechangeswith height,
environmentallapse rates are used as indicators of atmosphericstability and the
dispersionpotential of pollutants.
The relationshipbetweenenvironmentallapserates and stability can be seenin
Figure3.2. Line A indicatesa slight decreaseof temperaturewith height. Sincein this
case sourcesor sinks of thermal energy are present (there is little or no heating or
cooling of the ground and adjacent afu), air cools as it expands and its pressure
decreases.
This temperaturechangeis close to the adiabaticrate.As a consequence,
atmosphericconditionsaredescribedasneutral.A neutrallapserateoccursin response
to (1) cloudy conditionsthat inhibit incoming solar radiation and outgoing thermal
radiation,(2) windy conditionsthat rapidly mix heatedor cooled air near the Earth's
surface,and (3) nansitionalcircumstancesnear sunriseand sunsetwhen changesin
stability occur.Under such neutralconditions,dispersionis relativelygood.
ArRoulltrv
1
c
'o
I
lncreasing
ATMOSPHERIC
DISPERSION,
IRANSPORT,
AND DEPOSITION
77
be surfacebased (occur near the ground) or elevated(occur aloft). Atmospheric
processesmay producefrontal, advective,radiational,and subsidenceinversions.
In a frontal inversion,air from a warm front flows over cold air in an adjoining
air mass.Invertedtemperaturesoccur at the interfaceof the two fronts. Becauseof
the movementof theseair massesand the interactionbetweenthem, frontal inversionshave only limited effects on air quality.
An advectiveinversionforms when warm air flows over a cold surfaceor cold
air. They are commonly associatedwith land and seabreezesand may be surfacebasedor elevated.
Radiationaland subsidenceinversionsposesigniflcantair quality concernssince
they suppressvertical mixing over industrialized river valleys and urban areas,
resultingin elevatedpollutant levels.
3.1.4.1 Radiational lnversions
Temperature---------r.
In lapseratecon'
profiles
variations
illustrating
temperature
near-surface
Figure3.2 Vertical
stability.
atmospheric
ditionsandincreasing-decreasing
The lapse rate characterizedby line B shows a temperaturedecreasethat is
greater (-2"Cll0 m) than those under neutral conditions (near the adiabatic lapse
rate). A parcel of polluted air in such an environmentwill rise rapidly. The lapse
rate is describedas superadiabatic.Atmosphericconditions are very unstable,with
strong vertical air motion. Such instability occurson clear days with light winds at
midday. Pollutant dispersion,as expected,is excellent'
Line C representsan isothermal lapse rate (i.e., temperaturedoes not change
with height). If a parcelof warm polluted air were releasedinto this somewhatstable
environment,it would rise slowly and soon cool to the temperatureof its surroundings. Becausethe atmosphereis more stable,dispersionis more limited. Dispersion
potential under theselapse rate conditions can be characterizedas moderate.
Line D indicateslapserate conditionsin which temperatureincreaseswith height.
As such, the temperaturechangeis invertedfrom "normal." Under such conditions
the atmosphereis very stable.Becauseof walmer temperaturesabovethe ground,
a warm parcelof polluted air will quickly comeinto equilibrium with the temperature
of its surroundings.Vertical air motion is suppressedand the dispersionpotentialof
emitted pollutants is poor. Such stable ground-levelinverted lapse rate conditions
occur at night under clear skies with calm to light winds.
Atmospheric stability, describedabove, representschangesin near-surfaceair
temperaturesthat occur over the courseof a single day.They do not include largerscalemeteorologicalconditions associatedwith high-pressuresystems.
3.1.4
Inversions
As indicated,when lapseratetemperaturesincreasewith height,they areinverted
from the normal. Suchatmosphericconditionsare describedas inversions.They can
Radiationalinversionsare producedas a result of the radiationalcooling of the
ground.Since they form at night, they are also called nocturnalinversions.Radiational inversionsare, in most cases,ground-based.
Radiationalinversionsonly occuron clearnights.Surface-based
inversionsbegin
to form as the sun setsand intensify throughoutthe night until sunrise.As the Earth
is a net radiator of heat at night, it begins a cooling processthat subsequentlycools
air immediately above it. Relatively warm air overlays an increasingly deepening
layer of cool air beneath.This inversionlayer may be only 10 to 20 m (33 ro 66 ft)
deepover flat terrain. With the exceptionof ground-basedsources,such inversions
haveonly a limited impact on air quality.
Radiational inversionsin river valleys are of major environmentalimportance
becauseoftheir historically heavyindustrializationand pollutant emissionsand the
fact that such inversionsare intensifiedas a result of the effects of topography.
River valleys were formed by the erosiveforce of water as it moved from higher
elevationsto the sea.Valleys serveas conduits for water as well as cool, denseair
flowing downslopefrom the radiative cooling of ridges bordering the valley. On
reachingthe valley floor, this cool, denseair runs under warmer air and forces it
aloft.The flow is downslopeand downriver.Becauseof its denshyand volume, cool
air floods the valley floor. This river of cool air deepensover the nighttime hours
andreachesits maximum depthjust before sunrise.In some mountain valleys the
top of the inversionlayer may be 100 to 200 m (330 to 660 f0 above the valley
floor.A radiationalinversionin a mountainvalley and its vertical temperatureprofile
areillustratedin Figure 3.3.
The height of the inversion layer has a significant effect on how well or how
poorlypollutantsaredispersed.In river valleys,emissionsfrom sourceshaving stack
heightsup to 100 m may be trappedin the inversion layer. As such, dispersionis
yery poor and pollutant concentrationsnear the top of the inversionlayer are high.
Polluted air emitted from a source will rise to an altitude where its temperature
is the same as its surroundings. This occurs near the top of the inversion layer. As
a consequence,a layer of intensely smoky or hazy atr forms at this height. This is
most evident in the early morning hours in a river valley before inversion breakup.
AIR QUALITY
ATMOSPHERIC
DISPERSION,
TRANSPORT,
AND DEPOSITION
ALR = Adiabatic
lapserate
ELB= Environmental
lapserate
0)
Temperature
Flgure 3.3
4
river valleyand associated
Nocturnal,radiationalinversionin an industrialized
temperalureprofile.
During the winter when days are short, radiationalinversionsmay persist up to
16 to 18 h in northern latitudes; they are typically considerablyshorter during the
summer. Pollutant levels, as can be expected, are higher under more persistent
inversionconditions.
Radiational inversionsbegin to break up as the sun starts to warm the ground
and the air aboveit. Increasinglylarge convectioncells of turbulentair are formed,
which causecompleteinversionbreakupseveralhours after sunrise.At that time the
heavily polluted air massnearthe top of the inversionis brought to the ground.This
phenomenonis describedas fumigation becauseof the high ground-levelconcentrations produced.
Despite radiative cooling of the ground, surface-basedinversionsgenerally do
not form in urban areaslocated on flat tenain. This is due to the fact that urban
surfacesemit considerablequantitiesof heat that producea well-mixed layer of air
abovethem. Emitted heat,however,can be absorbedby the polluted air mass(often
describedas a dust dome) that forms above many cities. As heat is absorbedby
pollutants, a layer or two of warm air forms aloft (Figure 3.4). Though these are
elevatedinversions,they are, nevertheless,producedby radiationalcooling.
3.1.4.2
Subsidence Inversions
Subsidenceinversionsare formed over large geographicalareasas the result of
the subsidenceof air in high-pressuresystems.As air subsides(sinks) to lower
altitudes,it compressesair beneathit, causingtemperaturesto rise. Sinceturbulence
almost always occursnear the ground,air in this part of the atmosphereis relatively
unaffectedby subsidenceoccurring above.As a consequence,an inversion layer
(that may be 50 m (165 ft) thick) forms betweenthe subsidingair and the relatively
turbulentair below it (Figure 3.5).
0
Figure3.4
79
5
10 15
% Attenuation
300
36"
40"
Temperature("F)
Effectsof dust levelson environmental
lapserate over the cincinnati,oH, metropolitanarea.(FromBach,W., Geogr.Rev.,61, S7g, 1971. With permission.)
Figure3.5 Formation
ol a subsidence
inversion
anditsassociated
profile.
tenlperature
The height of the inversion layer varies.It is highest near the center of the cell
and lowest near the cell's periphery.As a result of turbulence,elevatedinversion
layersdo not reach the ground. Though commonly associatedwith high-pressure
systems,subsidenceinversionsonly have significanteffects on air quality when the
inversionlayer is relatively close to the ground (e.g., 300 to 400 m, -990 to 1300
ft) and persistent(3 to 5 days).
Most high-pressuresystemsare migratory; i.e., they move over large expanses
of the Earth's surfaceafter they form. These migrating systemscontribute to the
hazy summer conditions over the American Midwest, Southeast,and Northeast.
occasionally,they have inversionlayers that are relatively low and more persistent
thannormal. Such stagnatingsystemsmay result in productionof elevatedpollutant
levelsnear the ground. These occurrencesare called episodes.Fortunately,severe
AtA OUALITY
80
ATMOSPHERIC
DISPERSION,
IRANSPORT.AND DEPOSITION
pollution episodesassociatedwith migrating high-pressuresystemsare relatively
rarein North America and northernEurope.The mostsevereandpersistentinversions
occur in middle latitudes in autumn. Subsidenceinversionsof particular note are
thoseassociatedwith semipermanentmarinehigh-pressuresystems(seeChapter1).
The inversion layer comesclosestto the ground on the easterlyor continentalside
of such Systems.As a consequence,west coastsof continentshave relatively low
and persistentinversion conditions. Inversionsbelow 800 m (2600 ft) occur over
the southerncoastof Califomia approximately90Voof the time during the summer'
Such inversions are primarily responsiblefor the smoggy and hazy atmospheric
conditions over the Los Angeles Basin.
Topographicalfeaturessuch as water surfaces,with their large heat-absorbing
capacities,have lower maximum and averageMHs than land surfaceswith little
vegetativecover (e.g.,deserts).Not surprisingly,Phoenix,AZ,has a relatively high
,summertimeMH, with coastalcities among the lowest (Figure 3.6).
MHs are affectedby semipermanentmarine high-pressuresystems(note Los
Angelesin Figure 3.6) and migrating high-pressuresystems.Subsidenceof air and
formationof inversionscauseMHs to decrease.MHs are an imDortantvariable used
in air quality modeling (seeChapter7).
3.1.5 Mixing Heighl
Both micro- and mesoscaleair motions are affected by nearby topographical
features.Topographycan havesignificanteffectson both air movementand pollutant
levels.These include differential vertical airflows associatedwith forests, plowed
agriculturalfields, parking lots, etc. Such flows affect the behavior of smokestack
plumes.
In river valleys, downslope airflows at night intensify (deepen)surface-based
inversions,and valley winds during the day help move pollutants upslope and out
of the valley. Mountainsalso serveas barriersto air movement.In the Los Angeles
Basin,the San BernardinoMountainsretard drflow in northerly and easterlydirections,further intensifyingsmogandhazeconditions.Mountainsalsoincreasesurface
roughness,therebydecreasingwind speeds.
The smog problem over the Los Angeles Basin is also affectedby the adjacent
coolwatersof the Pacific.when seabreezesbring cool air in from the ocean,warmer
air is pushedaloft, further intensifying the elevatedinversion.
Mesoscaleairflow patternsoccur on relatively calm days in coastalareasas
,a result of differential heating and cooling of land and water surfaces.During
{lummer,when skies are clear and prevailing winds light, land surfaceswarm
,morerapidly than sea and lake water. The subsequentlywarmed air flows up
and waterward.As a consequenceof temperatureand pressuredifferences,air
hows landward at the surface from the water, forming u ,"u or lake breeze.Air
rnoving from the land cools and descendsto form a weak cirgulation cell. At
night, rapid radiational cooling of the land results in surfaceairflows toward
water,forming a land breeze.Theseland breezesare generallylighter than lake
and seabreezes.
Land, sea,and lake breezes,and the circulation patternsthat form with them,
occuronly when prevailing winds are light. They are overridden when winds are
strong.In the caseof the southcoastof california, seabreezesintensifysubsidence
inversions.They may also causeadvectiveinversions,which commonly occur in
latespring when large bodies of water are still cold relative to adjacentland areas.
As water-cooledair movesinland,it warms;theinversionis brokenup and replaced
by superadiabatic
lapserate conditions.The weakcirculationcells associatedwith
land,lake, and seabreezesmay allow pollutants to be recirculatedto some degree
andcarried over from one dav to the next.
The mixing height (MH) is the height of the vertical volume of air above the
Earth's surfacewhere relatively vigorous mixing and pollutant dispersionoccurs.A
deflnableMH is assumedto occur under unstableand neutral conditions.It cannot
be deEnedwhen the air mass above the surfaceis stable.Elevatedinversionsfrequently place a iap on the MH. AverageMHs for selectedU.S. cities are indicated
on the map of the U.S. in Figure 3.6.
The MH varies both diurnally and seasonally.It is markedly affectedby topography and high-pressuresystems.During the day, minimum MHs occur just before
sunrise.The MH increasesprogressivelyas the sun warms the Earth and the Earth
warms the air aboveit. Increasingly larger convectivecells are formed so that the
MH reachesits maximum value in early afternoon(commonly severalthousand
meters). Maximum values occur during summer, with minimum values in late
autumn and winter in the middle latitudesof the northernhemisphere.
St. Louis
Minneapolis 1400m
'1200m
Boston
1000m
Pinsburgh
1400m
Charleston
1600m
Atlanta
1500m
Figure 3.6
AveragesummertimeMHs for selectedU.S.cities.
3.1.6 Topography
AIR QUALITY
82
3.1.7
Pollutant Dispersion from Point Sources
Point sourcesmay occur at ground level, or as is often the case,pollutants are
emitted from smokestacksthat vary in height. The subsequenthistory of plumes
formed dependson (l) the physical and chemicalnatureof pollutants,(2) meteorological factors such as wind speed and atmosphericstability, (3) location of the
sourcerelative to physical obstructions,and (4) topographicalfactorsthat affect air
movement.As theseaffect plume rise, its spreadhorizontally and vertically, and its
transport,they also affect maximum ground-levelconcentrations(MGLCs) and the
distanceof MGLCs from the source'
g.1.7.1
Pottutants and Diffusion
In many cases,point sourceplumes are a mixture of gas- and particulate-phase
substances.Particleswith aerodynamicdiametersof >20 trrmhave appreciablesettling.velocities, and as a consequence,deposition occurs relatively close to their
soufces.Smaller particles,particularly those with aerodynamicdiametersof <lpm'
havevery low sbttlingvelocities and dispersionbehaviorsimilar to that of gasesand
vapors.The gaseousnatureof plumes,given sufficienttime, may allow for dispersion
by simple diffusion, whereby the random motion of moleculesresultsin pollutants
migrating from areasof high concentration(the centerof the plume) to areasof low
concentration(the plume's periphery).Diffusion causesplumes to spreadboth horizontally and vertically. As a consequence,the effect of diffusion can be seento
increasewith downwind distance.
3.1.7.2 Plume Rise and TransPort
Dispersion from a smokestacksource is significantly affected by its physical
height as well as plume rise. In Figure 3.7, the plume is seento rise to a maximum
height and then level off. The distancefrom the top of the stackto the centerof the
plume is describedas plume rise. The distancefrom the ground to the centerof the
plume (including the stack) is the effective stack height.
-c
'o
c
c)
Figure 3,7
Plumerise and effectivestackheight
ATMOSPHERIC
DISPERSION,
]RANSPORT.
AND DEPOSITION
Dispersion is enhancedwith increasingstack height and plume rise. GroundIevelconcentrationswill be lower, and at constantwind speed,the distanceaI which
MGLCs occur will be increasedas effective stack height increases.
The height of the plume at the point it levels off dependson the exit temperature
of stack gases,cross-sectionaldiameterof the stack, emissionvelocity, horizontal
wind speed, and atmospheric stability (as indicated by the vertical temperature
gradient).
The effectivestackheight for a sourcecan be increasedby building taller stacks.
Thll-stacktechnologyhas beenusedsince the 1960sby electricalutilities operating
largecoal-firedpower plants. Suchstacksare commonly 250 to 300 m (850 to 1000
ft) high, with some stacks-400 m (1300 ft). They were designedand are operated
on the principle that pollutants could be dispersedfrom such facilities without
causingunacceptabledownwind ground-levelconcentrations.
Wind speedin the horizontal dimensionsignificantlyaffectsboth plume rise and
ground-levelconcentrations.Higher wind speedsdecreaseeffective stack height.
However,due to the increasedvolume of air associatedwith increasingwind speeds,
ground-levelconcenffationsare usually reduced.Higher wind speedsdecreasethe
distanceat which MGLCs occur.At very high speeds(-80 km/h, 50 mi/h), plume
rise may be negligible; the plume may be brought to the ground immediately
downwindof the source.
Tall stacksare designedto take advantageof the higher wind speedsthat occur
aloft as frictional drag of the Earth's surfaceis diminished. Pollutant dispersionis
enhancedas a result of thesehigher wind speeds.
lPlume
rise, as indicated, is subject to atmosphericstability. Under unstable
"'i
tbnditions, significant plume rise occurs; under stable conditions, plume rise is
markedlyreduced.In the latter case,dispersionis decreasedand higher ground-level
boncentrations
can be expected.
3.1.7.3 Plume Characteristics
As a plume moves downwind of a source, it expands by diffusion in both
horizontaland vertical dimensions.Plumes take forms and beh&ior patterns that
reflectstability conditions in the atmosphere.Major plume types are illustrated in
Figure 3.8. In the flrst case (Figure 3.8(a)), the lapse rate is superadiabaticwith
relativelycalm winds. There is significantinitial plume rise and a subsequent"looping" motion. This motion results from portions of the plume being buoyed up by
convectiverising, with subsequentdescendingof air. Upward and downward air
tl0otionis considerable.Since eddies that produce this motion are oriented in all
f,irections,signiflcanthorizontal dispersiontakesplace as well.
j, A coning plume is illustrated in Figure 3.8(b). Coning plumes form when lapse
fptesareneutralto isothermal.As such,the atmosphereis slightly unstableto slightly
gtable.Such lapse rates occur on cloudy or windy days or at night. Atmospheric
(urtulenceis primarily mechanical;turbulenteddiesmay have different orientations
Ithatresult in dispersionin a relatively symmetricalpattern.
AIR OUALIW
ATMOSPHERIC
AND DEPOSITION
DISPERSION.
IRANSPORT,
3.1.8 Large-ScaleTransport and Dispersion
(a) Looping
3.1.8.1 Long-RangeTransport
V
.oo[\
t\
\
o[
E
F
'6
I
(b)Coning
3oo
f
\
th
(c) Fanning
300
l./
l/llr.trrfrr
l/
ol/
;
ln
TemPerature+
conditions.
stability
atmospheric
withdifferent
associated
Figure3,g plumeformandbehavior
Historically, pollutant concernshave focusedon dispersionof pollutants in
urbanareasand thosedownwind of largepoint sources.It was onceassumedthat
whenpollutantswerediluted to acceptablelevelsor plumeswereno longervisible,
they were not a problem. In the early 1970s,the phenomenonof long-range
transportwas identifiedas the causeof elevatednocturnalrural O, levels. Both
O, and its precursorswere transportedhundreds of kilometers from urban areas
(urbanplume).The phenomenonof acidic depositionand its attendantecological
effectsand associationswith the long-rangetransport of acid precursorsin North
America becameknown in the mid- to late 1970s.This was followed by the
recognitionthat Arctic hazeover Barrow,AK (in the northernhemisphericspring),
was associatedwith long-range transport of pollutants over the North Pole from
northernEurope and Asia. There was, in the late 1980s,increasingevidence that
troposphericpollutantswere being transportedinto the stratosphere.In more recent
times,we havecometo betterunderstandprocesses
by which long-livedsubstances
suchas carbondioxide (COr), CH4,halogenatedhydrocarbons,and particlesmove
aroundthe planet.
3.1.8.2 Urban Plume
i
A fanning plume is illustrated in Figure 3.8(c). It is produced under stable
conditionsin which the top of a ground-basedinversionis well abovethe stack.The
plume rises slightly, there being little vertical air movementand dispersion-Horiiontal motions, however,are not inhibited. As a consequence,the plume may be
characterizedby varying degreesof horizontal spreading.
Other plume forms and behaviors(not illustratedhere) occur.Lofting, fumigating, and trapping plumes are associatedwith inversions.A lofting plume may be
produceOwtten tne atmosphereabovea surfaceinversionis unstable,with the stack
abovethe surface-basedinversion.The plume rises upward as its downwardmovement is restrictedby the inversionbeneathit. Lofting plumes are producedat sunset
on clear nights, over open terrain. A fumigating plume is producedwhen a surfacebasedinversionbreaksup after sunrise.Pollutantsare brought to the ground by the
downward movement of convectivecells. When inversions occur both above and
below a smokestack,the plume is trapped;in appearanceit is somewhatsimilar to,
but deeperthan, a fanning Plume.
As plumes move downwind, they generally mix with air that is less polluted.
Such mixing occurs as a result of diffusion, advection,displacement,convection,
and mechanicalturbulence.Consequently,concentrationsdecreasewith downwind
distance.Depending on their height and atmosphericstability, plumes may reach
the ground *ithitt u1"* kilometers of a source or may remain airborne for extended
distances.Plumes can be seenbecauseof the presenceof light-scatteringparticles'
plumes becameless and Iess visible as particle concenfiationsdecreaseas a result
of dilution.
Large urban centers include numerous individual point and mobile sources.
TheseJontributecollectively to large polluted air massesthat affect air quality for
tensto hundredsof kilometers downwind. This air mass is describedas an urban
plume. The transport and dispersionof pollutants in an urban plume occur over
largergeographicalareasand timescalesthan those typically associatedwith individualsources.Of particularimportanceare airflows associatedwith migrating highandlow-pressuresystems.
3,1.8.3 Planetary Transport
*
A stable layer of air at the top of the PBL retardsvertical mixing and isolates
it from the free troposphereabove. Becauseupward air movement is somewhat
impeded,timescaleson the orderof a few hoursto a few daysareneededfor transport
of pollutantsout of the PBL. As a consequence
of convectiveenergyflows, baroclinic
(pressure-related)
instability, and heat releasefrom condensationof water vapor,
pollutantsare transportedto the top of the troposphere,with uniform mixing occurring in about a week.
Air in the troposphereis continuously stirred by convectiveair movement and
otheratmosphericphenomena.As a consequence,substanceswith lifetimes of sev'eral
months or more are well mixed within the troposphere.However, significant
concentrationdifferencesexist betweennorthern and southernhemispheres.
Atmosphericphenomenaat the equatorretard airflows from one hemisphereto
another.As a consequence,cross-equatorialmixing time is approximately I year.
AIi] OUALITY
B6
3.1.9
Stratosphere-Troposphere
ATMOSPHERIC
DISPERSION.
TRANSPORT.
AND DEPOSITION
Exchange
The troposphereand stratosphereare characterized,respectively,by decreasing
and increasingtemperatureswith height. They are separatedby an isothermallayer
of air (the tropopause).The increasingtemperatureswith height in the stratosphere
Serveto limit the upward and downward movementof atmosphericgasesbetween
the troposphereand stratosphere.Trace gas measurementshave shown, however,
that such exchangetakes place, albeit relatively slowly.
exchangeprocesses,long-lived
As a consequenceof stratosphere-troposphere
(CFCs),
CH+,and nitrous oxide (NrO)
chemicalspeciessuchas chlorofluorocarbons
originating in the stratospecies
are ttansportedinto the stratosphere,and chemical
Or, nitric oxides (NO"),
include
These
troposphere.
sphereare transportedinto the
chloride (HCl) from
(ClONOr)
hydrogen
and
nitrate
chlorine
and substancessuchas
of
CFCs.
photodestruction
the
The time required to exchange the mass of the entire tropospherewith the
is estimatedto be 18 years. Becauseof differencesin mass' it takes
stratolsphere
abouf 2 years for the entire stratosphereto mix with the troposphere.
A number of potentialpathwaysand mechanismshavebeenproposedto explain
exchange.The simplestof theseconsistsof a single Hadstratosphere-troposphere
tey-type cell in each hemispherewith a uniform rising motion acrossthe tropical
tropopause,polewardmovementin the stratosphere,and return flow into the troposphereoutside of the tropics. This is consistentwith low HrO vapor levels in the
tropical stratosphereand high 03 levels in the lower polar stratosphere.This circulation cell is describedas a wave-driven"extratropicalpump"'
exchangeare illustrated
This and a secondpathwayfor stratosphere-troposphere
in Figure 3.9. In the secondpathway,transportoccursfrom the lower stratosphere
to the tropospherein mid-latitudes and from the troposphereto the stratosphere
along surfaces of constant potential temperaturethat cross into the tropopause.
Exchangeof air tendsto occur in associationwith eventsknown as tropopausefolds.
In this phenomenon,the tropopauseon the polewardside of a jet stream(seeChapter
1) is distorted during developmentof large weather systems.Large intrusions of
stratosphericair occur, which becometrapped and eventually mix with the troposphere.Air from the tropospherecan also be trapped in the stratosphereduring
tropopausefolds. In a third proposedpathway,air is transportedconvectivelyfrom
the troposphereto the stratosphere.
3.1.10 Stratospheric Circulation
Due to strong inversion conditions the stratosphereis very stable, with little
vertical air motion. Becauseof differencesin stratospherictemperaturebetweenthe
equatorand poles, as well as those causedby diabatic heating (associatedwith O,
absorptionof ultraviolet (UV) ligh0, zonal (easte+ west) and meridianal (north <->
south) flows characterizethe stratosphere.Thermal gradientsresult in strong zonal
winds, which reach peak speedsnear solsticesand reverseafter equinoxes.
Despite diabatic heating, circulation in the stratosphereis wave driven. Atmospheric waves tlansport air poleward in the winter hemisphere.It subsidesat the
,
Extratropical
F
Path I
Overworld
pump
o
=
Cumulonimbus
clouds
STE
Path l l l
Underworld
Latitude
Figure3.9 Stratosphere-troposphere
pathways
(FromHolton,
exchange
andprocesses.
J.R.
et al.,Rev.Geophys.,
33,403,1995.Withpermission.)
poles,where it warms by compression.A circulation cell developsthroughout the
middleandupperstratosphere.
Thereis a slow rising of air in the summerhemisphere
andtropics and more rapid sinking over a smaller areain the winter hemisphere.
This stratosphericcirculation significantly affects the movement of chemical
species.It results in the movement of O, (most O, is produced in the tropical
stratosphere)and gasestransportedfrom the tropospherepoleward; movement is
strongin the winter hemisphereand less so in the summerhemisphere.
Such stratosphericairflows transporttrace gasessuch as CFCs to the Antarctic
andOr-depletedair northwardfrom the Antarctic. They also havesignificant effects
on substancesthat contribute to the formation of polar stratosphericciouds (see
+
Chapter4).
3.2 ATMOSPHERIC REMOVAL AND DEPOSITION PROCESSES
3.2.1Atmospheric Lifetimes
Referencewas made to averagelifetimes or residencetimes of various atmosphericpollutantsin Chapter2. All gas- and particulate-phase
pollutants have a life
historyin the atmospherebefore they are ultimately removed.By averagingthe life
historiesof all moleculesof a substance,or all particlesof a particular type or size,
onecan determinetheir averageresidenceor lifetime. Residencetime can also be
describedin the context of a pollutanfs half-life, i.e., the time required to reduce
its concentrationby 50Voof its initial value.
nihounurv
Atmospheric lifetimes can be calculatedusing mass balanceequations.Under
steady-stateconditions(emissionrate = removalrate),the residencetime or lifetime
of a substancecan be calculatedfrom the following equation:
t=QP
g/year)= -1 week
x = 2 x 1012g/(100x 1012
Lifetimes are also calculatedon the basis of the substance'sreactivity with sink
chemicalssuch as hydroxyl radical (OH.) and nitrate (NOr-). In suchcaseslifetimes
are cplculatedby dividing the productof the OH. concentrationandthe rate constant
(k) into L For the reactionbetweenCHo and OH',
Q.2)
an atmosphericlifetime of 5 years is calculated:
r = l/k[OH.]
(3.3)
= 1l[6.3 x 10-15
cm3molecule-rsec-rx (1 x 106OH' moleculescm-3)]
= 1.5ex 108sec= -5 vear
These atmosphericlifetime calculationsare basedon chemical kinetics. They
assumethat there areno competitorsfor OH. or, if there are, that OH'is in a steadystate concentration.If there are competing loss processes(such as photolysis or
reaction with Or), lifetimes may be shorter.
Though the atmosphericlifetimes of gas-phasesubstancesare determinedby
chemical reactionswith substancessuch as OH., their by-products,unreacted
specieswill ultimatelybe removedby deposition
molecules,and particulate-phase
processes.
3.2.2
Deposition Processes
vg = -F/c
(3.4)
Thedepositionvelocity is a positivenumberdespitethe fact that the flux is negative;
it is given as centimetersper second(cm/sec).Dry depositionvelocities are given
for coarse and fine particles and for gas-phasesubstancesin Table 3.2. Highest
depositionvelocities have been observedfor coarseparticles and gas-phasenitric
acid(HNOr). Becauseof its high solubility,HNO, is readilyabsorbedinto dew and
otheraqueoussurfacesand rapidly taken up by plants.
r Wet depositionincludes all processesby which airbornegasesand particles are
transferredto the Earth's surfacein aqueousform (rain, snow, fog, clouds, dew).
'Theseprocessesinclude (1) absorptionof gas-phasesubstancesin cioud droplets,
raindrops, etc.; (2) in-cloud processeswherein particles serve as nuclei for the
rcondensation
of HrO vapor to form cloud or fog droplets; and (3) collision of rain
dropletsand particlesboth within and below clouds.In the last case,collisions with,
andsubsequentincorporationof, gas-phasesubstancescan also occur.This process
is called washout.The removal of particles in the rain-making processis called
rainout.
i l
r,3.3METEOROLOGICAL APPLICATIONS: AIR POLLUTION CONTROL
'
As seenin previousdiscussionsin this chapter,a variety of meteorologicalfactors
raffectthe dispersionof pollutants from individual sourcesas well as urban areas.
'Thusmeteorology has signiflcant applications in air pollution control programs.
Theseinclude episodeprediction, planning, and community responses.These also
include determinationof whether, under worst-caseatmosphericconditions, proposedsourceswill be in compliancewith air quality standardsand visibility protectionrequirementsof preventionof significant deteriorationprovisions of clean
air legislation.Complianceis determinedfrom air quality modehng.
Table3.2 Dry Deposition
(cm/sec)for Particlesand SelectedGases
Vetocities
Pollutant
Particles
> 2 pm
. , Ut
[,,P6r11a,"a
Surface
Exterior
Exterior
Exteriorand interiorleaf
surfaces
.'HNO3
Primarilyexteriorleaf
surfaces
Primarilyleaf interiors;also
exteriorleaf sudaces
Primarilyleaf interiors;also
e)deriorleaf surfaces
Qar
vv2
Dry depositionis characterizedby direct transfer of gas- and particulate-phase
substancesto vegetation,water, and other Earth surfaces.This transfer may take
place by impaction, diffusion to surfaces,and, in the caseof plants, physiological
uptake of atmospheric contaminants.Particulate-phasesubstancesmay also be
removedby sedimentation.
89
Dry depositionis characterized,bya depositionvelocity (Vg). It is a proportionality constantthat relatesthe flux (F) of a chemical speciesor particle to a surface
iandits concentration(C) at somereferenceheieht:
(3.1)
where t is the residencetime, Q is the total massof substancein the atmosphere
(g), and P is the emissionor removal rate (giyear).
is 2 x 1012g and P = 100
If the total mass(Q) of a given atmosphericsubstance
x l0t2 glyear,
CH. + OH. + CHr. + HrO
ATMOSPHERIC
DISPERSION.
TRANSPORT.
AND DEPOSITION
NO,
:
o3
Range(cm/sec) _
0.5-2
<0.5
0.2-1 dry foliage,open
stomates;>1 for wet
surface
1-5 or greater
0.1-0.5whenstomates
open
0.1-0.8
AIR QUALITY
3.3.1
Episode Planning
In the late 1950sand early 1960s,the U.S. WeatherService (now the National
WeatherService) observedthat elevatedpollutant levels in urban areaswefe associated with slow-moving migratory high-pressuresystems.As a consequence,it
began a program of issuing air pollution potential forecastswhen such systems
coveredan area of at least 90,000 km2 (-35,000 miz) and were expectedto persist
at least36 h.
Despitethe fact that regulatory efforts to control air pollution have significantly
reduced health threats associatedwith ambient air pollution, such stagnanthighpressuresystemsoccur periodically,as do the semipermanenthigh-pressuresystems
over southernCalifornia. As such, they can causesignificant increasesin groundlevel concentrationsthat may adverselyaffect the health of individuals at special
risk (e.g.,asthmatics,thoseill with respiratoryor cardiovasculardisease,the elderly).
Therefore,there is a continuing need to forecast air pollution episodes.When the
Nationq] Weather Service forecastsa developingepisode,regulatory authoritiesat
the local, state,and nationallevels begin to implementepisodecontrol plans,which
may include requiring phased reductions in emissions of one or more primary
pollutants and issuing community health warnings (seeChapter 8).
3.3.2
Air Quality Modeling
New stationarysourcesregulatedunder National Ambient Air Quality Standard
provisions of clean air legislation are required to demonstratethat they will be in
compliancewith thesestandardsevenunder atmosphericconditionsthat are unusually favorable for elevatedground-levelconcentrations.Such compliance(in new
sourcereviews) can only be demonstratedby using dispersionmodels to evaluate
the source'simpact. Primary inputs to dispersionmodels are emissionand source
information, meteorologicaldata,and receptorinformation.Meteorologicaldataand
information required for thesemodels include stability class, wind speed,ambient
temperature,and MH. Point sourcedispersion,as well as other models,is described
in detail in Chapter 7.
READINGS
Calvert,S.
in Handbookof Air PollutionTechnology,
dispersion,
Boume,N.E.,Atmospheric
andEnglund,H., Eds.,JohnWiley & Sons,NewYork, 1984.
and Global
Chemistry
Brasseur,
G.P.,Orlando,J.J.,andTyndall,G.S.,Eds.,Atmospheric
Change,OxfordUniversityPress,Oxford,1999.
D.C, 1969.
Washington,
Briggs,G.A.,Plumerise,AEC CriticalReviewSeries,
MotionandAir Pollution,JohnWiley & Sons,NewYork, 1979.
Dobbins,R.A.,Atmospheric
B.G. andPins,J.N., Jr.,Chernistryof the Upperand LowerAtmosphere:
Finlayson-Pitts,
AcademicPress,Orlando,FL, 2000.
andApplications,
Theory,Experiments,
ed.,AcademicPress,Orlando,FL,
Holton,J.R.,Intoductionto DynamicMeteorology,3rd
1992.
ATMOSPHERIC
DISPERSION,
]RANSPORT,
AND DEPOSITION
91
Lutgens,F.K. and Tarbuck,E.G., The Atmosphere,Tth ed., PrenticeHall, Saddlebrook,NJ,
1998.
scorer,R.S., Meteorologyof Air Pollution: Implicationsfor the Environmentand lts Future,
Ellis Horwood, New York, 1990.
Turner,D.B., Workbookof AtmosphericDispersion Estimates:An Introduction to Dispersion
Modeling, Lewis Publishers/CRCPress,Boca Raton, FL, 1994.
QUESTIONS
i. What is the planetaryboundary layer? What role does it play in ftoposphericair
motion?
2. What is turbulenceand how is it formed?
3. How doesturbulenceaffect the dispersionof pollutants from a source?
4. What is the effect of wind direction on pollutant concentrationsdownwind of a
source?
5. The velocity of wind moving past a constantemission sourcechangesfrom 1 to
4 m/sec.What is the relative quantitativeeffect of this changeof wind velocity
on the pollutant concentration?
6. How is atmosphericstabiliry relatedto lapse rate conditions?
7. Describe dispersioncharacteristicsof the atmosphereunder the following lapse
rate conditions:-2,0, and 1"C/100m.
8. Describeradiational inversion formation.
9. Indicate differencesin the forms ofradiational inversionsin river valleys and over
cities on flat, open terrain.
10. How do sea,lake, and land breezesaffect air quality over a city?
11. Why can't polluted air in most casespenetratean inversion layer? Indicate the
physical principles involved.
12. How are subsidenceinversionsproduced?
13. Characterizesubsidenceinversionsrelative to their vertical temperatureprofile,
geographicalscale,and persistence.
14. What meteorologicalfactors affect plume rise?
15. Under what lapse rate conditions are looping, coning, and fanning plumes produced?
16. When do maximum ground-levelconcentrationsof pollutants ocqrr in mountain
valleys?Why?
17. What is an urban plume? How is it formed?
18. What air quality problems are associatedwith long-rangeffanspoft?
19. Generally,how long doesit take for a long-lived pollutant to be uniformly mixed
vertically in the troposphere?Horizontally throughoutthe troposphere?
20. By what mechanismsare pollutantstransportedinto the stratosphere?
21. Characterizeair circulation in the stratosphere.
22. What factors affect the lifetime of pollutants in the atmosphere?
23. What is dry deposition?What factors contribute to increaseddeposition rates?
24. Describe processesthat result in rainout and washout of pollutants from the
atmosphere.
25. What meteorologicalconditionsproduce pollution episodes?
26. How is meteorologicalinformation used in the implementationof episodeplans?
f,
i;
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l:
i1
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t
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ii
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