Air Quality Monitoring and Advanced Bayesian Modeling

317 Pages·2023·17.542 MB·English

by  Yongjie Li, Ka In Hoi, Kai Meng Mok, Ka Veng Yuen

AIR QUALITY
MONITORING
AND ADVANCED
BAYESIAN MODELING

AIR QUALITY
MONITORING
AND ADVANCED
BAYESIAN MODELING
YONGJIE LI
Department of Civiland EnvironmentalEngineering, University of
Macau, Avenida da Universidade, Taipa, Macau
KA IN HOI
Department of Civiland EnvironmentalEngineering, University of
Macau, Avenida da Universidade, Taipa, Macau
KAI MENG MOK
Department of Civiland EnvironmentalEngineering, University of
Macau, Avenida da Universidade, Taipa, Macau
KA VENG YUEN
State Key Laboratoryon Internetof Things forSmart City, Department
of Civil and Environmental Engineering,Universityof Macau, Avenida
da Universidade,Taipa,Macau
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CHAPTER 1
Introduction
Contents
1.1. Cleanversuspollutedair 1
1.2. Sourcesandimpactsofairpollutants 3
1.3. Airqualitymonitoringstrategies 6
1.4. Modelingandforecastingofairpollution 8
1.5. Aboutthisbook 10
References 11
Whenwesayavolumeofairis“polluted,”wemeanthatitcontainsunwantedsubstances
thatmightaffectthespecieslivinginthatvolumeofairorcausesomeothereffectstothe
ecosystem. As the key information conveyed to the public about the quality of air, an
array of indexes is used to describe the levels of pollutants in the volume of air that
weconcernabout.Theindexes,normallycalledairqualityindex(AQI),arederivedfrom
numerical expressions scalable to the pollutant concentrations. For instance, numeric
index from 0 to 100 (or 0 to 10 in some countries) is commonly used to indicate the
AQI,with0beingexcellent,thoughnotachievable,and100beingbad.Thereare,how-
ever, situations where AQI values can reach 500 or more, which is normally labeled as
“severe”or“hazardous.”TheAQIvaluesarenormallydeterminedfromthehourlycon-
centrationlevelsofseveralcriteriapollutantsautomaticallymeasuredbytheenvironmen-
tal protection agencies. Therefore, before one can derive these indexes, the
concentrationsoftheairpollutantshavetobemeasuredfirst;ifonewantstoknowwhat
will be the anticipated AQI tomorrow, then forecasting models have to be invoked to
predicttheconcentrationsofthepollutants.Thesetwotopicsarethefocusofthisbook.
In addition, of course, for regulatory and research purposes,much more detailed infor-
mation on air pollutants is needed than just those covered by AQI; such information is
also obtained from different measurement methods, which will also be covered by
this book.
1.1 Clean versus polluted air
Thequestionofhowpollutedthevolumeofairis,however,requiresquantitativeassess-
ment of the concentration levels of that particular pollutant. Before that, perhaps some
even more fundamental questions have to be answered first:
– What is the unpolluted “clean” air supposed to contain?
AirQualityMonitoringandAdvancedBayesianModeling Copyright©2023ElsevierInc.
1
https://doi.org/10.1016/B978-0-323-90266-3.00004-2 Allrightsreserved.
2 Airqualitymonitoringandadvancedbayesianmodeling
– Hasthecompositionof“clean”airbeenthesameinthegeologictimesincethefor-
mation of our atmosphere?
– What substances can be considered pollutants and need to be measured?
Thefirstatmosphere(c.4.6billionyearsago)oftheEarthisbelievedtoconsistofmainly
hydrogen (H ) and helium (He), which were swept away by solar wind or lost due to
2
gravitationescape(Jacobson,2002).Thentheoutgassingofcarbondioxide(CO ),water
2
vapor,andassortedgasesfromtheEarth’smantleformedthesecondatmosphere,which
isreferredtoprebioticatmosphere.Thebioticatmosphere,emergedaftertheappearance
oflivingorganisms(c.3.5billionyearsago),mainlyconsistedofmethane(CH ),molec-
4
ularnitrogen(N ),sulfurdioxide(SO ),andCO .Afterlivingorganismscapableofpho-
2 2 2
tosynthesis appeared (c. 2.3 billion years ago), oxygen and ozone were produced; the
oxygen buildup resulted in aerobic respiration, which led to more efficient production
of N , making it the major component in the atmosphere today.
2
Today,the dryair is mainly composedof N (78.1% by volume)and O (20.9% by
2 2
volume). The other relatively abundant gas is argon (Ar, 0.9% by volume), which is a
noble(andextremelynonreactive)gas.Othernonreactivegasesincleanairincludeneon
(Ne),He,krypton(Kr),H ,andxenon(Xe),whichhavevolumefractionsof(1–200)(cid:1)
2
10(cid:3)7.Alltheseareconsiderednonreactiveandrelativelyinvariableconstituentsofclean
gases,thusarenotconsideredpollutants.Anothergasthatisrelativelynonreactive(witha
long atmospheric lifetime of about 15 years) and invariable gas, CO (currently with a
2
volumefractionofaround0.04%,or400partspermillion,ppm),isanimportantgreen-
housegasthatcausesglobalwarming;itisthusconsideredasanairpollutantfromanthro-
pogenicactivities(e.g.,burningoffossilfuels).This“invariable”characteristicofCO is,
2
however,arelativedescription.Itsconcentrationdidincreasefromabout280ppmbefore
the Industrial Revolution to over 400ppm nowadays due to anthropogenic input; it
shows a clear annual pattern with lows in summer (efficient uptake by plant growth)
andhighsinwinter(releasefromdecaysofplantlitters).Watervaporisalsoanimportant
constituent in the atmosphere and it is highly variable, with volume fractions ranging
from a few parts per million all the way to approximately 4%. Water vapor is generally
notconsideredasanairpollutantbecauseitoccursmostlynaturally,butitspresenceand
the dynamics of it greatly affect the hydrological cycle and the meteorological
phenomena.
Most air pollutants we concern about are in trace amounts and their concentrations
arehighlyvariable(Jacobson,2002).Forinstance,carbonmonoxide(CO)hasavolume
mixing ratioof0.04–0.2ppmincleanenvironments,butitcanreach2–10ppminpol-
lutedenvironments.Themixingratioofsurfaceozonecanbeaslowas10ppb(partsper
billion)incleanenvironments,butashighas350ppbinpollutedregions.Sulfurdioxide
(SO )mixingratiosarenormallylessthan1ppbincleanareasbutcanbeashighas30ppb
2
inpollutedareas.Nitrogenoxides(mainlynitricoxide,NO,andnitrogendioxide,NO )
2
have mixing ratios of <0.3ppb in clean environments but can be up to 300ppb in
Introduction 3
polluted areas. Ammonia (NH ) normally has sub-ppb mixing ratios in clean areas but
3
canbeabove10ppbnearsources.Volatileorganiccompounds(VOCs)arenormallypre-
sentintheatmosphereinsub-ppblevel,exceptformethane(CH ),whosemixingratio
4
canbe1–2ppmevenincleanenvironments.Inpollutedregions,however,someVOCs
canhavemixingratiosof>10ppb.Particulatematters(PM)aresuspendedliquidorsolid
particles (Seinfeld and Pandis, 2016) that have sufficiently long lifetimes in the atmo-
sphere to exert various effects on the environment. PM are normally classified by a
cut-point diameter,with PM denoting particleswith aerodynamicdiameters<10μm
10
andPM andPM of<2.5and<1μm,respectively.TypicalPM massconcentration
2.5 1 2.5
in clean environments is normally less than 10μgm(cid:3)3, but it can be way above
100μgm(cid:3)3 in polluted urban areas.
Mostoftheseairpollutantshaverelativelyshortatmosphericlifetimes,rangingfrom
1daytoapproximatelyamonth,makingtheirconcentrationshighlyvariableandspatially
heterogeneous.Someoftheemissionsourcesoftheseairpollutantsarealsonotcontin-
uous, at least not emitted at a constant strength. For instance, the CO from vehicular
exhausts has much higher emissions during rush hours, while the oxidation of it by
OHradicals during noon time contributes to theremoval of thisair pollutant. Assuch,
most of these air pollutants show significant dynamics in concentrations and spatial dis-
tributions, making the monitoring of them a challenging task.
1.2 Sources and impacts of air pollutants
Most of the air pollutants of concern are more or less related to combustion processes,
with only a few exceptions. Burning of fossil fuel is a process to harvest the energy
releasedfromthebreakageofchemicalbondsinhydrocarbonsinthefuel,yieldingideally
CO andH Oincompletecombustion.Incompletecombustion,whichisalmostinev-
2 2
itably,actuallyreleasesasubstantialamountofCO,blackcarbon(BC),andflyash.While
COisagaseouspollutantthatcaninhibitthecapacityofbloodtocarryoxygentoorgans
andtissues,bothBCandashareimportantcomponentsinPM andmaycauseadverse
2.5
health effects and alter energy balance due to absorption of solar radiation. Some fossil
fuel, especially coal, contains a significant amount of sulfur, and the combustion of it
releasesSO astheby-product.Ifnotproperlytreated,emissionofSO inhighamount
2 2
willhavehealthimplicationsbecauseitcancauserespiratorysymptomsinsensitivepop-
ulation.Inaddition,SO isalsoanimportantprecursorforthesulfuricacid(H SO )that
2 2 4
isresponsibleforacidrainformation;evenafterbeingneutralizedbyalkalinespeciessuch
asNH ,thesulfuricacidisconvertedtoammoniumsulfate,(NH ) SO ,whichisoneof
3 4 2 4
the major inorganic components in PM .
2.5
Nitrogenoxides(e.g.,NOandNO ,often termedcollectivelyasNO )areformed
2 x
fromhigh-temperaturereactionofN andO intheair(termedasthermalNO )orthe
2 2 x
oxidationoforganicallyboundnitrogeninfuels(termedasfuelNO ).Inparticular,NO
x 2
4 Airqualitymonitoringandadvancedbayesianmodeling
canalso affectlung functioninpersonswithasthma, andoxidation ofitcanform nitric
acid(HNO ),anotherstrongacidthatcontributestotheformationofacidrain.Similarly,
3
neutralizationofHNO byNH formsammoniumnitrate,NH NO ,whichalsocon-
3 3 4 3
tributestoPM masssubstantially.NH isgenerallynotconsideredasacriteriaairpol-
2.5 3
lutant, but its participation in neutralization of the aforementioned inorganic acids
contributestotheformationofsecondaryPMcomponents.Thereforethemostimpor-
tant secondary inorganic aerosol (SIA) components include ammonium (NH+), sulfate
4
(SO2(cid:3)),andnitrate(NO(cid:3)).ThesourcesofNH includenaturallyoccurringdecompo-
4 3 3
sitionofaminoacidsinorganicwaste.AnthropogenicsourcesofNH includelivestock
3
farming,fertilizerusage,andbiomassburning.Morerecently,NH emissionfromvehi-
3
cleshasbecomeagreatconcern.ThisunexpectedsourceofNH isrelatedtothetreat-
3
ment of other air pollutants of vehicles, NO and NO , by after-the-pipe catalytic
2
reduction technology. Specifically, NH can be formed from the reduction of NO by
3
H in gasoline-powered three-way catalytic convertors or from the reduction of NO
2
and NO by urea in diesel-powered selective catalytic reduction devices (Farren
2
et al., 2020).
Unlikeothergaseouspollutants,O isnotdirectlyemittedbyanysources.Itisasec-
3
ondary pollutant formed from various reactions of gaseous components with the right
mixture and ripe condition. In the stratosphere (about 12km above surface), high O
3
concentration is desired because it helps block the short-wavelength ultraviolet (UV)
lights in the solar radiation from penetrating to the Earth’s surface and affecting living
organisms (including us) down here. Up there, O formation proceeds mainly via the
3
Chapmanmechanism(Chapman,1930),inwhichaO moleculeissplitintotwotriplet
2
stateO(3P)atomsuponirradiationbyUVlightswithwavelengthslessthan240nm;the
O(3P)combinesrapidlywithanotherO moleculetoformO ,withcollisionofathird
2 3
molecule (normally denoted as M and should be N or O because of their high abun-
2 2
dance) to remove excessenergy. The high-O layer uptherein the stratosphere isper-
3
turbed by some other species that can undergo catalytic destruction of O , including
3
H O, NO , and chlorinated and fluorinated carbons (CFCs). The perturbation by the
2 x
last one, CFCs, was found to be promoted by the polar stratospheric clouds (PSC) in
spring time over Antarctica (Molina and Rowland, 1974), resulting a “hole” with
extremely low concentrations of O in the stratosphere over the South Pole.
3
Inthetroposphere(fromthesurfacetoabout12km),NO playsacentralroleinthe
2
formation of O , whose presence in high concentration is not desired as it can damage
3
respiratory tissues in animals and also tissues in plants. Different from the Chapman
mechanism that requires formation of O(3P) from photolysis of O molecules (short-
2
wavelength UV lights are needed), O(3P) can also be formed down here in the
tropospherebyphotolysisofNO ,whichonlyrequireslightswithwavelengthslessthan
2
380nm(Finlayson-PittsandPitts,2000).NO isconvertedtoNOinthisreaction.The
2
presenceofanarrayofreducedgaseouspollutantsinthetroposphere,however,amplifies
Introduction 5
thisO formationmechanism.First,reducedgasessuchasCO,CH ,andotherVOCsare
3 4
oxidizedbyOHradicalstoformperoxyradicals,includingHO andRO .ThenHO
2 2 2
andRO canconvertNObacktoNO ,whichcanphotolyzetogenerateO(3P)forO
2 2 3
formationagain.ThereforeO formationinthetroposphereismanifestedbythemixture
3
ofNO andreducedgasessuchasCO,CH ,andotherVOCs,withthehelpofsunlight
x 4
thattriggerstheradicalreactions.Inaddition,O inthetropospherewillalsobeaffected
3
byoccasionaldownwardtransportofthehighconcentrationsofO inthestratosphere.
3
PMhasmultiplesources,complexcomposition,aswellasmanyeffectsontheenvi-
ronment. Some PM can be produced naturally, for example, biological aerosols that
include intact microorganisms (such as bacteria, fungi, and viruses) or debris of plants
and animals. Wind-induced dust suspension and bubble bursting on the ocean surface
arealsoprocessesthatproducePMinlargequantities.Volcanoeruptionisalsoanatural
phenomenonthatwillinputalargeamountofPMintotheatmosphereinashortperiod
oftime.Anthropogenicactivities,ontheotherhand,alsoreleasesubstantialamountsof
PM. These activities include combustion of various fuels, including fossil fuel and bio-
massusedinsomehouseholdsettings,aswellasotherindustrialprocesses.Thecompo-
sition of PM is highly complex, which includes primary (directly emitted) components
suchasBC,crustalmaterials,andsodiumchloride(NaCl,inseasprayaerosols);somePM
components are secondarily formed, which include SIA components such as sulfate,
nitrate, and ammonium, as well as secondary organic aerosol (SOA) components from
theoxidationofVOCs.ThereareseveraladverseeffectsofPM,butinsomeoccasions,
itmightalsobringsomebenefittotheenvironments.First,PMcancauserespiratoryand
cardiovasculardiseasesinhumanbeings.Second,highPMconcentrationcanreducevis-
ibility greatly via light scattering. Third, some PM components, for instance BC and
brown carbon (BrC, light-absorbing organic compounds), can absorb solar radiation
andwarmtheclimate.Onthesamesubjectmatter,someothercomponents(e.g.,sulfate)
cancooltheclimatebylightscattering(directeffect)orcloudformation(indirecteffect).
Finally,transportoflargeamountsofdustparticles,whichcontainmicronutrientssuchas
iron, from the desert to the oceans is beneficial for the marine ecosystem.
ThemostabundantVOCintheatmosphereismethane(CH ),withamixingratioof
4
1–2ppm. This mixing ratio of CH nowadays is approximately 2.5 times that of prein-
4
dustrialera(about0.8ppm).Ontheotherhand,thereactionrateconstantbetweenCH
4
andOHradical(k )isontheorderof10(cid:3)14cm3mol(cid:3)1s(cid:3)1,twoordersofmagnitude
OH
lowerthanthosebetweenmostotherVOCsandOHradical.Withhighabundanceanda
slow reaction rate constant with OH (as the main cleansing agent in the atmosphere),
CH is normally separately discussed from other VOCs. The main source of methane
4
is from the fermentation process of anaerobic bacteria, which are widely distributed in
wetland, rice paddy, ruminating animals, etc. Anthropogenic sources of CH include
4
burning of fossil fuel and biomass, landfill, and leakage from gaseous fuels. The annual
CH emission is over 500Tgyr(cid:3)1 (teragrams per year). Although CH is generally
4 4