Chemical Modeling for Air Resources. Fundamentals, Applications, and Corroborative Analysis

276 Pages·2013·7.029 MB·English

by  Jinyou Liang (Auth.)

Chemical Modeling for Air
Resources
Fundamentals, Applications,
and Corroborative Analysis
Jinyou Liang
AMSTERDAMdBOSTONdHEIDELBERGdLONDONdNEWYORK
OXFORDdPARISdSANDIEGOdSANFRANCISCOdSINGAPORE
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Preface
Air is an invaluable resource for humans. It participates in maintaining human life
chemically and shields humans from harmful radiation, but also contains toxic
constituentstobecleaned.Tounderstandachemicalphenomenonintheair,nearbyor
far away, or to assess implications of marketing a new chemical, or to evaluate the
investmentofimplementingathoroughcleanairpolicy,chemicalmodelingprovides
a powerful tool for integrated analyses.
Built on over 20 years of experience in developing, applying, and analyzing
chemical models for air resource research and regulatory purposes at the Chinese
Research Academy of Environmental Sciences, Harvard/Stanford/Zhejiang Universi-
ties, and California Environmental Protection Agency of the USA, I wrote this book
during the summer of 2012. This book is written for graduate students and junior
researchers in a manner similar to assembling puzzle blocks: many pieces have
been arranged, while some remaining pieces are identified for interested readers to
research.
Toprovideaconcisetutorialonchemicalmodelingforairresources,thisbookis
divided into three parts:
l The first part focuses on fundamentals required for air resource chemical modeling. The
chemicalcompositionoftheairisdescribedinChapter1,chemicalreactionsintheairare
discussed in Chapter 2, radiation in the air is considered in Chapter 3, and modeling
chemicalchangesintheairisdescribedinChapter4.
l The second part focuses on application cases of air resource chemical modeling.
The ozone hole is considered in Chapter 5, acid rain is discussed in Chapter 6, climate
change is the subject of Chapter 7, surface oxidants are described in Chapter 8, partic-
ulate matter is discussed in Chapter 9, and other toxins in the air are considered in
Chapter 10.
l The third part, Chapter 11, introduces methods to corroborate analyses of data from
models and observations for serious simulations, such as in support of governmental
regulations.
At the end ofeach chapter,a handfulofexercises are provided.
Additional information is available from Introduction to Atmospheric Chemistry
by Professor Daniel J. Jacob at Harvard University, Fundamentals of Atmospheric
Modeling by Professor Mark Z. Jacobson at Stanford University, air resource docu-
mentsfromregionalenvironmentalprotectionagencies,andanumberofprofessional
journals,suchasAtmosphericEnvironment,JGR-Atmospheres,ChinaEnvironmental
x Preface
Sciences,aswellasothernationaljournals.Readerswhostewardairresourcesfrom
a chemical perspective at regional, national, or global level will hopefully find this
book helpful.
Jinyou Liang
California,USA
April2013
(E-mail address: [email protected])
1
Chemical composition of the
atmosphere of the Earth
ChapterOutline
1.1 Atmosphericcompositionfromobservationandtheory 4
1.1.1Troposphere 5
1.1.2Stratosphere 8
1.1.3Mesosphereandabove 9
1.2 Tracechemicalsobservedinthetroposphere 9
1.2.1Naturaltracechemicalsinthetroposphere 9
1.2.2Anthropogenicemissionsourcesintheregionaltroposphere 10
1.2.3Anthropogenicorganicchemicalsintheregionaltroposphere 14
1.2.4Traceelementsintheregionaltroposphere 14
1.2.5Tracechemicalsintheglobaltroposphere 15
1.2.6Isotopictracersinthetroposphere 15
1.3 Tracechemicalsobservedinthestratosphere 16
1.4 Greenhousechemicalsintheatmosphere 17
1.5 Toxicchemicalsinbreathingzones 18
Summary 19
Humans can only survive for 1–2 minutes without taking oxygen (O ) into the
2
bloodstreamand,intheexploreduniversetodate,thereisnodirectreservoirforO
2
otherthantheatmosphereoftheEarth.Thus,theEarth’satmosphere,whichmainly
consistsofN andO ,isthemostimportantresourceforhumans,thoughitistheleast
2 2
commercializedresource compared with food,land, andwater.
Chemicals in the modern atmosphere are versatile, and their evolution from
primitiveEarthisstilllargelyamystery.Inthemodernatmosphere,O servesasthe
2
onlyfueltomaintainthebiochemicalenginesofhumansandmostanimals,andH Ois
2
the mostimportant gasto adjust airtemperature in the lower atmosphere to comfort
humans and animals living near the surface, while CO and H O are necessary
2 2
nutrients for plants and crops to grow. Numerous chemical species, besides N , O ,
2 2
H O,CO ,the“noblegases”(He,Ne,Ar,Kr,Xe,Rn),andH ,havebeenidentifiedin
2 2 2
the atmosphere since industrialization, when analytical instruments such as chro-
matographsandspectrometerswereinvented.Amongthem,ozone(O )isfoundtobe
3
necessary in the middle atmosphere to protect humans from harmful ultraviolet
radiation during daytime. However, O is harmful to humans when present near the
3
Earth’ssurface.Whenachemicalintheair,suchasO ,hasaso-called“dose–response
3
relationship”,mostlydeterminedfromanimalexperiments,itiscalledanairtoxin.
ChemicalModelingforAirResources.http://dx.doi.org/10.1016/B978-0-12-408135-2.00001-X
Copyright(cid:1)2013ZhejiangUniversityPressCo.,Ltd.PublishedbyElsevierInc.Allrightsreserved.
4 Part1:Fundamentals
Therearestillmanychemicalsthataresuspectedtobepresentintheatmosphere
butnotdetectableduetolimitations ininstrumentation or theoretical methods.
1.1 Atmospheric composition from observation and theory
Inthesolarsystem,theEarthisauniqueblueplanetlookingfromspace,andtheblue
color isaresultofscattering ofsunlightby the atmospheric chemicals ofthe Earth.
The Earth’s atmosphere extends from the surface to an ambiguous outer bound,
~100 km above average sea level (ASL). If using the indicator “1 mole air¼0.79
N þ0.21 O ”, then the Earth’s atmosphere may be characterized by four to six
2 2
verticallayersupward,namelythetroposphere(0–10km),stratosphere(10–50km),
mesosphere (50–85 km), ionosphere (80–90 km), thermosphere (85–100 km), and
exosphere (100–500 km), as shown in Figure 1.1. The troposphere may be further
dividedintotheplanetaryboundarylayer(PBL;0–1.5km)atthebottomandthefree
troposphere (1.5–10 km) at the top. The thickness of the planetary boundary layer
shows strong diurnal and spatial variations, and is a hot topic for meteorologists,
atmospheric scientists, and environmental professionals. The vertical structure of
atmospherictemperatureisregulatedbychemicalcompositionandradiationsofthe
Sun and Earth, as well as other physical processes, such as surface characteristics,
Figure1.1 VerticalthermalstructureoftheatmosphereoftheEarth.Airtemperaturenear
thesurfaceispertinenttosubtropicallandareasduringspringandfall.Solidline:from
measurements;dashedline:linearprojection.
ChemicalcompositionoftheatmosphereoftheEarth 5
relative positions of planets and moons, and resulting dynamic patterns in atmo-
spheric layers. Table 1.A1 at the end of this chapter lists vertical profiles of
temperature as well as pressure and O at 2-km intervals from the surface to 46 km
3
ASL in the modern atmosphere, and more constituents of the atmosphere are dis-
cussed below.
1.1.1 Troposphere
Thetroposphere,where~90%ofairmassovertheEarthresides,referstothebottom
~10kmoftheatmosphere(Figure1.1).Inthetroposphere,atmospherictemperature
descendsupwardwithaslopeof~10Kkm(cid:2)1fordryairand~7Kkm(cid:2)1forwetair.At
night,airtemperatureatthesurfacemaybelowerthanthatupto~100m,duetothe
combinationoflong-waveradiationofEarthandtheso-calledgreenhouseeffect.In
the troposphere, numerous field campaigns have been conducted to investigate air
composition over developed areas, such as North America, Europe, East Asia,
Australia and New Zealand, their downwind areas, such as the Atlantic Ocean and
PacificOcean,andremoteregions,suchastheArcticandAntarcticareas.Whilemost
observations havebeen madenear thesurface,significant efforts, such asthe useof
balloons, flights, rockets, and satellites, have also been made to observe the air
compositionabove,especiallyinrecentdecades.Inpopulateddevelopingcountries,
suchasChinaandIndia,fieldcampaignshavealsobeenconductedrecentlytosurvey
the chemicals responsible for air pollution, such as O , acid rain, and particulate
3
matter.
On a global, annual average basis, the modern tropospheric air composition
excludingH O,CO ,CH ,andN OislistedinTable1.1,whichistermed“dryair”.
2 2 4 2
ItcanbeseenthatN isthemostabundantchemical,followedbyO ,andinturn
2 2
by noble gases and H . The chemical composition of the dry air, in terms of the
2
mixingratio,changeslittleintheopenatmosphereoftheEarth,orasdefined,though
theO mixingratioisperturbedbyhumans, animals,plants,andcrops,andmay be
2
modulatedbygeochemicalprocesses.Thereareanumberofhypotheseswithregard
to how the chemical composition of the dry air has arrived at its current status. For
example,intheverybeginning,thedryairoftheEarthcouldhavebeenpurelyCO ,
2
similartothecurrentstatusofMars;biogeochemicalprocessesmighthavegradually
Table1.1 Dryaircomposition
Dryair Molarmixingratio
N 7.81E-01
2
O 2.10E-01
2
“Noblegases” 9.32E-03
H 6.00E-07
2
Sum 1.00Eþ00
Note:1E-01denotes1(cid:3)10(cid:2)1,andmolarmixingratiosofthenoblegasesHe,Ne,Ar,Kr,
Xe,andRnare5E-8,1.5E-5,0.93E-2,1E-6,5E-8,and2E-19respectively.
6 Part1:Fundamentals
fixedcarbonfromtheairtoformfossilfuelsundergroundandleavingO intheair.
2
TheprocessinvolvedisthephotosynthesisinplantsthatconvertsCO andH Ointo
2 2
O ,whileotherprocessesarethesubjectofEarthsystemmodeling.Mixingratiosof
2
N , H , and noble gases in the dry air are speculated to result from complex
2 2
biogeochemicalprocesses.Atpresentlevels,thesegases,exceptRn,havenoreported
adverseeffectsonhumanhealth,andhumansandanimalsmayhaveadaptedtotheir
levels in the air. As an industrial resource, N is routinely used to make nitrogen
2
fertilizers and is used as a liquid agent for small surgery, and He is used to fill
balloons.
Besidesthedryair,H Oisanimportantcomponentoftheairinthetroposphere.
2
Ononehand,itisthereservoirofprecipitationsthatprovideeconomicdrinkingwater
andwatersuppliesforagricultural,industrial,andrecreationalpurposes.Ontheother
hand,itisanaturalandthemostimportantgreenhousegasinmodernairthatraises
thetemperatureofsurfaceairbyover30KsothattheEarth’ssurfaceishabitablefor
humansandanimals.ThemixingratioofH Ovaporinthetroposphererangesfrom
2
<0.01 percent to a few percent, depending on elevation, latitude, longitude, surface
temperature and other characteristics, such as closeness to bodies of water such as
ponds,rivers,lakes,estuaries,seas,andoceans.Theairmaycontainasmallamount
ofliquidwaterasrain,cloud,fog,haze,orwetaerosol;whenairiscoldenough,such
asinnontropicalareasduringwinterorintheuppertroposphere,itmayalsocontain
an even smaller amount of solid water as snow, hail, graupel, frost, cirrus cloud,
contrails, or other icy particles suspended in the air. Table 1.2 lists typical seasonal
saturatedwatervapormixingratiooverthenorthernhemisphere,whichrangesfrom
0.1% to 4%. Over global oceans, the relative humidity near the surface is close to
100%.Overtheland,therelativehumidityvariesfrombelow5%overdesertstoover
90% in coastal areas. Thus, water vapor is the third or fourth most abundant gas in
surface air.
In general, the H O mixing ratio is higher over the tropics than over polar areas,
2
higherinsummerthaninwinter,higheroverfarmlandsandforeststhanoverdeserts,
and higher near the surface than further away from the surface; these phenomena
reflectthe facts that H Oevaporates faster at higher temperatures and H O vapor is
2 2
transported inthe troposphere following airstreams termed general circulations.
Table1.2 Typicalseasonalsaturatedwatervapormixingratio
Latitude DJF MAM JJA SON
0 0.033 0.035 0.033 0.033
15 0.041 0.035 0.037 0.035
30 0.017 0.026 0.041 0.026
45 0.006 0.013 0.026 0.015
60 0.002 0.004 0.017 0.007
75 0.001 0.001 0.007 0.003
Note:Saturatedwatervaporpressure(pascals)wascalculatedas610.94(cid:3)exp{17.625(cid:3)T((cid:4)C)/[T((cid:4)C)þ243.04]}.DJF,
December,January,February;MAM,March,April,May;JJA,June,July,August;SON,September,October,November.
ChemicalcompositionoftheatmosphereoftheEarth 7
Figure1.2 ObservedatmosphericCO mixingratio.
2
Obtained from Longinelli etal.(2010).
CO ,CH ,andN Oarethethreemostimportantgreenhousegasesinthemodern
2 4 2
troposphere,asregionalandglobalindustrializationhasacceleratedtheirincreasing
trends,especiallyinrecentdecades.Anthropogenicactivitiesinvolvingcombustion
harness energy from fossil fuel and biomass and emit CO into the atmosphere,
2
mostlytothetroposphere,exceptforaviation.Globally,anthropogenicemissionof
CO has increased dramatically since the beginning of industrialization over
2
a century ago, and amounted to ~40 billion tons per year recently. Freshly emitted
CO ispartlyfixedbyplantsoverthelandandinsurfacewaters,andpartlydissolved
2
into water bodies. Atmospheric CO may also transform some rocks on
2
a geochemical time scale. The remainder stays in the atmosphere, mainly in the
troposphere, and raises the mixing ratio of CO there. Figure 1.2 shows the annual
2
increaseofCO overworldoceansintheyears1996–2007(Longinellietal.,2010).
2
As the lifetime of CO in the troposphere is an order of magnitude longer than the
2
mixingtimeoftroposphericair,CO iswellmixedinthetroposphereexceptatthe
2
surface with sinks or near emission sources. In fact, research has suggested that
theCO mixingratiorosefrom~280ppmvin1750to~310ppmvin1950,according
2
toice-coreanalyses,andto~380ppmvin2010basedonmeasurementsataground
stationof~3kmASLattheMaunaLoaObservatoryinHawaii(Intergovernmental
Panel on Climate Change (IPCC), a Nobel Laureate, 2007). If anthropogenic CO
2
emissionfollowsthecurrenttrend,theatmosphericCO mixingratiomayreach600
2
ppmbefore2100;theexactresponseofatmosphericCO tofossilfuelconsumption
2
dependsoncomplexfactorsunderactiveresearch.Theincreaseoftheatmospheric
CO mixing ratio has two opposite effects on humans: on one hand, a higher CO
2 2
mixingratiomayincreasecropyieldsandwarmupcoldregionsifotherconditions
8 Part1:Fundamentals
are fixed; on the other hand, a higher CO mixing ratio may have harmful conse-
2
quences,suchasthelossofcoastalwetlands,morefrequentstormsordroughts,and
more stagnant air near the surface.
The CH mixing ratio in the troposphere is currently ~1.8 ppm, with a slightly
4
highermixingratiointhenorthernhemisphere,wheremostsourcesarelocated,than
inthe southern hemisphere dueto itsrelativelyshortlifetime(~10 years) compared
with the timescale of interhemispheric air exchange (~1 year). CH is the major
4
componentofnaturalgas,andisusedwidelyasacleanfuelforresidential,traffic,and
industrialneedswhenavailable.Forcomparison,theCH mixingratiowasestimated
4
to be ~0.8 ppm in the middle of the eighteenth century. Tropospheric CH may
4
originate from leakages during the production, storage, transportation, and
consumptionoffossilfuels,andmayalsobeemittedfromricepaddiesandswamps
duringcertainperiods,aswellasfromothersources.CH isapotentgreenhousegas,
4
e.g.witha100-yearglobalwarmingpotential21timesthatofCO ,accordingtothe
2
IPCC; it also contributes significantly to the photochemical production of O in the
3
troposphereon a global scale.
N Oisratherstableinthetroposphereanditscurrentmixingratiois~0.32ppm.
2
In nature, it is a laughing gas, and is also emitted from farmlands. According to
a recent survey in California, synthetic fertilizers and on-road vehicles have
become dominant sources for N O emission there. It is estimated that tropospheric
2
N O has increased by ~10% from preindustrial 1750. N O is a potent greenhouse
2 2
gas, with a 100-year global warming potential 310 times that of CO , according to
2
the IPCC.
1.1.2 Stratosphere
Thestratospherecontains~9.9%ofairmassovertheEarth,andrangesfrom~10to
~50 km ASL with ascending temperature up to ~270 K. Due to precipitation in the
troposphere, H O can scarcely survive through vertical transport to reach the
2
stratosphere.Inthestratosphere,O maybephotolyzedbysolarultravioletradiation
2
to form ozone (O ), which results in the so-called “O layer”. The O layer itself
3 3 3
absorbs solar ultraviolet radiation with a little longer wavelength, to close the
Chapman cycle of O formation in the natural stratosphere. As a result, solar UV
3
radiation at the top of the stratosphere is much stronger than at the bottom of the
stratosphere for wavelengths less than ~300 nm. Thus, humans and animals are
effectively protected by the O layer from harmful, solar UV radiation with wave-
3
lengths shorter than ~300nm.
In the modern atmosphere, chemicals such as N O and chlorofluorocarbons
2
(CFCs),whichdecomposeslowlyinthetroposphere,mayaccumulatetoasignificant
amountandenterthestratosphereviastratosphere–troposphereexchangeevents.Due
to strong solar radiation in the stratosphere, these chemicals photolyze to form NO
andhalogenradicals,whichthenperturbtheChapmancycletoaffectthethicknessof
the O layer. The most important observation related to the O layer in the strato-
3 3
sphere is the so-called “O hole”, initially observed over Antarctica during early
3
springintheearly1980s.Table1.3liststypicalseasonalcolumnO overtheEquator,
3