JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D1, PAGES 152%1547, JANUARY 20, 1998
The effects of sulfur
emissions from HSCT
aircraft:
A 2-D model
intercomparison
DebraK. Wciscnstcin,
• MalcolmK. W. Ko,• IgorG. Dyominov,
• GiovanniPitari,•
LucrcziaRicciardulli? GuidoVisconti,• andSlimant Bckki•,•
Abstract.
Four independently formulated two-dimensional chemical transport
models with sulfate aerosolmicrophysicsare used to evaluate the possible effects
of sulfuremissionsfrom high-speedcivil transport (HSCT) aircraft operatingin
the stratospherein 2015. Emission scenariosstudied are those from Baughcum
and Henderson[1995],while assumptions
regardingthe form of emitted sulfurare
similarto thoseof Weisensteinet al. [1996].All modelsshowmuchlargerincreases
in aerosolsurface area when aircraft sulfur is assumedto be emitted as particles of
10 nm radiusrather than as gasphaseSO2. If we assumean emissionindex (EI)
for SO2of 0.4 gm (kgfuelburned)
-1 in 2015,maximumincreases
in stratospheric
sulfateaerosol
surfacearearangefrom0.1 /•m2 cm-3 to 0.5/•m2 cm-3 with sulfur
emittedasSO2gasandfrom1.0/•m2 cm-3 to 2.5/•m2 cm-3 with sulfuremittedas
particles. Model differencesin calculatedsurfacearea are deemedto be due mainly
to differencesin model transport. Calculated annual averageozoneperturbations
dueto aircraftemissions
with EI(NOx)=5, EI(H20)-1230, and EI(SO2)=0.4 range
from-0.1% to -0.6% at 45øN for sulfur emissionas SO2 gas and from-0.4% to
-1.5% with sulfur emissionas 100% particles. The effect of zonal and temporal
inhomogeneities
in temperatureon heterogeneous
reactionsrates is accountedfor in
the Atmosphericand EnvironmentalResearchmodel and the Universirk degli Studi
L'Aquila model and significantlyincreasesthe calculated ozone depletion due to
HSCT, particularly for the caseswith concurrent increasesin aerosolsurface area.
Sensitivitiesto polar stratosphericclouds,backgroundchlorineamount, additional
heterogeneousreactions,and backgroundaerosolloading are also explored.
1.
Introduction
turb stratospheric chemistry, aerosol concentrations,
High-speedcivil transport aircraft, known as HSCTs,
could be flying commercialroutes by 2015. These aircraft would operate at cruise speedsbetween Mach 2.0
and Mach 2.4, which necessitateflying in the stratosphere at altitudes between 18 and 21 km. HSCT enginesare expected to emit large amounts of water vapor and carbon dioxide and relatively small amounts of
nitrogenoxides(NOx), sulfur dioxide,carbonmonoxide, and hydrocarbons. These emissions could per-
and ozone. Early studies[e.g., Johnstonet al., 1989]
indicated that NOx emissionshave the largest effect
on ozone, leading to significant decreases.Formal assessmentsof the effectsof HSCTs have been performed
under the NASA AtmosphericEffects of Aviation Program (AEAP), which has reportedpredictedchanges
in ozonefrom a numberof two-dimensional
(2-D) atmosphericmodels, basedon forecastemissionscenariosof a
globalfleetof 500 HSCT aircraft [Albrittonet al., 1993;
$tolarskiet al., 1995]. Thesemorerecentassessments
chemistryoc•Atmospheric
and Environmental
Research,
Inc., Cambridge, haveincludedthe effectsof heterogeneous
Massachusetts.
2Department
forAtmospheric
Research,
Novosibirsk
StateUniversity,
Novosibirsk,Russia.
curringon the backgroundsulfateaerosollayer [World
Meteorological
Organization(WMO), 1993],whichhas
3Dipartimento
di Fisiea,Universith
degliStudiL'Aquila,L'Aquila, been found to significantlydecreaseozonesensitivityto
Italy.
nCenter
forAtmospheric
Science,
Depmment
of Chemistry,
University
of Cambridge,England.
HSCT NOx emissions
[ Weisenstein
et al., 1991;Bekkiet
al., 1991]. However,they havenot considered
possible
perturbationsto the sulfateaerosollayer by emissionof
SNowatNationalCenterforAtmospheric
Reseagh,
Boulder,Colorado.
6Permanently
at Serviced'Aeronomie
duCNRS,InstitutPierre-Simon sulfur from HSCT engines.
Some 2-D studies of the effect of HSCT
Laplace,UniversitcParis,Paris,France.
sulfur emis-
sions have been conducted by individual modeling
Copyright 1998 by the American GeophysicalUnion.
groups.Bekki and Pyle [1993]modeledthe impact of
a fleet of HSCTs, assumingan emissionindex (EI) for
NO• of 20 (i.e., 20 g NO2 per kilogramof fuel burned)
Papernumber97JD02930.
0148-0227/98/97JD-02930509.00
1527
1528
WEISENSTEIN
ET AL.:
INTERCOMPARISON
OF EFFECTS
OF HSCT
SULFUR
and EI(SO2) of 2 (i.e., 2 g SO2 per kilogram of fuel cleswould lead to substantialcoagulationin the wake
burned). They concludedthat the SO2 emissions
could [KarcherandFahey,1997;Danilinet al., 1997].The depotentially double the aerosolsurfacearea of the north- tailed chemistryand physicsof sulfur in aircraft plumes
ern hemisphere lower stratosphere. Bekki and Pyle are not well known. Nevertheless,a recentstratospheric
[1993]found decreasedsensitivityof ozoneto HSCT measurement
in the plumeof the Concordeaircraft[Faemissionsfor their model scenariobut pointed out that hey et al., 1995] indicatedthat a substantialfraction
the increased aerosol levels also increased the model senof the emitted sulfur was convertedto aerosolpartisitivity to chlorine abundancein the lower stratosphere. cles within 16 min after exhaust. A measurement of
Pitari et al. [1993]modeledan emissionscenariowith aerosolsin the exhaust trail of a military aircraft at 23
EI(NO•)=15 and EI(SO2)=l.0. They found increases km [Hofmannand Rosen,1978]indicatednearlycomin aerosolsurfacearea in the northern hemispherelower
plete conversionof emitted sulfur to aerosolsafter 18
stratosphereof up to 60% along with enhancedozone
hours.
depletion. Tie et al. [1994]also usedEI(NO•)=15
and EI(SO2)=l.0. They predicteda 10-20%increasein
The 2-D modelsemployedin this study will adopt assumptionsregarding the atmosphericstate of the emitted sulfur at the point when the emitted material is
aerosol surface area and found that the associated ozone
changeswere small in comparisonwith the total impact diluted to grid box size (a time period of roughly 1
of HSCT emissions.The above studies were performed week). This approachwill enable us to calculatethe
by putting the sulfur emissionsalong the flight paths globalimpact of aircraft sulfur emissionsin lieu of knowas gasphaseSO2. Weisensteinet al. [1996]modeled ing the details of chemistry in the engine, nozzle, and
an HSCT scenariowith EI(NO•)=5 and EI(802)=0.4,
wake regimes.The amount of sulfur put into eachgrid
box is given by the product of the adopted EI for S02
and the fuel burn in that grid box. We considerthree
particles/90%SO2 gasmix. Increasesin sulfateaerosol assumptionsregardingthe phase of the emitted sulfur:
surfacearea were found to be up to 50% for sulfur emis- (1) all sulfurremainsin the form of SO2gas,(2) 10%
sion as SO2 gas and up to 200% when emissionswere of the emitted sulfur is convertedto aerosolparticles
assumedto be all particles. Ozone depletion due to of 10 nm radius,and (3) 100% of the emitted sulfuris
HSCTs was enhanced by the increasedsulfate surface convertedto 10 nm aerosolparticles. Greater changes
area, from 0.1% without sulfur emissionsto 1.3% with in the aerosolsurface area of the stratosphereare to
sulfur emissionsas 100% particleswith 3 ppbv of chlo- be expected under assumption 3. The 100% conversion
rine on an annual averagebasis at 47øN.
of aircraft-emitted sulfur to aerosol particles within 1
Because the previously discussedmodeling results week cannot be ruled out on the basis of atmospheric
used different emission scenarios and contained varimeasurementsto date, so we take this case as the upations in chemical reaction rates and background gas per limit. Assumption 2 representsthe lower limit of
concentrations,the resultscannot be compareddirectly. aerosolformation in the plume as determined by the
This paper will presentresultsfrom four independently Concordemeasurements
[Faheyet al., 1995]within 1
formulated 2-D stratospheric models, each of which hour of emission. Assumption I is included for historiincludes sulfate aerosol microphysicsand uses identi- cal reasons and is similar to theoretical calculations for
cal emission scenarios and standardized chemical rates
aerosolformation in aircraft plumes when sulfur is as[fromDeMote et al., 1994]. Thesemodelsare the At- sumedemitted as SO2 and reactionwith OH is the only
evaluating the impact with sulfur emissionsassumedto
be all gas phase SO2, all 10 nm particles, and a 10%
mosphericand EnvironmentalResearch(United States) oxidationmechanism[Brownet al., 1996a].
We will start with descriptionsof the four models to
2-D model,the Universityof Cambridge(England)2-D
model, the Universitk degli Studi L'Aquila (Italy) 2-D be used in this comparison,including the similarities
model, and the NovosibirskState University (Russia) and differencesof their microphysical,chemical, and
2-D
model.
While
these models can calculate
aerosol
microphysics
on the scaleof their modelgrid boxes(typically 10ø latitude by 2 km in the vertical), they cannot accountfor microphysicsand heterogeneouschemical processingthat may occur in the aircraft plume or
wake. Conditionsin the wake, particularly in the first
few secondsafter emission,are very differentfrom ambient conditions. Sulfur dioxide may be convertedto
803 or H2804 in the early stagesof wake evolutionor
in the engineor nozzle[Brownet al., 1996b].As a result of the high supersaturationspossiblein an aircraft
wake, H2SO4 can form new sub-micronsizeaerosolparticles through homogeneous
nucleationor by condensation onto soot particles[Brown et al., 1996c;Kiircher
and Fahey,1997]. A very high densityof aerosolparti-
dynamical formulations, in section 2. Section 3 will
describethe HSCT scenarioused and present results
of changesin sulfate surface area and ozone. Section
4 will present additional sensitivity studies that have
been performed by subsetsof the four models. Section
5 will summarize
the results and draw conclusions.
2. Model Descriptions and Comparisons
2.1.
Atmospheric
and Environmental
Research
Model
The Atmospheric
andEnvironmental
Research
(AER)
sulfurmodelis describedby Weisensteinet al. [1997].
Aerosolprocesses
modeledincludehomogeneous
nucleation, coagulation,condensation/evaporation,
and sed-
WEISENSTEIN
ET AL.: INTERCOMPARISON
imentation. There are 40 aerosolsize bins ranging from
0.39 nm to 3.2 •umby volume doubling. Only binary
sulfuric acid-water
aerosols are considered.
The sulfur
model is not coupled to the ozone model in that the
OH field in the sulfur model is fixed at precalculated
values. The sulfate surface area density calculated by
the sulfur model is treated as input to the photochemical model. The photochemicalmodel with full Ox,
HOx, CHOx, NOx, C1Ox, and BrOx chemistry is de-
OF EFFECTS
OF HSCT SULFUR
1529
include the organicchlorinespeciesCFC13 (CFC-11),
CF2Cl• (CFC-12), CF•C1CFCI• (CFC-113), CHF•C1
(HCFC-22), CH3CFC12 (HCFC-141b), and CH3CC13
and the organicbromine speciesCH3Br, CF3Br (H1301)and CF•C1Br (H-1211), whichare calculatedonly
for a backgroundatmosphererepresentingthe year 2015
(3.0 ppbvor 2.0 ppbvof ely and 17 pptv of Bry).
Interaction of all these gaseouscomponents is governed by 157 photochemicalreactions, including 26 rescribedby Prather and Remsberg[1993]and Weisen- actions that describe interactions among the sulfate
stein et al. [1996].Model temperatureand circulation components. The diurnal variation of photolysisrates is
are prescribedaccordingto climatology and do not re- recalculated every tenth model day. Five heterogeneous
spondto changesin aerosolsor chemicalspecies.
A temperature probability distribution is employed
for the calculationof homogeneous
nucleationrates and
all temperature-dependentreaction rates following the
methodology
of Considineet al. [1994].Monthly zonal
mean temperature statisticsin I K incrementswere derived for the time period from 1979 to 1986 on the basis of daily data from the National Centers for EnvironmentalPrediction/NationalCenterfor Atmospheric
reactions
are used to account
for interactions
between
area due to HSCT emissions;however, inclusion of the
temperature probability distribution for calculation of
The temperature stratification of the atmosphere is
determined by the heat balance equation. Calculation
of atmosphericheating and cooling rates takes into account heat fluxes due to convection, turbulent heat exchange, and radiative transfer in three spectral ranges:
gaseouscomponentsand aerosolparticles(seereactions
(1)-(4) in section2.5 and reaction(5)in section4.4 below). The rates of heterogeneous
reactions(1)-(3) and
(5) are calculatedby usingthe formula k = •/VT$/4,
whereff is the reaction probability, VT is the thermal velocity of gas molecules,and $ is the surfacearea density
of sulfate aerosols. Values of ff for heterogeneousreactions (1)-(3) are taken from Table 3-2 of WMO [1993]:
Research
reanalysisproject[Kalnayet al., 1996]. The if1-0.1, log10(ff2
) - 1.87- 0.074W, where W is the
tropicalpipe circulation[Plumb,1996]is employedas weightpercentof H2SO4in aerosolparticles(calculated
described
by Weisenstein
et al. [1996],usingsmallval- accordingto the formulaof Hansonet al. [1994]),and
ues of horizontal diffusion in the tropical lower strato- if3 - 0.1ff•. The rate of heterogeneous
reaction (4) is
sphere. Grid resolution is 9.5ø latitude by 1.2 km in calculated according to the method of Hanson et al.
the vertical. Updatesto the sulfurmodel[Weisenstein [1994]and is proportionalto the volumeof the aerosol
et al., 1997]sincethe publicationof previousHSCT as- particles. The constantvalue 0.4 [Hansonand Ravissessment
results[Weisenstein
et al., 1996]havesome- hankara,1995]is adoptedfor the probabilityof heterowhat reduced the calculated increases in aerosol surface
geneousreaction(5).
reaction rates has increased the calculated ozone depletion for the same scenarios.
2.2.
Novosibirsk
State University
Model
Included here is a fairly detailed description of the
NovosibirskState University(NSU) model,asthis model has not previously been described in an English
languagejournal. The NSU model is a zonally averaged 2-D interactive model for self-consistentcalculation of diabatic circulation, temperature, and gaseous
and aerosolcomposition of the troposphere and stratosphere. The photochemicaland radiative parts of the
UV and visible (0.175-0.9 •m), near infrared (0.9-4.0
•m), and infrared (•4.0 •m). The radiative fluxesin
the UV and visible part of the spectrum are computed
by taking into account scattering on air molecules and
aerosolparticles, reflection from clouds and the Earth's
surface,and absorption by oxygen, ozone, and nitrogen
dioxide. In the near infrared, absorption by water va-
por (in the 0.94, 1.1, 1.38, 1.87, 2.7, and 3.2/•m bands)
and by carbondioxide(in the 2.0 and 2.7 •m bands)is
modelare describedby Dyominov[1988, 1991, 1992], considered. Infrared fluxes account for longwave radiaDyominovand Zadorozhny[1989], Ginzburgand Dy- tive absorption by CO•, 03, H•O, CH4, N20, CFC13,
ominov[1989],and Dyominovet al. (unpublishedma- and CF•Cl•. The fine spectral structure of these con-
H20, NO, NO2, NO3, N•Os, HNO3, HNO4, N•O,
CO, CH4, CH•, CH•O, CH•O•, CH•CO, CH•CO•,
stituents is taken into account in absorption bands centered around 15 •m for CO•; 9.6 •m for 03; 6.3 •m
for H•O; 7.6 •m for CH4; 4.5, 7.78, 8.57, and 17 •m
for N20; 9.22 and 11.82 •m for CFC13; and 8.68, 9.13,
and 10.93 •m for CF•Cl•. For H•O, rotational bands
CH3CO3NO• (peroxyacetylnitrate), C1, C10, HOC1,
are also taken
C1ONO•, HC1, CC14, CH3C1, Br, BrO, HOBr, HBr,
BrONO2, CH3SCH3 (dimethylsulfide,or DMS), CS•,
H•S, OCS, S, SO, SO•, SO3, HSO3, and H•SO4. Ad-
Dynamical processesin the atmosphere are representedin the model by the residual circulation and eddy
diffusion. The method of computing the residual circu-
ditional chlorine and bromine sourcegasesare included
in the model but not calculated interactively. These
lation is describedby Ginzburgand Dyominov[1989]
and Talroseet al. [1993]. It consistsof solvingthe
terial, 1992). In this work, circulation and temperature are calculated self-consistentlywith 45 minor gas
constituents:
03, O(3p), O(1D), H, OH, HO2, H202,
into account.
1530
WEISENSTEIN
ET AL.'
INTERCOMPARISON
OF EFFECTS
OF HSCT
SULFUR
zonally averaged equations describing conservationof from the same three-dimensional model and below 20
momentum, mass,and energy,with the momentumflux mbar from standard references. A temperature distridivergencedue to gravity wavesapproximated by means bution is applied at each model grid point by assum-
of a Rayleighfriction coefficient[Garcia and Solomon, ing a statisticalstandarddeviation[Pitari, 1993]. The
1983]. The momentumflux due to breakingplanetary temperature distribution is applied to the calculation of
waves is parameterized as in the model of Hitchman the ratesof the heterogeneous
reactionsC1ONO2+H20,
andBrasseur[1988].The eddydiffusioncoefficients
are C1ONO2+HC1, and HOCI+HC1.
computed by using the method describedby Brasseur
et al. [1990]. The modelgrid resolutionis 5ø latitude 2.4. University of Cambridge Model
The University of Cambridge2-D model is described
2 km vertical resolution from 36 to 50 kin.
by BekkiandPyle [1992,1993].It treatsaerosolpartiThe 2-D model of aerosol composition of the tropo- clesin 25 sizebinsbetween0.01 and 2.5/•m with volume
and 1 km in the vertical for altitudes up to 36 kin, with
sphere and stratosphereis describedby Golitsyn et al.
doublingbin spacing. Microphysicalprocessesincluded
[1994].The resultsof calculations
with this modelare in the Cambridgemodelareheterogeneous
andhomogealsodescribed
by DyominovandElansky[1995],Dyomi- neousnucleation,condensation,evaporation,coagulanovet al. [1995],and DyominovandZadorozhny
[1996]. tion, and sedimentation.The homogeneous
nucleation
The
aerosol
2-D
model
can calculate
distributions
of
is parameterized by assumingarbitrarily that new sul-
three types of aerosol particles' ice, nitric acid trihy-
furicacidparticlesareformedat a constantrate [Bekki
drate (NAT), and sulfateparticles,thoughonly sulfate and Pyle, 1992]in the tropicaluppertropospherein orparticles are treated in the present study. There are der to providea sourceof nucleiin this region[Brocket
30 aerosolbins in the model, with particle radii ranging al., 1995].Hamill et al. [1982]testedvariousnucleation
from 6.4 nm to 5.2/•m by volumedoubling. Microphysical processes
simulatedare nucleation(both homogeneousand heterogeneous),condensation,evaporation,
coagulation, sedimentation, and washout. The classical
approach is usedfor the calculation of the binary homo-
mechanismsand found that a simple nucleation treatment led to overall aerosolproperties similar to those
obtained with sophisticated schemes. The model in-
cludesonly DMS and carbonylsulfide(OCS) as sulfur
sourcegases,ignoring surfaceemissionsof SO2. n The
geneousnucleationrate, as discussed
by Frenkel[1975] Cambridge model is a classicalEulerian model. The
and Hamill et al. [1982]. The heterogeneous
nucle- grid resolution is 9.5ø latitude and 3.5 km in the vertiation rate is calculated accordingto the method given cal. The temperature and circulation are calculated interactivelyfrom forcingterms, which includesolarheat-
by Hamill et al. [1982]and employedby Pitari et al.
[1993]. Concentrations
of Aitken nuclei(0.012/•m _<r
_<0.4/•m) and largenuclei(0.5/•m _<r _<2.0/•m) at a
surfaceof the Earth wereassumed
equalto 1800cm-3
and 40 cm-3, respectively.Condensation
and evaporation are calculated in the model following Turco et
ing by 02 and 03 and longwavecoolingby CO2, H20,
03, N2 O, and CH4. The chemicalschemeincludeswhat
are thought to be the most important reactionsof Oz,
NOy, HOz, ClOy, BrOy, CHOz, and SOz. A tempera-
ture distribution is employedfor the calculation of hetal. [1979]and Toonet al. [1988]. Changesof particle erogeneousreaction probabilities. Monthly zonal mean
size distribution resulting from BrownJancoagulation temperature statistics are taken from Cospar Internaare described with classical relations, as described by tional Reference
Atmosphere[Labitzkeet al., 1985]cli-
Asaturovet al. [1986]. Sedimentationof aerosolsand matologies. The reaction probabilities are as described
troposphericwashoutfollowPitari et al. [1993].
for the NSU model, with 3' of ClONO2+HC1 set to a
tenth of the reactionprobability of C1ONO2+H20. The
2.3. Universitlt degli Studi-L'Aquila (Italy)
reaction HOCI+HC1
Model
is not included.
and Differences
The UniversithdegliStudi-L'Aquilamodel(hereafter 2.5. Model Similarities
referredto as the Italy model) is describedby Pitari et
All four modelsemployfairly standardclassicalapal. [1993]. The sulfateaerosolmodulecalculatespar- proachesto aerosolmicrophysics.The processesof conticle densities in 11 size bins between 0.01 and 10.25
densation,evaporation,coagulation,and sedimentation
/•m radius by radius doubling. Microphysicalprocesses are treated similarly in all the models. None of the
modeled include heterogeneousnucleation, condensa- model simulationspresentedin this paper include PSC
tion, evaporation,coagulation,and sedimentation. All chemistry.While somemodelsuseheterogeneous
nucleaerosols
are assumed
to contain
a solid
core.
Polar
ation and someusehomogeneous
nucleation(the NSU
stratosphericcloud (PSC) particle distributions(both model usesboth), nucleationis a minor processin the
NAT and ice) can be calculatedby the Italy modelbut stratosphere
(comparedwith condensation
andcoagulaare not included here. The grid resolution is 10ø lati- tion) and probablydoesnot lead to largedifferences
in
tude by 2.8 km altitude. The circulation is taken from the model resultsto be presentedhere. The modelsalso
the group'sspectralquasi-geostrophic
three-dimensional differ in the resolutionand range of aerosolsizescon-
model[Pitari et al., 1992]but is not interactivein these
sidered. The number of size bins varies from 11 to 40 in
calculation.
the models,with the AER and NSU modelsincluding
Temperatures above 20 mbar are taken
WEISENSTEIN
ET AL.: INTERCOMPARISON
OF EFFECTS
OF HSCT SULFUR
1531
aerosolradii less than 0.01/•m and the Italy and NSU reactions(2)-(4) in the AER and Italy modelsare taken
models including the largest sizes. The AER, Cam- from Hansonet al. [1994],while all modelsusea conbridge, and NSU models have the same size resolution, stant 3' of 0.1 for reaction (1).
using a volume doubling scheme. The Italy model has
a coarserresolution,usinga radius doublingscheme.A 3. Model Intercomparison
lower size resolution has been found to produce a shift
The sulfate aerosol surface area calculated by each
in the sizedistributiontowardlargerparticles[Larsen, of the models for annual averagebackground(non1991].
volcanic) conditionsis shown in Figure 1. That de-
The most important differencesbetween the models rived from Stratospheric Aerosol and Gas Experiment
are not in their microphysicalformulations but in their (SAGE) data and adopted by modelersto represent
transport circulations, temperature fields, and chemical
cleanbackgroundconditions[WMO, 1993]is shownin
formulations. The Cambridgeand NSU modelscalcu- Figure If, and the April 1991 surfacearea [Yue et al.,
late temperature and circulationinteractively, while the 1994]derivedfrom SAGE II is shownin Figure !e. The
Italy model takes these fields from an off-line calculafirst half of 1991 (beforethe Mount Pinatubo eruption
tion. The AER modelprescribesthem (usingobserved in June) representsa recent year that is near backclimatologicaltemperatures). Temperaturefields par- ground conditions, though it probably includes some
tially determine aerosol composition and the vertical residualeffectsof volcanicactivity [Thomasonet al.,
extent of the aerosol layer. Results from perturba1997]. The actual distributionof aerosolsurfacearea
tion studies in 2-D models always contain some depen- during background conditions is not well known and
dence on model transport, and there is no "standard" would of coursevary from year to year.
or "best" 2-D climatologicaltransport circulation availAll modelsreproducethe magnitude and structure of
able. Thus each modeling group usesa set of winds and the WMO surface area, with lowest values in the tropieddy diffusion coefficientsthat each has found to provide calculated values of ozone and other trace gases
that compare favorably with observations. The combination of transport rates and diffusion coefficientsthat
meet these criteria is not unique, and each set leads to
somewhat different results in perturbation studies. In-
cal lower stratosphereand highestvaluesin the high latitude lower stratosphere. In comparisonwith WMO the
aerosolsurfacearea of the Cambridgemodelis 30% low
in the tropical lower stratosphere,while the Italy model
is 30% high in this region. The NSU model matches
WMO most closely. The AER and NSU models predeed,the modelcomparison
presentedby WMO [1995] dict greater surface area in the northern polar region,
indicates intermodel differences of up to 0.7 ppbv in on an annual average basis, than in the southern polar
Cly concentrations
at 50øNand 22 km despiteidentical region, while the Italy model and WMO show greater
boundary conditions.
surface area in the southern polar region. The CamThe method of obtaining diurnal averagesof concen- bridge model is relatively symmetrical between hemitrations of short-lived radical speciesdiffers between the
spheres.Model tropospheresvary considerablydependmodels and probably accounts for some of the differing on the surfacesourcesof sulfur includedin the modences in sensitivity of ozone to NO•, C10•, and HO•
els. The Cambridge model includesonly DMS and OCS
concentrations. Though chemical rate coefficientsare surface sources, while other models also include CS2,
taken from JPL-94 [DeMote et al., 1994] in all four H2S, and SO2 surface emissions. Surface emissionsof
models, the model calculated OH concentrations are
SO2 are evenly distributed with latitude in the Italy
different, and therefore rates of conversionfrom SO• to
and NSU modelsbut are weightedtoward the northern
H•SO4 are different. The modelsincludeheterogeneous hemisphere in the AER model.
reactions on sulfate aerosols,namely,
Each model was run with identical parameters for
N205 q-H20 -+ 2HNO3
(1) aircraft emissionsand similar backgroundatmospheres,
with a chlorine content of 3.0 ppbv and bromine content
C1ONO2+ H20 -+ HNO3 q- HOC1
(2)
of 17 parts per trillion by volume(pptv). The aircraft
C1ONO2+ HC1-+ HNO3 + C12
HOC1+ HC1 -+ H20 q- C12
(3)
(4)
emission scenarios employed here are those described
by $tolarskiet al. [1995]. Theserepresent500 aircraft
operatingin the year 2015and burning82x109kg of
of whichreaction(1) is the most important on a global fuel annually, with the geographicalfuel use distribubasis. Reactions(2)-(4) are important in the cold re- tion derivedby Baughcumand Henderson[1995]. The
gionsat high latitudes in winter and under high aerosol HSCT aircraft cruiseat Mach 2.4, with a cruisealtitude
conditions. The importance of reactions on cold sulfate of 18-21 km. Engine emissionsare specifiedby an emisaerosol has been demonstrated in analysis of the ob- sion index, or El, defined as grams of pollutant emitted
served partitioning of the radicals in both satellite and
per kilogram of fuel burned. We present results for two
aircraftdata [Michelsenet al., 1997].The useof a tem- valuesof EI(NOx), 5 and 15. An EI(NOx) of 5 repperature probability distribution in the AER and Italy resents a significant reduction in emission index from
modelsleads to enhancedimportance of these reactions current supersonicaircraft (valuesof about 20 are typdue to their strong temperature dependence. Rates for ical), though technologicaladvancesin the last several
1532
WEISENSTEIN
ET AL.' INTERCOMPARISON
35
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d 30 ---0.2--'"•
I
I
0.2--_
90øS 60øS 30øS
EQ
30øN 60øN 90øN
Latitude
f
5 ,,i,,i,,i,,i,,i,,
30-
•
10-
-
5 -
I
t
I
i
i
I
,
90øS 60øS 30øS
,
I
,
EQ
,
I
,
,
I
i
i
30øN 60øN 90øN
Latitude
Ol
,I•I•'tIiIiItIl
o
90øS 60øS 30øS
EQ
30øN 60øN 90øN
Latitude
Figure 1. Calculated
annualaverage
background
aerosol
surface
area(/tin2 cm-a) fromthe(a)
AER model,(b) NSU model,(c) Italy model,and (d) Cambridge
model.Alsoshownare (e)
surfaceareasderivedfrom SAGE II observations
of April 1991[Yueet al., 1994],and (f) WMO
[1993]background
surfacearearecommendation.
arearateof increase
of about0.1 /•m2
yearshave shownthat an EI(NOx) of 5 is achievable ingto a surface
[Stolarskiet al., 1995]. EI(H20) of 1230is usedin all cm-a day-•.
calculations.
The EI of SO2 is taken to be 0.4, based on projec-
tions that sulfur content in jet fuel will declinefrom its
3.1.
Surface Area Changes
Figure 2 showsthe calculatedchangein the annual
currentaveragevalueof 0.8 [Baughcum
eta!., 1994]. averageaerosolsurfacearea densitydue to emissionof
Because these 2-D models are not formulated to calcu-
sulfur from HSCT aircraft, assuming that it remains
late troposphericaerosols,we have not includedsulfur SO2until mixingdilutesthe plumeto grid box size(asemissions from subsonic aircraft or from HSCT aircraft
sumption1). The spatialvariationof the perturbation
below 13 kin. The aircraft sulfur emissions amount to
corresponds
to the geographicaldistributionof fuel us2.4x 10•ø gramsSO2per yeardeposited
between13 and age,whichis concentratedin the northernhemisphere.
22 km altitude. At 45øN and 19 km, which represents
the peak of HSCT traffic, sulfur emissionsamount to
about 30 moleculescm-3 s-1 of 802. If emissionsare
assumedto be 100% particlesof 10 nm radius,this value
Maximumchanges
in surface
arearangefrom0.1 /•m2
cm-a in the Cambridge
modelto 0.2/•m2 cm-a in the
Italy model,0.3/•m2 cm-a in the AER model,and0.5
/•m2 cm-a in the NSU model.After the aircraftsulfur
represents
about90 particlescm-a day-•, correspond- is mixed into the globalgrid box as SO2, reactionwith
WEISENSTEIN ET AL.' INTERCOMPARISON OF EFFECTS OF HSCT SULFUR
35
!
,
I
,
,
I
,
,
I
,
,
I
,
,
I
,
35
,
•
i i I ' ' I I I I i , I I I I [
b
_
• •5
1533
30
_
20
•
15
Z :.5
•:I10•
•--/•.•
---!
5
90os
60os
30osEQ3•øN
•0'øI•
• øN
Latitude
35
' , i , , i , , i , , i , , ] , ,
90øS 60øS 30øS EQ 30øN 60øN 90øN
Latitude
35
'
'
i
,
,
i
,
,
i
,
,
i
,
,
i
,
c
30
_
• 25
_
_
20
•
15
10
5-
90øs60øs30øsEQ 30øN
60øN
• øN
Latitude
90øS
30øN
,,60øSm,,30øSi, ,EiQ,
, i , 60øN
, t , 90øN
,
Latitude
Figure2. Calculated
annual
average
increase
in aerosol
surface
area(pm2 cm-a) dueto HSCT
emissions
withEI(SO2)=0.4,
assumed
emittedasSO2gas.Results
arefrom(a) AERmodel,(b)
NSU model,(c) Italy model,and (d) Cambridge
model.
OH is the only pathwayto produceH2SO4. Therefore variesfrom 0.1 pm2 cm-3 for the AER model to 0.4
model differencesmay reflect variations in calculated ftm2 cm-3 for the Italy model.
OH concentration,as well as the distribution of availCalculated increases in aerosol surface area under asable sulfateparticleson whichgasphaseH2SO4may sumption3, whenall emissionsare assumedto be particondense.As a percentage
of the background
surface cles,are shownin Figure 4. The maximum surfacearea
area, calculated perturbations in the northern hemi- increases
are 1.0 pm2 cm-3 in the Cambridge
model,
spherelowerstratosphere
are up to 25% in the Italy
model,up to 30%in the Cambridgemodel,up to 40% in
the NSU model,and 2.5 pm2 cm-3 in the Italy model.
the AER model, and up to 50% in the NSU model. The
These perturbations are more than double those found
1.5 pm2 cm-3 in the AER model,2.0 pm2 cm-3 in
Cambridgeand AER modelsshowtheir largestpercent under assumption2 and representincreasesof up to
change
in the tropicsbecause
the calculated
background 200%overbackground
for eachmodel.The Italy model
is quite low there.
Increasesin aerosolsurfacearea under assumption2,
calculates as much as a 300% increase over the background surface area. Note that the area of maximum
when 10% of the emitted sulfur is assumed to be con-
perturbationis morelocalizedunderassumption3 than
under the other two assumptionsbecauseaerosolsare
Figure3 for the fourmodels.The aerosolparticlesgen- input directlyalongthe flight tracksas smallparticles
erated in the aircraft plume are assumedto be of radius with high surface-to-volumeratio.
10 nm (0.01 pro). This assumptionis consistentwith
Differences
in transportratesbetweenthe modelsapverted to aerosolparticles in the plume, are shownin
modelcalculations
of the wakeregimeby Faheyet al.
pear to account for most of the intermodel differences in
[1995]andDanilin et al. [1997].Calculated
maximum surfacearea perturbation.Relativechangesin stratochanges
in aerosolsurfacearea underthis assumption sphericaerosolmassamongthe four modelsmirror rela-
are0.4pm2 cm-3 fortheAERandCambridge
models, tive changesin surfacearea. The smaller calculated sur0.6pm2 cm-3 fortheItaly model,and0.8pm2 cm-3 facearea (and aerosolmass)perturbationsare related
forthe NSUmodel.Corresponding
percentage
changes to shorterresidence
timesand fastertransportratesin
are65%fortheAERmodeland75%fortheCambridge, the modelstratosphere.
The Cambridgemodelhasthe
Italy, and NSU models.The inclusionof aerosolparti- smallestincrease
in surfaceareaunderassumptions
1,
cles in the emission scenario increases the surface area 2, and 3 and usesan Eulerian transport schemewith
perturbationfor each of the models,but the amount faster advectionand larger diffusioncoefficientsthan
1534
WEISENSTEIN
ET AL.' INTERCOMPARISON
3O
OF EFFECTS
OF HSCT SULFUR
•o.•'
•
3O
/
•••o%
z"0.7_
5
i
i
I
i
i
I
i
•
90øS 60øS 30øS
I
i
Eq
i
I
i
i
I
t
i
t
30øN 60øN 90øN
5
•
i
,
,,
i
Latitude
35
i
,
90øS 60øS 30øS
[
Eq
i
i
.
,
[
30øN 60øN 90øN
Latitude
, [ I ' [ I [ ] I [ ] I [ [ I [
[•I'[I][I]'
[I[]I'
o
30-
•5
!.•"•'•
.e
15
•
90øS 60øS 30øS
EQ
30øN 60øN
ON
•
I
i
,
I
,
i
90øS 60øS 30øS
I
i
t
EQ
I
!
,
I
•
i
30øN 60øN 90øN
Latitude
Latitude
Figure 3. Calculated
annualaverage
increase
in aerosol
surface
area(/•m2 cm-3) dueto HSCT
emissions
with EI(S02)=0.4, assumedemitted as 10% particlesand 90% S02 gas. Resultsare
from (a) AER model,(b) NSU model,(c) Italy model,and (d) Cambridgemodel.
35
, , i , ,' i , , i , , i , , i , ,
••_•25
-0.1
20
ß
ß
.q •o
5
i , I , t I , ,EQI , 30øN
, I , 60øN
, i • ;0øN
90øS 60øS 30øS
Latitude
c
35l, , , , , , , , , , , , , , , , ,
35
/ , ,'l,',i , , i , , i , , I , ,
10'
5
,
,
I
,
,
I
,
90øS 60øS 30øS
,
I
,
EQ
Latitude
,
i
,
,
i
,
30øN 60øN 90øN
,
i
i
,
,
I
,
90øS BOøS 30øS
i
I
,
EQ
,
I
[
,
I
,
30øN 60øN 90øN
Latitude
Figure 4. Calculatedannualaverage
increase
in aerosol
surface
area(/•m2 cm-a) dueto HSCT
emissions
with EI(S02)=0.4, assumedemitted as 100%particlesof 10 nm radius. Resultsare
from (a) AER model,(b) NSU model,(c) Italy model,and (d) Cambridgemodel.
WEISENSTEIN ET AL.' INTERCOMPARISONOF EFFECTS OF HSCT SULFUR
1535
lx104
[ .....• ........ • ........ • ........
lx10
3 a
..
lx102
'•
\\\
,•lx101
•
lx10ø
lx10-2
lx10_
3•'::i,,•,,
[•/f,,
,."i
....
, ........
, ,, ,•....
0.•1
0.01
0.1
1
Pmicle Radius(microns)
lx104
lx104
lxlO3
lx103!.
lxlO2
•x•o 2
• lxlO 1
_o
• lx101
•
•
lxlOø
lx10ø
lxlO d
lx10-1 •
lx10-2
lx10-2
lx10-3
0.01
lx10-3
0.1
1
2.5
ParticleRadius(microns)
0.01
0.1
1
ParticleRadius(microns)
Figure5. Calculated
aerosol
number
density
dN/d(logr) asa function
of particle
radius
for
the(a)AER,(b)Italy,and(c)Cambridge
models.
Thesolid
linerepresents
thebackground
distribution,
thedashed
linerepresents
a simulation
including
HSCTemissions
ofsulfur
asSO2
gas,andthe dottedlinerepresents
a simulation
including
HSCTemissions
of sulfuras 10 nm
particles.
the othermodels.The Italy modelhasa smallincrease cludemoresmallparticlesand fewerlargeparticles.
in surface
areaunderassumption
i but the largestin- The shape of the size distributions in the AER and
creasein surfacearea under assumption3. It usesa Cambridge
models
aresimilar,thoughthetotalparticle
relatively
slowtransportrate,whichyieldsa greaterin- numberdensityis smallerin the Cambridge
model.
creasein stratospheric aerosolmass due to HSCT emisWith additionof HSCT sulfuras gasphaseSO2the
sions.Surfaceareaincreases
underassumption
1, how- Italy modelshows
a decrease
in numberdensityforparever,alsodependonthe background
particlesizedistri- ticleslessthan0.08/•mradiusandan increase
for parbution. With the coarserresolution of aerosolsizesin ticlesgreaterthan 0.08/•m. The Cambridge
modelrethe Italy modelthe background
distributionis shifted sultsare similar,but decreases
in numberdensityare
towardlargerparticles.Condensation
on largerparti- evidentonly for particleslessthan 0.03 /•m. Such
cles providesa smaller surfacearea increasethan does changes
in particlenumberdensityare typicalwhen
condensationon smallerparticles.
condensation
ontoexistingparticlesis the onlymechaTo illustrate how the aerosol size resolution and cal-
nismfor convertinggasphaseH2SO4into aerosols.The
culated backgroundaerosoldistribution influencethe
AER modelshows
evidence
fornewparticleproduction
HSCT surfaceareaperturbation,
we showin Figure5 by nucleation,asthe numberof particleslessthan 0.001
calculated
aerosol
sizedistributions
fortheAER, Italy, /•m is increasedby additionof HSCT gasphaseemisand Cambridgemodelsunderbackground
and HSCT sions. The gas phase sulfur emissionsincrease nucleconditions at 45øN and 20 km in March. Under back- ationin the stratosphere
by 150%,thoughonly 6% of
ground conditions the AER model shows a maximum the emittedsulfurundergoes
nucleation,
while94%conin numberdensityat 0.04/•m radius,whilethe Italy densesontoexistingparticles.With HSCT emissions
as
andCambridge
models,
whichdonot consider
particles 100%particles,
coagulation
is the mainprocess
for in-
smallerthan 0.01/•m, showspeak numberdensitiesat
teractionbetweenthe backgrounddistributionand the
0.08 and 0.06 /•m, respectively.The Cambridge
and newparticles.The AER and Cambridgemodelsshow
AER models
usea finersizeresolution
thanthe Italy similarsizedistributionsfor particlesbetween0.01 and
modeland showbackgroundsizedistributionsthat in- 0.1 /•m. The Italy modelsshowa greaterredistribu-
1536
WEISENSTEIN
ET AL.- INTERCOMPARISON
OF EFFECTS
OF HSCT SULFUR
tion of injected sulfur into particle sizes between 0.01 showno ozonecolumn changeat the equator and 0.6%
and I/•m. When the background distribution contains to 0.7% depletion near the north pole. Ozone column
particles smaller than 0.01 /•m, as in the AER model, changesat the south pole are about -0.2% to -0.3• for
these small particles preferentially coagulate with the all models.
With addition of sulfur emissions from HSCT airinjected 0.01/•m particles, somewhatreducingthe surface area of the combinedbackground/HSCTdistribu- craft, all models show greater ozone depletion, which
tion in comparisonwith the distribution expected with- increases with concurrent increases in aerosol surface
out this interaction. Note, however, that it is not possi- area. Increasingaerosolsurface area has two effectson
ble to distinguishthe relative importance of differences the ozone impact of HSCT. It reduces the amount of
in transport and differencesin aerosolsize resolution on NOy in the activeformsof NO and NO2, thus decreasthe basis of these calculations.
ing the importance of the NOx cycle to ozone loss and
The sensitivity of these results to the assumed size reducing the impact of aircraft-emitted NOx. It also
of aerosol particles at plume breakup has been investi- increasesthe amount of chlorine and HOx in the active
gated with the AER and NSU models. Calculations
were performed with particle sizes ranging from 0.4
nm to 80 nm. The maximum impact on aerosol sur0.0
..............
face area was found for particles in the 2.5 to 20 nm
-0.5
range. Smaller particles coagulate faster with background aerosols,reducingthe surfacearea increase,and
-1.0
•.•. ............
,,'27,,,,.--,;
-1.5
larger particles are removedmore rapidly by sedimenta-2.o
tion. Input of 80 nm particles resulted in lessthan half
-2.5
the surfacearea perturbation calculated for 10 nm par-90
-60
-30
0
30
60
90
ticles under assumption 3. A calculation with 0.4 nm
Latitude
particles produced a peak aerosolsurfacearea enhance0.5
ment which was 25% lessthan that with 10 nm particles
o.o
under assumption3. A modeling study of the far-wake
regime[Danilin et al., 1997]indicatedthat the particle size distribution
at I week after emission is sensitive
to the wake dilution rate. By employing a very fast,
a very slow, and an intermediate dilution rate, several
possibleparticle size distributions were obtained, with
peak number densitiesoccurring between4 and 20 nm.
Our choiceof 10 nm particlesfalls within this range and
producesapproximately the largest expectedimpact.
-1.5
-2.0
-2.5
-90
-60
-30
0
30
60
90
30
60
90
Latitude
0.5
0.0
-0.5
3.2.
-1.0
Ozone Changes
-1.5
Calculated changesin annual average column ozone
as a function of latitude due to emissionof NOx, H20,
and sulfur from HSCT aircraft are shownin Figure 6 for
each of the four models.
-2.0
-2.5
-90
model: the solid line representsaircraft emissionswith
EI(NOx)=5 and no sulfur emissions,while the three
dashedlinesincludesulfuremissions
with EI(SO2)=0.4
under assumptions 1, 2, and 3. Each model used its
-60
-30
0
Latitude
Four results are shown for each
0.5
0.0
_
-w-'•.•.•-•.•'•"'•w•.-''•-'•.•'.•'-•'-'-•;•'-''•.-•'•'•.•-'•-•.•
,..•
-o.5
-1.0
-1.5
own calculatedbackgroundaerosolsurfacearea (those
-2.o
shownin Figure l) for the subsoniccase(usedas the
-2.5
baseline)and the HSCT casewithout sulfur emissions.
-90
-60
-30
0
30
60
90
Latitude
Without sulfur emissions,ozonecolumnchangesdue to
this HSCT scenariorangefrom -0.1% to +0.2% nearthe Figure 6. Calculated changesin annual average colequator and from 0% to -0.7% for middle and high lat- umn ozone due to an HSCT emission scenario with
itudes. All models show more ozone depletion at high EI(NOx)=5, EI(H20)=1230, and differentassumptions
from the (a) AER model, (b) NSU
northern latitudes than at high southern latitudes as on sulfuremissions
a result of the asymmetry of the HSCT source. The model, (c)Italy model,and (d) Cambridgemodel. The
Italy model showslittle changein column ozone between 30øSand 50øN and depletionsof up to 0.3% at
higher latitudes. The Cambridgemodel showssmall
ozonedepletionin the tropicsand up to 0.5• depletion
near the north pole. Both the AER and NSU models
solid line represents a case with no SO2 emissions,the
long dashedline representsEI(SO2)=0.4 with assumption 1, the short dashedline representsEI(SO2)=0.4
with assumption 2, and the dot-dashed line represents
EI(SO2)=0.4 with assumption3. BackgroundCly is 3
ppbv.
WEISENSTEIN
ET AL.: INTERCOMPARISON
OF EFFECTS
OF HSCT SULFUR
1537
With these models,ozonedepletion is greatestat high
northern latitudes in spring, with more than 2% ozone
depletion between60øN and 90øN in March and April.
Ozone depletion in the southern hemispherehigh latitudes approaches1% in spring. The midlatitudes and
tropics show little seasonalcontrast. The Cambridge
additional ozone column depletion under assumption 1 model showsonly modest changesin ozone depletion
is lessthan or equal to 0.3%. Further depletion under due to particle rather than gas phase sulfur emissions,
assumption2 is only an additional0.1-0.2% (compared with the greatestchangein the tropics and at high latwith resultsunder assumption1) except for the Italy itudes in spring.
model, which calculatesadditional ozone depletion of
Figure 8 showsprofiles of ozone changesat 45øN in
0.4% in the northern hemisphere. Under assumption3 June for the four models with no sulfur emissions and
additional ozone depletion beyond that calculated un- with sulfur emissionsunder the three assumptionsfor
der assumption2 is up to 0.6% for the AER model, 0.3% EI(NO•)=5. For all of the models,HSCT emissions
for the NSU model, up to 1.4% for the Italy model, and without sulfur produce an ozoneincreasein the tropo0.1% for the Cambridgemodel.
sphereand an ozonedecreasein the middle stratosphere
The Cambridgeand Italy modelsshowsimilar ozone above20 km, but the responsein the lowerstratosphere
column depletion under assumption1. The Italy model, (10-20km) variesmarkedlybetweenmodels.This varihowever,showsthe largest depletion under assumption ation reflectsthe sensitivity of ozone in this region to
3 (as muchas 2.3% on an annualaveragebasis),while catalytic loss by the HO•, NO•, and C10• cyclesand
the Cambridgemodel showsthe smallestdepletion un- to transport. HSCT emissions add H20, and thereder assumption3 (only 0.5% at most). This difference fore H O•, and N Ox, increasing ozone loss by their reis consistent with the differences in calculated aerosol
spective catalytic cycles. But NO• emissionsalso tend
surface area perturbation and treatment of heteroge- to reduce the HO• and C10• cycles by forming HNO3
neouschemistry. The AER and NSU modelsyield sim- and C1ONO2. The increase in the NO• cycle appears
ilar ozone responsesto HSCT emissionswith no fuel to dominate the ozone responsein this region in the
sulfur, but the AER model yields greater ozonedeple- AER and NSU models, while reductions in the HO•
tion with fuel sulfur. Maximum annual average ozone and C10• cyclesdominate in the Cambridge and Italy
column changesunder assumption3 are 1.1% for the models. In this sensitiveregion of the stratosphere,difNSU model and 1.8% for the AER model. The NSU
ferences in model formulations of both chemistry and
model calculates a larger aerosol surface area pertur- transport substantially influence calculated changesin
bation than the AER model, but the AER model cal- ozone, leading to model differencesin the sign and the
culates a larger ozone responsebecause it includes a magnitude of the ozone responseto HSCT between 10
temperature distribution. We can separate differences and 20 km.
forms of C10 and HO2, leading to ozonedepletion. For
the EI(NOx)=5 scenariothe latter effect is more significant, and there is additional ozone depletion. The
impact of sulfur emissionon ozone column changeis
smallestin the tropicsand greatestat northern high latitudes in all modelsexcept the Cambridgemodel. The
in calculated
aerosol surface area from differences in cal-
culated ozone depletion by using the calculated surface
area density from various modelsin a single model. Use
of the surfacearea density calculated by the Italy model
under assumption3 in the AER model resultsin maximum annual averageozonedepletion of 2.5%. Use of
the Cambridge model surfacearea calculated under assumption3 in the AER model resultsin a maximum of
1.7% annual averageozone depletion.
To further emphasizethe differencebetween assumption 1, with all sulfur emissionas gas, and assumption
3, with all sulfur emission as particles, we examine in
Figure 7 the calculated changesin ozone column as a
When emissionsof sulfur are also considered, all mod-
els predict a more negativeresponseto HSCT emissions
in the troposphere and lower stratosphere. The sensitivities vary, however. At 15 km the difference in the
percent ozonechangebetweenthe scenariowith no sulfur emissions and that
with sulfur emissions as 100%
particles is 1.1% in the Cambridge model, 1.2% in the
NSU model, 2.7% in the AER model, and 5.5% in the
Italy model. In the middle stratosphere,where the NOx
cycle constitutesa large fraction of the total ozoneloss,
all models calculate
emissions.
The
less ozone loss due to HSCT
altitude
at which
increases
sulfur
in aerosol
surface area begin to cause ozone enhancementrather
than depletion varies from 20 km in the NSU model to
with EI(NOx)=5 for the AER, Italy, and Cambridge 24 km in the Italy model and has some dependenceon
models. With emissionof sulfur as all gas the AER absolute surface area. The Cambridge model showsthe
model shows ozone loss across all of the extratropics, greatest ozone increasein the middle stratospheredue
while the Italy model shows some ozone increase in to aerosolemissions,which largely offsetsthe calculated
spring from 30ø to 50øN and in the tropics. The Cam- depletion at lower altitudes and accountsfor the modest
bridge model showsless than 0.3% ozone column de- changein column ozone due to aerosolemissions.
Figure 9 showschangesin annual averageozone colpletion at all latitudes and seasonsexcept poleward of
function
of latitude
50 ø in summer
and season due to HSCT
and fall.
With
emissions
emission of sulfur as all
umn
due
to
HSCT
emissions
for
a
scenario
with
particles, ozone reductionsin the AER and Italy mod- EI(NO•)-15. The Cambridgemodel did not calculate
els are dramatic, occurring at all latitudes and seasons. this scenario, so only three model results are shown.
1538
WEISENSTEINET AL.- INTERCOMPARISON
OF EFFECTSOF HSCT SULFUR
Gas
•' i : ' ;' ,,,., ,, , ,/., ,
,• :: ,,'•'
-•.o--'
90øN
a
-•
60øN
%':-L::.:2,':'
................
os.........
'-'
30øN
....
.._o.•.'.-.-:.'.
-'-o.o-............
......................
0.4--'
EQ
Particles
90øN:../.'..-,'.,.
•
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I'•'
:,",',
..,,V,,
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o•-...........
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0.2
......
.............
0.2;
.......
" '•0
4 ............
'................
f:::
9"-n'7-::
........
:-'::':6:
•'-::-_-.:•:•:--9_
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---0.2
..................
o.z
.............
o.
•
- "•ø'•F=:•
- "•EQf'--ø'2
..................
0z-- 02;i
30øS _
-
30os
.... 0.2
................
0.2.
............0.2;
....
I.... 0.4...............
o
60øS
90øS
"-0..•..
.....
,
, ', .',. ,,
J F M A M J J A S 0 N D
Gas
90øN
;' ;''! ' •:•o'""
'.,'"'""":
-...... ......
0.6._/
•q;'
'•...-0.4
............
o.4,-'-
c
60øN
_
_
•
'-.
90os
.•_
Month
Particles
0øN
,,, ;,•;-,
,
r
,,- ,: ;, - '...-,
0øN-'•::'}_Z•'-•::::::::
....:::'l..•......
:::::::::::::::::::::::::::::::::::::::::::
::::::::::::::::::: .... :--' ...... 1.2--:::::-':-.
,OøN................
0/t::::::::::: .... : .........
0.6- ..... : ............
30oS
:'-'..'-'-o.6.......
---0.2.......'..................
.--0.4.....
,
J J AS0 ND
90øS
,.-'2..
J
F
M A M
,----.,
J J
Month
A
900Ni
30øN
•
.,•
EQ -
0.3.....
..................
'- .............0.1.......................
0.1--
60øS
,,
90øS
•
J
ß-.... , I
,
i
i
t,,
i
i
¾ Id A. lvl: J J
Month
N
D
,. ,
......
__0.3.......
_
EQ•
.......
0.3'
...........
"'
'•'-'•
•............... -0. L.
ß
,
30ONI
"....... 0.1.,
:'
30øS
,. ,
0
,,-;:.:.?.
......
::::;;.':,:
.........
'....
-
'"0.•:"-0.4
........."
".............
0 2........... 0.2-;,
,
S
•"-o.._.,'
'• ",,.
"--o.•..,"
,."-
e
- ",
,
'•-
Particles
.. , , ,•-
Gas
90øN
0.•"
0.4......,.....
...0.6
...........
0.6..
Month
60øN
'
•, ,,,,,, ,,' • ,,' : ,, ,- ',,• ,...',,,:',.:::::.P'...... ..-; "-i--",:.',,.,:,,'
60øS-.
60øS
J"• YAM
,• --,,
.....
0.4
.......
::/::::-•:h::
....
•'q- :-•-:b:•:
' .....
::7::::
......
_
90øS
::
,, xx ,::5
.--.'o ',
J F M A M J J A S 0 N D
.......
-•
30øS
.....o.&
....
.....
',,,,.....
..,o
'o',......
c..:
,
Month
30øN
•.,,.
.,•.•'
•"'
'•"•'
••-•"'
''••'
.......
....
6oøN['"'"'""
I- q'''
"-'•
"'-:'•
"--'•--'•'-•
---•-- • ''•'
__
•..
_ 4•..........
i
i
A. S
i
'to
,
0
N
:D
,.
.......
,
C'0.3-;:::-:,
60øS
• ,,......
,•---0.3..
3oos ,::t:_:.:•: "'
/.--a:;'-'o
90øS
/ J , F ,"'
M A
M
J J
Month
A
S
0
N
D
Figure7. Calculated
percent
change
incolumn
ozone
asa function
oflatitude
andseason
due
to HSCTemissions
withEI(NO•)-5,EI(H20)-1230,
andEI(SO2)=0.4
forthe(a,b) AER,(c,
d)Italy,and(e,f) Cambridge
models
under
assumptions
(left)1and(right)
3. Thebackground
Clu is 3 ppbv.
With sulfur emissionsomitted the ozone responseis
surface area reduce the amount of emitted N O•: that remains in active forms. The NSU model does, however,
significantly
largerthanthat for EI(NO•)=5. The re3 than
sponse
is dueprimarilyto increases
in NO• catalytic showgreaterozonedepletionunderassumption
lossof ozone.The responseto sulfuremissions
is quite under assumption I or 2.
Changes
in Juneozoneprofileat 45øNfor the three
differentfrom that seenwith EI(NO•)=5. While the
southernhemisphere
still showsincreased
ozonedeple- modelswith EI(NO•)=15 are shownin Figure10. All
tion with increasingsurfacearea, the northernhemi- models show substantial ozone increases in the tropo-
dueto emission
of NO•, but varyingfrom1.7%
sphere
shows
a varietyof responses
among
themodels. sphere
The AER and Italy modelsshowonly a smallresponse in the NSU model to 2.5% in the AER model and 4.0%
ozoneincreaseis
in the northernhemisphereto sulfuremissions
under in the Italy model. The tropospheric
somewhat
reduced
by
sulfur
emissions.
Note, however,
assumptions
I and 2 but showa substantial
negative
that
tropospheric
ozone
changes
contribute
little to the
response
to sulfuremissions
underassumption
3. In
the
the NSU model the sulfur emissionsreduceozonedeple- total columnresponse.In the lowerstratosphere
tion in the northernhemisphere.
Sulfuremissions
can AER and Italy modelsshowenhancedozonedepletion
while the NSU modelshowsre"buffer"or reducethe effectsof high NO• emissions
by due to sulfuremissions,
aircraft,sinceheterogeneous
reactions
onthe increasedducedozonedepletion.It is likely that the NSU model
WEISENSTEIN ET AL.' INTERCOMPARISON
OF EFFECTS OF HSCT SULFUR
1539
35
30
25
•20
/' ?'1
.E 15
lO
o
t..••.%......
%ø.?\
.... I ..... I .... I,,,,I
.... I ....
-3.5 -3 -2.5 -2 -1.5 -1 -0.5
.... I ....
o
0.5
1
-2
Percent Difference
I ....
-1.5
-1
I ....
-0.5
•
0
'''1,,•
0.5
PercentDifference
35
30
25
• 20
•
El5
ß
..•
ø• .
lO
IIIi
-1
Percent Difference
iii
-0.5
0
0.5
PercentDifference
Figure 8. Calculated
percent
change
in ozonedueto HSCTemissions
with Ei(NOx)=5,
EI(H20)-1230,anddifferent
assumptions
onsulfuremissions
in Juneat 45øNfor (a) AER
model,(b) NSUmodel,(c) Italy model,and(d) Cambridge
model.Thesolidlinerepresents
a
casewithnoSO2emissions,
thelongdashed
linerepresents
EI(SO2)=0.4withassumption
1, the
shortdashed
linerepresents
EI(SO2)=0.4withassumption
2, andthedot-dashed
linerepresents
EI(SO2)-0.4withassumption
3. Background
Clyis3 ppbv.
wouldshowlessof a positiveresponse
in this regionif
a temperature distribution were included and the effect
of heterogeneousreactionswere enhanced.
Table I listspercentchangesin columnozoneat 45øN
4. Sensitivity Studies
4.1. Temperature
Probability
Distribution
To illustratethe importanceof a temperatureprobability distribution
in the calculation
of ozonechanges
inand Cambridgemodelsdue to an HSCT fleet with NOx ducedby HSCT emissions,
weshowin Figure11 results
emissionindicesof 5 and 15. Global ozonechanges fromthe AER andItaly modelswithouta temperature
are about one half to two thirds of the changesat distributionfor the EI(NOx)--5 scenario.Theseozone
45øN for most scenarios.Calculatedchangesin global columnchangesas a function of latitude and seasoncan
ozone are less than or equal to 0.2% depletion with be comparedwith thoseshownin Figure 7, whichinEI(NO•)=5 and no sulfur emissionsbut as much as cludethe distribution.Exceptfor the Italy modelwith
0.8% with sulfur emissionsas 100% particles. Global gasphasesulfuremissions,
ozonecolumnchanges
are
ozone changeswith EI(NO•)=15 are up to 0.4% de- smallerwithoutthe distribution,particularlyfor the
pletion without sulfur emissionsand up to 0.5% de- casewith 100%particleemissions.
The seasonality
of
pletion with sulfur emissionsas 100% particles. Com- ozonedepletionis alsochanged,with maximumdepleparingthe EI(NO•)=5 andthe EI(NO•)=15 scenarios, tion found in fall without a temperature distribution
we see much larger ozonedepletionfor EI(NOx)=15 and in springwith a temperature distribution. Northwithout sulfur emissionsfrom aircraft, but with sulfur ern midlatitudeozonedepletionis stronglyaffectedas
emissions
as 100%particles,ozonedepletionis larger well. By includinga temperaturedistribution,annual
for EI(NO•)=5. Thus attemptsto controlNO• emis- averageozonedepletionat 45øN changesfrom 0.81% to
sions from aircraft without concomitant reductions in
1.14% in the AER modelwith particle emissions
and
fuel sulfur may not yield desiredcontrolson ozonede- from 0.90% to 1.45% in the Italy model with partiand on a globalaveragebasisfor the AER, NSU, Italy,
pletion.
cle emissions.Ozonecolumndepletionresultsfor the
1540
WEISENSTEIN
ET AL.'
INTERCOMPARISON
0.5
OF EFFECTS
OF HSCT
S'ULF'UR
els is important for calculating ozone and ozonetrends,
especially under conditions of perturbed stratospheric
sulfur, suchas followingvolcaniceruptions. This sensitivity has been noted previously by Murphy and Ravis-
o.o
-0.5
-1.o
hankara[1994],Borrmannet al. [1997],and Michelsen
et al. [1997].
-1.5
-2.0
-2.5
-9O
-60
-30
0
30
60
90
Latitude
0.5
Polar Stratospheric
Clouds
The effect of including polar stratospheric clouds
,
b
(PSCs) in a 2-D model alongwith HSCT sulfur emis-
0.0
sions has been investigated by the Italy model. The
method of treating PSCs is described by Pitari et al.
-0.5
-1.0
4.2.
:.._.
',,,•..%•..
[1993]and involvesheterogeneous
nucleationof NAT
-1.5
ß
-2.0
-2.5
-9O
-60
-30
0
30
60
90
particles on sulfate aerosolsand nucleation of ice particles on NAT particles. The processesof condensation,
evaporation, and sedimentation are also modeled. As
shownin the paper by Pitari et al. [1993],the inclu-
Latitude
sion of PSC aerosolshas the net effect of mitigating
the ozone column decreaseproduced by supersonicair0.0
craft. The net columnchangeis the result of two oppo-0.5
site ozone anomalies, with the positive one below about
-1.0
15-20 km altitude where C1Ox and HOx catalytic cycles
%.
-1.5
have a larger impact on ozonerelative to the NOx cycle.
-2.0
The NO2 increaseresultingfrom direct aircraft emis-2.5
sion
lowersthe C10/C1ONO2 ratio, thus producinga
-90
-60
-30
0
30
60
90
Latitude
C10 decreasewith respect to the backgroundatmosphereand also removing more OH via the three-body
Figure 9. Calculated changesin annual average colreaction
formingHNO3. The relative weightof the assoumn ozone due to an HSCT
emission scenario with
ciated
ozone
increaseis larger if the backgroundstratoEI(NOx)=15, EI(H20)=1230, and different assumpspheric
amount
of NOx is smaller, as is the case when
tions on sulfuremissions
from the (a) AER model, (b)
PSC
particles
are
present and a net redistribution of
NSU model, and (c) Italy model. The solid line representsa casewith no SO2 emissions,the long dashedline NOy is producedby particle sedimentation.Chlorine
representsEI(SO2)=0.4 with assumption1, the short processingby heterogeneouschemicalreactionson PSC
dashedline representsEI(SO2)=0.4 with assumption2, surfaces is another effect to be taken into account. If
and the dot-dashedline representsEI(SO2)=0.4 with the surfacearea were kept constant, no additional ozone
assumption3. BackgroundCly is 3 ppbv.
changeswould be produced by the aircraft emissions.
However, both NOx and SOl emissionsfrom HSCT may
EI(NOx)=5 scenariofor the AER and NSU modelsare increasethe PSC particle surfaceavailablefor heterogequite similar without a temperature distribution in ei- neous chemistry by increasing the saturation ratios of
ther model.
HNO3 and H2SO4 and then speedingup the processes
For scenarioswith particle emissions,and thus greatly of particle growth and nucleation.
enhanced aerosolsurfacearea distributions, the effect of
Calculationswith the Italy model including PSCs for
including a temperature distribution for calculation of both the baseline simulation and the HSCT simulation
heterogeneousreaction rates is quite important, result- with EI(NOx)=5 show substantiallyless ozonedepleing in a doubling of the calculated ozone depletion in tion than calculations without PSCs. With the effect of
both polar regionsin spring. Reactions(3) and (4), in- a temperature distribution also included, calculated anvolving reactions with HC1 on sulfate aerosol surfaces, nual averagechangesin column ozone poleward of 40øN
are responsiblefor most of this sensitivity, with reac- are 0.0% with sulfur emissionstreated as SO2 gas and
tion (3) accountingfor about two thirds of the effect -1.0% with sulfur emissionstreated as 100% particles.
and reaction (4) accountingfor the remainder. The Correspondingozonechangeswithout PSCs are-0.3%
Cambridge model also usesa temperature distribution and -2.0%. The ozone responseto HSCT emissionsis
but doesnot calculate greatly enhancedozonedepletion less sensitiveto a temperature distribution with PSCs
with HSCT particle emissions,possiblyas a result of the included becausereaction probabilities of C1ONO2 and
cruderparameterizationof reaction (3) and omissionof HOC1 on PSC surfacesare assumedto be temperature
reaction (4). Our resultsimply that use of a tempera- independent. Results of HSCT sensitivity to PSCs in
ture distribution in 2-D models and of accurate temper- 2-D models is expected to be sensitive to model formuature fields and fluctuations in box and trajectory mod- lation, and indeed, among 2-D models that have per0.5
ß
,
•
I
,
,
I
•
•
I
,
,
I
•
•
I
-
,
,
WEISENSTEIN
ET AL.' INTERCOMPARISON
OF EFFECTS
OF HSCT
SULFUR
1541
35
30
25
Illl]llllll,,,111,,
-2.5 -2-1.5-1-0.5
0 0.5
1 1.5 2 2.5
3
-2-1.5-1-0.5
Percent Difference
0 0.5
1 1.5 2 2.5
Percent Difference
35
30
25
•20
•= 15
10
-2-1.5-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
Percent Difference
Figure 10. Calculated percent changein ozone due to HSCT emissionswith EI(NOx)=15,
EI(H20)=1230, and differentassumptions
on sulfur emissions
in June at 45øN for (a) AER
model, (b) NSU model, and (c) Italy model. The solid line representsa casewith no SO2
emissions,
the long dashedline representsEI(SO2)=0.4 with assumption1, the short dashedline
represents
EI(SO2)=0.4 with assumption2, and the dot-dashedline represents
EI(SO2)=0.4 with
assumption3. BackgroundCly is 3 ppbv.
formed similar experiments, no clear consensushas been
reached[Stolarskiet al., 1995].
4.3. Background Cly
The sensitivityof the HSCT ozoneresponseto Cly
is important becauseit is expectedthat Cly will de-
creasein the future as the atmosphererespondsto reduced emissionsof CFCs and other chlorine-bearing
compounds.A Cly concentrationof about 2.0 ppbv
is forecastfor 2050 [WMO, 1995]. Figure 12 showscalculated annual averageozone column changesdue to
HSCT emissions
with EI(NOx)=5 and 2 ppbv of Cly
Table 1. Calculated Change in Annual Average Ozone Column Due to EmissionsFrom a Mach 2.4 HSCT Fleet
in 2015
AER
Sulfur Emission*
45øN
NSU
Global
Italy
45øN
Global
Cambridge
45øN
0.02%
Global
45øN
Global
-0.02%
-0.22%
-0.26%
-0.27%
-0.36%
-0.14%
-0.17%
-0.21%
-0.30%
None
-0.35%
-0.21%
-0.32%
-0.16%
0% particles
10% particles
-0.57%
-0.72%
-0.33%
-0.40%
-0.50%
-0.59%
-0.27%
-0.32%
-0.11%
-0.44%
-0.09%
-0.23%
100% particles
-1.14%
-0.63%
-0.83%
-0.44%
EI(NOx)=15
-1.45%
-0.77%
None
-0.65%
-0.30%
-0.94%
-0.35%
-0.14%
-0.12%
0% particles
10% particles
100% particles
-0.71%
-0.75%
-0.95%
-0.36%
-0.39%
-0.52%
-0.57%
-0.62%
-0.71%
-0.25%
-0.29%
-0.36%
-0.17%
-0.20%
-0.74%
-0.15%
-0.15%
-0.46%
The backgroundinorganic chlorine amount for 2015 was 3 ppbv.
*Sulfur emissionswith EI(SO•.)-0.4 are treated as 100% gas, 90% gas and 10% aerosolparticlesof 10 nm radius, or
100% aerosolparticles of 10 nm radius. A casewith no sulfur emissionsis also shown.
1542
WEISENSTEIN
ET AL.- INTERCOMPARISON
90øN
Gas
,
OF EFFECTS OF HSCT SULFUR
Particles
90øN
/
60øN
60øN
'6}-.
2
"......-1.0-----•
.....
30øN
•
.-0 .Z...........
.
-0.2- .............
•.0•
Eq
30øN
-
•2.
-
30øS
60øS
_
_
90øS
F
M
A
M
J
J
A
S
O
..... .•0.4.................
..........................
o•--•
N
.•,--"-"-'0
4
-04-
J
F
M
A
M
", '.......-0.4...........
--0.
•0.0••0
_
60øS - ---0.2 .......
O
N
D
EQ
30øS
• .•,............. • .o•
---o.•
......
•:::.....o.•
.........
•-•.....
:
.......o.•:•
.........
:_o_.?:::
....--•-..•:::.o.
- -...... 0.2.......•'
.......... .2.....' _
60øS
,----,, .......
Y
S
Particles
...... '__-:::•..........
30øN
'0/
-
F
A
60øN
..
J
J
90øN
_
90os
J
Month
30ON -
30os
.--
90øS
D
Gas
..,
04 ..........
-0.2 ....... '-..... 0.2-
._____
0.
Z..........
0Z •i
Month
60øN
----'"•
0.8-
'- -............................-0.6..........
EQ
.... 0.2.............0.2.................-0.2-.
J
0 8..........
...........
.............
..
30øS
60øS
0 5 ...........
A
Y
J
J
A
S
O
N
D
90øS
J
F
M
Month
A
M
J
J
A
S
0
N
D
Month
Figure 11. Calculatedpercentchangein columnozoneas a functionof latitude and seasondue
to HSCT emissions
with EI(NO•)-5, EI(H•O)-1230, andEI(SO•)-0.4 for the (a, b) AER and
(c, d) Italy modelsunderassumptions
(left) 1 and (right) 3, omittingthe effectof a temperature distributionon heterogeneous
reactionrates. To be comparedwith Figure 7 includinga
temperaturedistribution.The backgroundCly is 3 ppbv.
0.5
for the AER
and NSU models.
Without
sulfur emis-
0.0
sionsthe calculatedozonedepletionis slightlylargerin
-0.5
midlatitudes
for 2 ppbvof Cly than for 3 ppbvof Cly.
-1.0
With emissionof sulfurfrom aircraft, ozonedepletionis
increased,but by a lesser amount than that seen with
3 ppbv of Cly.
Figure 13 showscalculatedannual averageozonecolumn changeswith EI(NOx)=15 and 2 ppbv of background Cly for the same models. Again, calculated
-1.5
-2.0
-2.5
-90
-60
-30
0
30
60
90
Latitude
0.5
ozonedepletionis largerfor 2 ppbvof Cly than for 3
ppbvof Cly withoutsulfuremissions.
This effectis due
b
0.0
-0.5
-1.0
'"'-".
'"'
•":'•.
':';'--•!•l•'•l••'•"•"
- • "a.
•.•
_
to reduced production of C1ONO= when NOx is emitted by aircraft in a low-chlorine environment. North-
ern hemisphereozonedepletionwas increasedby sulfur
-1.5
emission
with 3 ppbv of Cly in the AER modelbut is
unaffected
by sulfuremissions
with 2 ppbvof Cly. In
-2.0
-2.5
-90
-60
-30
0
Latitude
30
60
90
the NSU model, ozonedepletion in the northern hemispherewas reducedby sulfur emissionwith 3 ppbv of
Figure 12. Calculatedchangesin annualaveragecol- Cly, andwith 2 ppbvof Cly it is reducedevenfurther.
ozone due to an HSCT
emission scenario with
Table 2 lists calculatedozonecolumnchangeat 45øN
EI(NO•)=5, EI(H20)=1230, and differentassumptions and global average ozone column change with
on sulfur emissionsfrom the (a) AER model and (b) EI(NO•)=5 and15and2 ppbvof Cly. With EI(NO•)=5
NSU model. The solid line representsa case with
andemission
of sulfuras 100%particles(assumption
3),
no SO2 emissions, the long dashed line represents
the calculated ozone depletion at 45øN from the AER
EI(SO2)-0.4 with assumption1, the short dashedline
representsEI(SO2)=0.4 with assumption2, and the modelis 1.14%with 3 ppbvof Cly and 0.85%with 2
dot-dashedline representsEI(SO2)=0.4 with assump- ppbvof Cly. Fromthe NSU model,calculatedozonedetion 3. Backgi•ound
C1u is 2 ppbv. To be compared pletionat 45øNis 0.83%with 3 ppbvof Cly and0.62%
with Figures 6a and 6d.
with 2 ppbvof Cly. With EI(NO•)=15 andsulfuremisumn
WEISENSTEIN
0.5
'
'
i
,
'
i
'
'
ET AL.'
i
'
'
i
INTERCOMPARISON
'
'
i
,
OF EFFECTS
on sulfate
'
aerosols.
OF HSCT
This
SULFUR
reaction
is included
1543
in the
a
NSU results presented above with a reaction probabil-
0.0
-•.o
-1.5
-2.0
-2.5
-90
-60
0
-30
30
60
90
polar regions,though valuesof about 0.4 are still found
over most of the stratosphere. This heterogeneousreaction has the effect of increasingactive HOx, C1Ox and
Latitude
0.5
0.0
ity (?) of 0.4 based on the measurementsof Hanson
andRavishankara
[1995].More recentmeasurements
of
this reaction have provided a ? which is a function of
temperatureand water activity [Hansonet al., 1996].
Values of ? are as large as 0.8 for aerosolswith acid
weight percent of lessthan 70%, which occursin the
b ' i '
BrOx, mainlyasa resultofHOBr photo]ysis[Randeniya
et al., 1996],leadingto enhancedozonedepletion.
-•.o
AER model resultsincludingreaction (5) and using
-1.5
the
parameterizationof Hansonet al. [1996]are shown
-2.0
in Figure 14 for EI(NO•) of 5 and 15. Comparedwith
-2.5
calculationsthat omit this reaction (seeFigures6a and
-90
-60
-30
0
30
60
}0
Latitude
9a), ozonecolumn changesare smaller without sulfur
emissions,especiallyfor EI(NO•)=15. Sulfur emissions
Figure 13. Calculated changesin annual average colcause a greater perturbation with reaction (5) than
umn ozone due to an HSCT
emission scenario with
EI(NOx)=15, EI(H20)=1230, and different assump- without it. With EI(NO•)=5 and sulfur emissionsas
tions on sulfur emissionsfrom the (a) AER model 100% particles, calculated ozone depletion is 1.3% at
and (b) NSU model. The solid line representsa case 45øN with a global averageozone depletion of 0.7%.
with no SO2 emissions,the long dashed line represents This is 20% more depletion than that without reaction
EI(SO2)=0.4 with assumption1, the short dashedline
representsEI(SO2)=0.4 with assumption2, and the
dot-dashedline representsEI(SO2)=0.4 with assumption 3. BackgroundCly is 2 ppbv. To be compared 4.5. Background Aerosol Loading
with Figures 9a and 9b.
Variations in background aerosol amount have been
found to affect calculated ozone change due to HSCT
sionsas particles, ozone column changesare about the emissions[Weisensteinet al., 1993]. There is uncersamewith 2 ppbv of Cly as with 3 ppbv of Cly.
tainty regarding the true "nonvolcanic"aerosollevelsin
4.4.
BrONO2
+ H20
Heterogeneous
Reaction
the stratosphere, since periods free from volcanic influence have been seenonly rarely in the past two decades
Another sensitivity that we consider is the effect of
including the heterogeneousreaction
[Hitchmanet al., 1994],mostrecentlyin 1979 [Thomason et al., 1997]. The total stratosphericsulfur mass
estimatedfor 1979by Kent and McCormick[1984]was
BrONO2 + H20 -• HNO3 + HOBr
570 kt. Models employing OCS photolysisas the major
(5)
Table 2. Calculated Change in Annual Average Ozone Column Due
to Emissions From a Mach 2.4 HSCT
Fleet in 2050
AER
Sulfur Emission*
45øN
NSU
Global
45øN
Global
None
-0.43%
-0.24%
-0.42%
-0.20%
0% particles
10% particles
100% particles
-0.55%
-0.63%
-0.85%
-0.31%
-0.35%
-0.46%
-0.47%
-0.52•
-0.62%
-0.23%
-0.27%
-0.33%
None
-1.03%
-0.51%
-1.41%
-0.55%
0% particles
10% particles
100% particles
-0.98%
-0.97%
-0.96%
-0.51%
-0.51%
-0.52%
-1.01%
-0.91%
-0.78%
-0.44%
-0.41%
-0.35%
The background inorganic chlorine amount for 2050 was 2 ppbv. Other
long-lived trace specieswere retained at their 2015 levels.
*Sulfur emissionswith EI(SO2)=0.4 are treated as 100% gas, 90% gas
and 10% aerosolparticlesof 10 nm radius, or 100% aerosolparticlesof 10
nm radius.
A case with
no sulfur emissions is also shown.
1544
WEISENSTEIN
ET AL'
INTERCOMPARISON
OF EFFECTS
OF HSCT SULFUR
change due to HSCT emissionsis smaller at higher
aerosol levels, both with and without sulfur emissions.
8 o.o
With EI(NOx)=5 the increasedbackgroundaerosolsur•
face area results in reductions
-1.0
in HSCT
ozone column
• -1.5
depletionat 45øN from-1.3% to-1.0% with sulfur emis-
•
sionsas 100%particles.With EI(NOx)=15 the changes
-2.0
are larger, reducing ozone column depletion at 45øN
-90
-60
-30
0
30
60
90
Latitude
0.5
-1.0% to-0.6% with sulfur emissionsas 100% particles.
Followingthis logic, we can argue that during times of
volcanic enhancementof the aerosollayer, HSCT emission of NOx will lead to smaller ozone depletion, and the
,
b
• 0.0 _
from -0.5% to -0.1% without sulfur emission and from
•,•,•,•
effect of sulfur emissions from aircraft
• -13
will be reduced.
...........
• -2.0
-'•0
5. Summary
-60
-30
0
30
60
)0
Latitude
Figure 14.
Calculated changes in annual average column ozone from the AER model due to an
HSCT emissionscenariowith (a) EI(NO•)=5 and (b)
EI(NO•)=15 and including the heterogeneousreaction BrONOe+H20.
The solid line represents a case
with no SO2 emissions,the long dashedline represents
EI(SO2)=0.4 with assumption1, the short dashedline
representsEI(SO2)=0.4 with assumption2, and the
dot-dashedline representsEI(SO2)=0.4 with assumption 3. BackgroundCI• is 3 ppbv. To be compared
with Figures 6a and 9a.
source
ofstratospheric
sulfate
arefound
tocontain
only
and Conclusions
Four independentlyformulated two-dimensionalmodels of trace speciesand sulfate aerosolshave been applied to predict the effect of HSCT emissionson the
stratospheric aerosol layer and on ozone. Modelpredicted changesin aerosolsurface area vary by a factor of 3 among the models for identical sulfur emissions. The main cause of model differencesappears to
be transport, which directly affects the residence time
of the emitted sulfur and thus the magnitude of the
surface area perturbation. However, the transport rate
affects the background calculated surface area as well,
and thus percentageperturbations are fairly consistent
0.5
abouthalfthisamount
[ChinandDavis,1995;Weisen-• o.o
• -0.5
steinet al.,1997].
•
Deep tropospheric convectionhas been found to have 5 -1.0
a largeeffectoncalculations
ofuppertropospheric
SOe =•
e -1.5
concentration[Chatfieldand Ur•tzen, 1984; Langner •.-2.0
and Rodhe,1991]and potentiallyon stratosphericaero-90
sol loading[Weisensteinet al., 1997]. This processhas
been accountedfor in the AER model by imposing an
SOe concentrationof 40 pptv in the tropical middle and
-60
-30
0
3O
6O
9O
Latitude
0.5
upper troposphere[Weisensteinet al., 1997]and leads
o.o
to a factor of 5 increase in aerosol mass density and a
factor of 2 increase in surface area density in the tropical lower stratosphere. Total stratospheric aerosolmass
-0.5
roughly doubles. We will presentresultsfor the HSCT
impact using a "high background" aerosolsurfacearea
obtained from the AER model employingenhancedupper tropospheric SOe.
Changes in calculated surface area due to HSCT
emission of sulfur are smaller with the "high background" model. Increasesin aerosolsurfacearea density
are reduced by 25-30% in relation to the results shown
for the AER model in Figures 2-4. The percentage
changeover backgroundis significantly less, especially
in the low latitudes where most of the backgrouridsurface area increaseoccurs. Calculated changesin ozone
with high backgroundsulfur and reaction(5) included
are shown in Figure 15. The predicted ozone column
-2.o
-1.0
-1.5
,
-2.5
-90
•
I
-60
,
,
I
-30
,
,
I
0
,
,
I
30
,
,
I
60
,
,
90
Latitude
Figure 15. Calculated changesin annual averagecolumn ozone from the AER
model due to an HSCT
emis-
sionscenariowith (a) EI(NO•)=5 and (b)EI(NO•)=15
for high backgroundaerosol(see text) and including
the BrONO2+H20 reaction. The solidline representsa
casewith no SO2 emissions,the long dashedline repre-
sentsEI(SO2)=0.4 with assumption1, the shortdashed
line represents
EI(SO2)=0.4 with assumption2, and the
dot-dashedline representsEI(SO2)=0.4 with assumption 3. BackgroundCly is 3 ppbv. To be compared
with Figure 14.
WEISENSTEIN
ET AL.: INTERCOMPARISON
OF EFFECTS
OF HSCT
SULFUR
1545
among the models. When sulfur emissionsare assumed
to be in the form of gas phaseSO2, maximum increases
in aerosolsurfacearea range from 25% to 50% above
their respectivecalculated background amounts. When
10% of the sulfur is assumedto be in particles of 10 nm
bution become a standard part of 2-D models, as it accounts for some important processesthat are otherwise
neglected and to which ozone is sensitive. Introduction
of PSCs into the Italy model leads to smaller ozone
depletion due to HSCT emissions,though this result is
radius,surfacearea increasesare greater,up to 65% or
75%. When all sulfur is assumedemitted as particles,
surfacearea increasesare muchgreater, up to 200% or
300%. The conclusion
of Weisenstein
et al. [1996]that
reduced from 3 ppbv to 2 ppbv, ozone depletion due to
NOx emission alone is increased, but ozone depletion
emissionof sulfur by aircraft, if it is converted to particles within the wake, could strongly perturb stratospheric aerosol concentrationsis supported by all four
models used in this study.
The models show larger variations in ozone response
to HSCT. This finding is to be expected becausethe
ozone responseis sensitiveto the partitioning of H Ox,
NOx, C1Ox, and BrOx radicals, to transport rates, and
to the backgroundaerosolsurfacearea. The balance between these competing processes,unfortunately, generally dependson model formulation. There is no consensus on the best way to formulate a numerical model of
atmosphericozone. Even with perfect knowledgeof the
atmosphere, modelerswould be forced to make compromisesin constructingmodelsto keep the computational
requirements modest. We will treat variations in ozone
responsebetween the models as sensitivities to model
formulation. Where qualitative consensusexist among
the models, we will draw more robust conclusionsregarding ozone responseto HSCT.
All models agree with the conclusion of Weisen-
expectedto be modeldependent.With backgroundCly
due to sulfur emissionsis reduced(with EI(NO•)=5)
or reversed(with EI(NOx)=15). Inclusionof the heterogeneousreaction BrONO2+H20 on sulfate aerosols
leads to a slight reduction in ozone depletion without
HSCT sulfur emissionsand an increasein ozone depletion with
sulfur
emissions.
This
reaction
should
be in-
cluded in future modeling studies. Increasingthe background aerosolsurfacearea leads to lessozone depletion
due to HSCT NOx emissionsand less ozone depletion
due to HSCT
sulfur emissions.
The recommendation for future modeling studies of
HSCT impact on ozoneis that sulfur emissionsmust be
included in assessmentcalculations, and knowledge of
chemical and microphysical processeswithin the plume
and far wake are crucial for accurate modeling. The
fractional conversion of emitted sulfur to aerosol particles depends on processeswithin the engine, nozzle,
and near-fieldplume [Kiircherand Fahey,1997;Brown
et al., 1996b],and the resultingparticle size distribution dependson dilution within the far wake [Danilin
et al., 1997]. The fractionalconversionis bracketedin
this study by 10% and 100% conversion,based on di-
stein et al. [1996]that for the HSCT scenariowith rect measurements
in the Concordeplume[Faheyet al.,
EI(NOx)=5, which representsa very modestemission 1995]. Becausethe aerosolinstrument did not meaof NOx, aircraft emission of sulfur will increase ozone
depletion. This increase is due to increased heterogeneousprocessingon the enhancedaerosollayer, increas-
ing ozonelossdue to the HOx and C1Oxcatalytic cycles.
Ozone depletion is largest if all sulfur is assumedemitted as particles. With this assumption, annual average
ozonedepletionat 45øN is calculatedto be 1.1% for the
AER model, 0.8% for the NSU model, 1.5% for the Italy
model, and 0.4% for the Cambridgemodel with 3 ppbv
sure particles smaller than 0.08/•m diameter and models predict a large number of such particles, the actual
conversionis likely to be larger than 10% in the young
plume (between16% and 40% at 16 min, accordingto
modelingstudiesby Karcher and Fahey[1997],Danilin
et al. [1997],and Yu and Turco[1997]). The model-
ing of heterogeneousprocessesbetween ambient species
and particles within the plume and far wake is, we believe, unnecessaryfor prediction of the global impact of
of Cly. When EI(NOx) is increased
to 15, the models' aircraft emissionsbecause the amount of air processed
calculated responsesto emissionof sulfur differ. This within the wake is too small. Heterogeneousprocessing
variable sensitivity can be understood as differencesin of aircraft-emitted species within the wake may prove
the relative importance of NOx catalytic loss of ozone to be important [Karcher,1997]. The impact of sulfur
(whichis reducedby addedsulfur) and HOx and ClOx emissions from subsonic aircraft is not considered here
catalyticlossof ozone(whichis increasedby addedsul- (seeFriedl et al., [1997]for a discussion
of possibletropospheric and climatic effects of subsonic sulfur emisfur) in eachmodel.
A number of model sensitivities are explored to see sions). However,future subsonicaircraft are expected
how the ozone responseto HSCT emissionsmay vary to fly at higher altitudes, depositinga larger fraction
under different conditions or with additional model pa- of their emissions(includingsulfur)in the stratosphere.
rameterizations. An important result of these model
calculations is that inclusion of a temperature distribution
in 2-D
models
to account
for deviations
from
These higher-flying subsonic aircraft should definitely
be considered in modeling the impact of aviation on
the future global environment.
the zonal mean temperature can significantly increase
Acknowledgments.
Work at AER was supported by
calculated ozone depletion due to HSCT emissions,es- the NASA AtmosphericEffectsof Aviation Program (AEAP)
pecially when sulfur emissionsare included. We rec- NAS1-19422 and the SAGE II programNAS1-20666. Novosiommend that use of a temperature probability distri- birsk State University acknowledgessupport from the Rus-
1546
WEISENSTEIN
ET AL.: INTERCOMPARISON
OF EFFECTS
OF HSCT SULFUR
Anderson, Aerosol particle evolution in an aircraft wake:
sian Foundation for Basic Research under grant 94-05-16661
Implications for the HSCT fleet impact on ozone, J. Geoand support from the U.S. National Aeronauticsand Space
phys. Res., 102, 21,453-21,463, 1997.
Administration under grant NAG5-3409. We thank two
DeMore, W. B., S. P. Sander, D. M. Golden, R. F. Hampson,
anonymousreviewers for their helpful comments.
M. J. Kurylo, C. J. Howard, A. R. Ravishankara, C. E.
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