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Aerospace Science and Technology 8 (2004) 307–320www.elsevier.com/locate/aesc

Reduced environmental impact by lowered cruise altitude for liquihydrogen-fuelled aircraft

Minskad miljöpåverkan genom sänkt marschflyghöjdför vätgasdrivna flygplan

Fredrik Svenssona,∗, Anders Hasselrota, Jana Moldanovab

a Swedish Defence Research Agency, FOI, Aeronautics Division, FFA, Department of Aviation Environmental Research, SE-172 90 Stockholm, Swedenb IVL Swedish Environmental Research Institute, Box 470 86, SE-402 58 Göteborg, Sweden

Received 8 July 2002; received in revised form 9 February 2004; accepted 9 February 2004

Available online 17 March 2004

Abstract

Environmental concerns, as well as the expected depletion of fossil fuel resources, have become the driving forces for researchdevelopment towards the introduction of hydrogen energy into air traffic. The present paper is a summary of a study that was cwithin the European sponsored project CRYOPLANE, co-ordinated by Airbus Germany.

The objectives of this study are to re-optimise and compare two equivalent medium-range aircraft – one kerosene-fuelled and2-fuelled – for reduced cruise altitude, from an environmental point of view. By lowering the cruise altitude, the contribution to global wmight be reduced at the expense of increased fuel consumption and pollutant emissions.

In order to assess the global impact, in terms of global warming, from the emissions discharged on a certain mission, a simplemodel is employed.

The results suggest that introduction of cryoplanes will improve the environmental performance, particularly in terms of global wProvided that an increase in fuel consumption in the order of 10% and an increase in TOM of a few percent are accepted, the resuthat cryoplanes should cruise at an altitude of about 2–3 km below where conventional aircraft cruise today in order to considerably reduthe environmental impact. 2004 Swedish Defence Research Agency. Published by Elsevier SAS. All rights reserved.

Sammanfattning

Miljöaspekter och sinande oljekällor utgör drivkrafterna för forskning och utveckling rörande ett införande av vätgas som bränsleflygtrafik. Denna artikel är en sammanfattning av en studie som genomfördes inom ramarna för det EU-finansierade projektet CRYvilket koordinerades av Airbus Tyskland.

Målsättningen är att optimera och jämföra tvåekvivalenta medeldistansflygplan – ett konventionellt och ett vätgasdrivet – ur miljösynpunktdå flyghöjden reduceras. Genom att sänka flyghöjden skulle bidraget till växthuseffekten från vätgasdrivnaflygplan kunna minskas pbekostnad av att bränsleförbrukningen och avgasemissionerna ökar.

För att bedöma den globala påverkan i form av växthuseffekt av utsläpp vid en specifik flyghöjd används en parametriserad moResultaten föreslår att ett införande av vätgasdrivna flygplan skulle minska miljöpåverkan, särskiltbeträffande global uppvärmning

Förutsatt att en ökning av bränsleförbrukningen i storleksordningen 10% och en ökning av flygplanets startvikt om några procentvisar studiens resultat att vätgasdrivna flygplan bör flyga på en marschhöjd ungefär 2–3 km lägre än vad konventionella flygplan gatt väsentligt minska bidraget till växthuseffekten. 2004 Swedish Defence Research Agency. Published by Elsevier SAS. All rights reserved.

* Corresponding author.E-mail address: [email protected] (F. Svensson).

1270-9638/$ – see front matter 2004 Swedish Defence Research Agency. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.ast.2004.02.004

308 F. Svensson et al. / Aerospace Science and Technology 8 (2004) 307–320

Keywords: Optimisation; Cruise altitude; Environmental impact; Aircraft performance; Global warming

Keywords: Optimering; Marschhöjd; Miljöpåverkan; Flygplanprestanda; Global uppvärmning

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1. Introduction

In the past, using hydrogen for civil aviation was invesgated sporadically, mainly by the USA and Russia [2,43,These early efforts were in part initiated because of the coil price dictation of 1973 [9]. More recently, environmenconcerns, as well as the expected depletion of fossil fuesources, have become the driving forces for research anvelopment towards the introduction of hydrogen energyair traffic. During the last decade Airbus Germany andRussian aircraft manufacturer Tupolev have put much eon resolving this issue [30–32]. The present paper is a smary of a study that was carried out within the Europesponsored project CRYOPLANE, co-ordinated by AirbGermany.

The main reason for changing the source of energy fkerosene to the energy carrier hydrogen, in the civil aviaindustry, is to reduce the environmental impact, particulain terms of global warming. Another reason is to macivil aviation independent of one single energy source.using hydrogen as aircraft fuel, the emissions speciesreduced to water vapour (H2O) and low quantities of oxideof nitrogen (NOx ), provided that the energy is producfrom renewable energy sources. All emissions containcarbon and sulphur are eliminated.

The primary contributor to global warming is carbdioxide (CO2). In addition, essentially depending on tcruise altitude, H2O and NOx can play an important role. Iorder to reduce the environmental burden from an aircone may either perform technical design improvementsinstance, on the airframe or on the engine, or one may aoperational measures. The way of operating the aircraftbe changed in several ways.

In this work the influence of lowering the cruise altitudas a means of reducing the environmental impact, is intigated. The option of changing the cruise altitude has bstudied before, particularly for kerosene-powered aircraf15,45,55], but also to a minor extent for cryoplanes [31].a lower altitude the air has a higher density, which caularger loads on the airframe. The wings therefore neebe stiffer, thus increasing the structure weight [20]. Onother hand, the load on the fuselage would be lower duthe lower pressure difference between the cabin and ouair, which would decrease the structure weight [20]. Flylower would reduce the radiation exposure, but the aircwould be more exposed to gusts that would increase thtigue and make the ride less comfortable.

The objectives of this paper are to re-optimise and cpare two equivalent medium-range aircraft, one kerosfuelled and one LH2-fuelled, for reduced cruise altitudfrom an environmental point of view. By carrying out d

-

tailed flight simulations, differences, in terms of requirchanges in configuration and optimum cruise altitudeminimum environmental burden, are identified for both acraft types.

2. Methodology

In order to assess the environmental improvement thpossible to be achieved by changing the aircraft configtion and flight procedures, two different simulation tools,ANO [48] and Hurdy-Gurdy [18], are employed. PIANO iscommercial software that may be used for performancemations, preliminary design studies and flight emission caculations. Hurdy-Gurdy is a software developed at FOIoff-design flight mission studies.

The simulation procedure consists of a sequence oferations. Firstly, a datum aircraft is established in PIANby defining geometries, weights, aerodynamic performa(drag polar) and engine performance. The aircraft weigand the aerodynamic characteristics are taken from [while the engine performance is obtained using the gasbine performance code TurboMatch [52]. Then the aircis re-optimised for the datum as well as reduced cruisetitudes, and changes in aircraft performance, fuel burnemissions are determined. In order to translate the chain emissions and fuel burn into a measure of global warmeach optimised configuration is simulated in Hurdy-Gurdyto calculate the amount of emissions emitted at differentitudes. In addition, a GWP model (Section 4) is used. Tprocedure is carried out for both the kerosene- and the L2-fuelled aircraft. Differences between these two aircraft typedue to a reduction in cruise altitude are estimated. Fthese results, conclusions about the optimum cruise altifrom the environmental point of view can be drawn, boththe kerosene and the hydrogen aircraft.

In Table 3 some fundamental performance data fortwo investigated aircraft, which are based on the Airbusaircraft, are summarized. The cryoplane features a low-wconfiguration with the fuel stored on top of the fuselagein a tank outside the pressure cabin in the tail cone offuselage. Further details about the aircraft are given in54].

In order to optimize the aircraft configurations in PIANa target function needs to be defined, i.e. what shouldminimized, as well as a set of main design parameallowed to vary within some specified design constraiIn this study, a function weighting the FW (fuel weighby about 2/3 and the MTOW (maximum take-off weighby 1/3 is used as target function. The design parame

F. Svensson et al. / Aerospace Science and Technology 8 (2004) 307–320 309

Nomenclature

Abbreviations and chemical formulae

APU Auxiliary power unitBPR By-pass ratioCH4 MethaneCL ClassCMR Cryoplane, medium rangeCO Carbon monoxideCO2 Carbon dioxideDLR Deutsches Zentrum für Luft- und RaumfahrtFL Flight levelFW Fuel weightGWP Global warming potentialHO2 Hydroperoxyl radicalH2O Water vapourISA International standard atmosphereKE Kerosene equivalentLH2 Liquid hydrogenMa Mach numberMAP Model of Aircraft PlumeMR Medium-rangeMTOW Maximum take-off weightNO Nitrogen oxideNO2 Nitrogen dioxideNOx Oxides of nitrogen (NO+ NO2)N2O Nitrous oxideO3 OzoneOH Hydroxyl radicalOPR Overall pressure ratioOWE Operating weight emptyPIANO Project Interactive Analysis and Optimisation

(software)ppb(V) Part per billion, volumeppm(V) Part per million, volume

RK Reference kerosene-fuelled aircraftSSLT Static sea-level thrustTOM Take-off massUHC Unburned hydrocarbons

Notations

A Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . m2

c Mixing ratio of CO2 (after addition) . . ppm(V)c0 Mixing ratio of CO2 (before addition) . ppm(V)Ei Warming effect of trace gasi . . . . . . . . . W m−2

EI Emission index of pollutantemission . . . . . . . . . . . . . . . . . . . . . . g kg−1 fuel−1

h Altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . kmMW Molecular weight . . . . . . . . . . . . . . . . . . . g mol−1

m Mixing ratio of CH4 (after addition) . . ppm(V)m0 Mixing ratio of CH4 (before addition) . ppm(V)n Mixing ratio of N2O (after addition) . . ppm(V)n0 Mixing ratio of N2O (before addition) . ppm(V)RF Radiative forcing . . . . . . . . . . . . . . . . . . . . W m−2

SFC Specific fuel consumption . . . . . . . . g kN−1 s−1

t Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . yr or dT Time horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . yrTET Turbine entry temperature . . . . . . . . . . . . . . . . . KP Production rate . . . . . . . . . g(species) g(NOx)−1

Q Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . kgS Sensitivity function . . . . . . . . . . . . W m−2 DU−1

Vmol Volume of a mol (273 K, 101325 Pa) . . . . . . m3

X Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DUY Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DUλ Climate sensitivity parameter . . . . . . K W−1 m2

τ Residence time . . . . . . . . . . . . . . . . . . . . . . yr or d�T0 Change in surface temperature . . . . . . . . . . . . . K

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allowed to vary in the optimization procedure are wing arwing aspect ratio and maximum take-off weight.

It is assumed that the aircraft stays at a constant altiduring the whole cruise phase, which is slightly in contdiction to real flight procedures. The Mach number is kconstant at 0.8 for all cruise altitudes. Most correct wouldto keep the total flight time constant when lowering the flialtitude, but since this complicates the assessment, the Mnumber is kept constant instead. This assumption will rein the flight time decreasing to a small extent (< 3%) whenthe cruise altitude is reduced.

3. Assumed technology level

In this section the assumptions made concerningtechnology level are stated. Since the paper deals with fu

technologies, the aircraft and engine data are meant to rethe technology level of year 2010.

3.1. Performance and emissions data of engines

In order to assess properly the performance and emissdata, both the effects of a future technology and the effof switching to hydrogen need to be addressed. All engand emission data are based on the data of the V2A5 engine. The reason for choosing this engine is thais a typical, reasonably modern, medium-thrust turboengine, for which there are sufficient public data availato create a reasonable model of its performance.engine performance at take-off (design point) and at crare calculated using the gasturbine performance codTurboMatch [52] (a description of the engine model is givin [50]). In addition, the inherent engine characterist


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Table 1Installed performance data for anup-scaled V2527-A5 engine burninkerosene and hydrogen, respectively. The design point is take-off: staticlevel, ISA + 10 K, BPR= 4.8, OPR= 28.5. Cruise: Ma= 0.8, altitude=10 668 m, ISA. The thrust requirement at cruise is calculated for an aveaircraft flight mass for a design rangemission assuming a cabin factor65%

Kerosene Hydrogen

Take-off Cruise Take-off Cruise

Thrust requirement [kN] 136 20.9 136 22.6TET [K] 1471 1155 1443 1174SFC [g KN−1 s−1] 9.900 16.826 3.510 6.033SFCKE [gKE KN−1 s−1] 9.900 16.826 9.780 16.80

(fuel flow and thrust versus Mach number and altituprovided in PIANO for different engine types are employThese characteristics are corrected to match the performanobtained by TurboMatch. Installed performance forV2527-A5 engine, both burning kerosene and hydrogedisplayed in Table 1. The emission index of NOx , UHC andCO of this engine is available in the public domain [21] ais given in Table 2.

When changing to hydrogen two fundamental differenthat affect the performanceoccur, namely, reduced fuemass flow and changed composition of the gases expanthrough the turbine(s) [4]. These two have opposite effon the engine performance. While the decreased massreduces the thrust, the changed gas composition cauhigher exhaust pressure, thus, if the nozzle is not choa higher exhaust velocity. In total these changes are sligbeneficial in terms of performance. The performance gcan be seen in Table 1 through lower SFC in termskerosene equivalent1 (SFCKE) for the hydrogen engine foequal TET. At cruise altitude this profit is eliminated duethe higher thrust requirement for the cryoplane.

The present and the future research (at least fornext decades) concerning civil aircraft engines will focon reducing both fuel consumption and emissions of Nx .Since measures to reduce the SFC, such as increthe OPR and the TET, tend to increase NOx formationand vice versa, lowering both fuel consumption and Nxemissions simultaneously is a very challenging task. Varattempts to assess the SFC and NOx reduction potentiaare made, both in the context of theoretical studies anobjectives of commercial development projects [10,12,Depending on the time frame, the potential in SFC reducfor kerosene-fuelled engines is believed to be betweeand 20%, while the corresponding numbers for NOx rangefrom 50 to 80%. The references differ slightly, but generathe reductions are compared with technology levels ofmid-1990’s. In the present study a SFC improvemen8% compared with the V2527-A5 engine performance d(Table 1) will be assumed for both engines to reflec

1 KE = Kerosene equivalent, i.e. the mass of kerosene that correspto the mass of hydrogen of the same energy content.

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Table 2Selection of measured certificate data for the V2527-A5 engine [21]

Mode Power setting EI of NOx EI of UHC EI of CO(%) (g kg fuel−1) (g kg fuel−1) (g kg fuel−1)

Take-off 100 26.5 0.041 0.53Climb out 85 22.3 0.041 0.62Approach 30 8.9 0.061 2.44Idle 7 4.7 0.105 12.43

Table 3Fundamental performance data of the two investigated aircraft

Conventional – RK 200 Cryoplane – CMR1 20

Engine type V2527-A5 V2527-A5(impr. & scaled) (impr. & scaled)

SSLT [kN] 136 136Design range [nm] 4000 4000No of seats (2 CL, MR) 185 185Wing area [m2] 170 170MTOW [ton] 89.3 87.6OWE (2 CL, MR) [ton] 48.7 61.4Payload [ton] 16.8 16.8Fuel capacity [ton] 23.9 9.4Fuel density [kg m−3] 803 71

technology level of 2010. Furthermore, a NOx emissionindex improvement of 60% is assumed for the kerosefuelled engine. The emission indices of CO and UHCassumed to be unchanged.

As for the NOx emissions when burning hydrogen, theretically there is, in spite of the higher stoichiometric flamtemperature of hydrogen, a potential to achieve lower emsions as compared with engines using kerosene. Thereason for this is that the hydrogen flame has a wider flmability range; particularly the lean limit is substantialower than that encountered for kerosene flames [58]. Thfore the entire operating range may be shifted furtherthe lean region, with considerably reduced NOx emissionsas a consequence. The effects on performance and polemissions of changing to hydrogen are covered in moretail in [50].

Employing the concept of micromix hydrogen combution for an APU, experimental tests have illustrated thatmole fraction of NOx emissions can be reduced by abo86% compared with conventional kerosene-fuelled combustors for full power conditions [7]. By means of modellinwork it was shown by [8] that to decrease the SFC by 1the NOx emissions of a hydrogen-fuelled engine wouldcrease by about 8%. Assuming that it would be possiblattain the same reduction in NOx emission for main engineas was shown by the APU test results, taking into accothe SFC improvement as well as converting to the emisindex, it would be reasonable to assume a reduction potial of about 84%. This is the number that is applied forpower settings in this study.

To conclude, the emission indices that are used instudy, aimed at reflecting a technology level of 2010, arfollows. For the kerosene engine, the NOx emission index is


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The produced amounts of the primary combustion pructs CO2 and H2O are proportional to the fuel consumptioIn the present paper, the used emission indices of keroare 3.16 for CO2 and 1.24 for H2O. For LH2 the emission in-dex of H2O is 3.21 (kerosene equivalent) or 8.94 (absoli.e. per kg LH2).

3.2. Aircraft performance

The airframe technology level of 2010 is governedassumptions of 7.5% reduction in structure weight a4% improvement in aerodynamic efficiency compared wthe current A321 aircraft [40]. By having a lighter anmore aerodynamically efficient aircraft to fulfill a flighmission, the total aerodynamic drag in the cruise segmwill be lower, and the net performance will be remarkaimproved.

4. Global warming potential (GWP) model

Each pollutant produced by aviation has a differentpact on the atmosphere, depending on the quantity emits chemical and light-absorption properties, and on whatitude it is discharged. In order to allow a simple assessmof the global impact, in terms of global warming, from temissions discharged on a certain mission, a simple pararic model is derived following the methodology proposed[29,31], taking into account model results and recommentions from [22]. The methodology is explained in the appdix. This simplified treatment avoids the necessity of rning a sophisticated 3D climate model with its formidaresources; it rather tries to comprise the influence of themerous chemical, physical, and dynamic interactionsparametric sense.

The model provides layer-specific values of the glowarming potential (GWP), which is a measure of hpollutants contribute to the global greenhouse effect relato the effect of CO2 [24]. The original GWP concept (whicis largely adopted here) relates the effect of an instantanpulse emission of a given tracer to a corresponding pemission of CO2, in terms of mass (see Appendix A). Athe lifetime of the tested tracer is different from that oCO2, a time horizon has to be defined to yield a uniqquantitative comparison. The time horizon is somewarbitrary, but a value of 100 years has been establisheda standard. Fuglestvedt et al. [13] provided a critical rev

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Fig. 1. GWP numbers per kilogram pollutant versus altitude.

of the use of GWP as a climate impact measure forpollutants that are not homogeneously mixed throughthe troposphere and that are acting indirectly through tsecondary products. While there are indeed several cavan accepted alternative measure suited for this kindstudies has not yet been proposed.

The application of the GWP concept used here is diretowards the effect of aircraft H2O and NOx (Fig. 1). Watervapour causes a direct radiative impact on the climsystem, whereas the influence of NOx emissions is indirecthrough its chemical impact on the atmospheric ozonemethane concentration: at altitudes below about 15 km, Nx

emissions cause an increase of the upper tropospheric oand thus contributing to the greenhouse effect, while atsame time increasing the atmospheric OH concentrawhich decreases the CH4 (and CO) lifetime (e.g. [25])The latter lowers the atmospheric CH4 concentration, andthereby reduces the greenhouse effect. Sensitive paramto describe these effects in the parametric model,(ozone) production rates, chemical lifetime, atmosphmixing times, and altitude specific radiative forcingthe species in question have been chosen to represensituation in summer atmosphere at mid-latitudes. Asbe expected from such a highly parameterised model,uncertainties are considerable, in particular with respecthe effect of NOx emissions.

For each pollutant, the global warming potential canestimated by multiplying the emission value in kilograby the measure of that pollutant for each altitude (Appendix A). Total GWP number for a certain missionthen the sum of the individual GWP due to CO2, H2O andNOx , emitted at different altitudes over the whole missiTotal GWP calculated in this way indicates the mass of C2which would produce the same effect upon the heat balaof the earth as the various substances actually emittetogether, taking a total period of 100 years into account.

According to Fig. 1, per kilogram pollutant, NOx hasthe greatest impact on global warming. For altitudes be4 km it has a negative global warming effect (coolingearth), and as the altitude increases its impact increuntil about 10 km altitude, where it peaks and starts


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The global warming effect of H2O from aircraft cruisingin the troposphere is negligibly small compared to thaCO2, hence it is here assumed to be zero below 10Above 10 km it has a small, but with altitude increasinimpact on global warming (see Appendix A). This isagreement with conclusions drawn in previous studies38] suggesting that the impact of water vapour emissfrom subsonic aviation on the radiative balance is negligsmall.

The model has drawbacks for application on aircemissions. The main drawback lies in its unsuitabilitya means of characterising short-lived emissions, e.g. Nx ,the effects of which show strong regional and seasovariations [1,22]. Another drawback is that the model dnot take into account any possible contrail formation arisfrom the emitted H2O. However, it appears that particompensation occurs between increasing contrail coveragfrom cryoplane traffic (due to higher H2O emissions) anddecreasing optical thickness of cryoplane contrails (dufewer but larger ice particles, as there are less condensnuclei present in the exhaust). The net result of conformation on global warming may, therefore, be in the saorder of magnitude for both cryoplanes and conventioaircraft. According to [44], switching from conventionaircraft to cryoplanes is likely to reduce the global averaircraft climate impact due to contrails by roughly 20This may imply that the conclusions of our study would nfundamentally change, if contrail effects had been includAs for many other aspects of aircraft related climate impacall matters in this issue are not yet clarified.

In the end, in spite of its limitations and simplifieapproach, this model is believed to be an appropriatefor the purposes of this study.

5. Results

Before the aircraft are re-designed, an important difence between the two aircraft in terms of energy consution needs to be highlighted. If operation of both aircris simulated under datum conditions, i.e. a design range o4000 nm (7408 km), assuming a cabin factor of 100%a maximum cruise altitude of 39 000 ft (11.9 km), the fuconsumption expressed in kerosene equivalent is abouthigher for the cryoplane than for the conventional, in spof the fact that the MTOW is slightly less (by about 2%) fthe former. The explanation for this is partly that the zlift drag is higher for the cryoplane (3.5% penalty in drpolar compared with the equivalent conventional configu

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Fig. 2. Effect of cruise altitude on aircraft characteristics.

Fig. 3. Effect of cruise altitude on fuel burn and emissions.

tion), due to a less efficient aerodynamic design, and pathat the average flight weight is higher, owing to a higstructure weight for the cryoplane than for the conventioairplane.

All results in the following sections refer to fuel cosumption and emissions of climb, cruise and descent frange of 2500 nm with a cabin factor of 65%. For pracal reasons, the fuel consumption and emissions relateoperation below the take-off screen height (35 ft) are nocluded. This limitation is not expected to affect the qualtive results and conclusions.

5.1. Effect of reduced cruise altitude on aircraftcharacteristics, fuel burn and emissions

In order to get an understanding of how the aircrconfigurations, fuel burn and emissions change duecruising at an altitude lower than the optimum (in terof energy consumption), both aircraft are optimised fordatum as well as for three lower cruise altitudes. In Fig. 2changes in aircraft characteristics, and in Fig. 3 the changein fuel burn and emissions, are presented as functionflight levels, for both the conventional airplane andcryoplane.

It may be observed in Fig. 2 that as the cruise altituis reduced, the wing area increases for both the airplawhile the wing span is constant or slightly decreasedtotal, the take-off mass increases to a smaller extent, w


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can be explained as follows. When cruising at an altitdifferent from the optimum, in terms of minimised enerconsumption, the required fuel flow increases in ordeovercome the increased drag (see Fig. 3). Increased fueduring the cruise phase – which represents the main shathe flight – results in an increased fuel burn for the missAs a consequence of the increased fuel burn, the airtake-off mass increases, due to both the increasedweight required for the mission and the increased structweight, including the need for bigger lifting surfaces athe take-off weight has increased. Hence, when cruisinreduced altitude, the aircraft become heavier and the warea increases.

The effect of increased wing area, and to a smaextent also the take-off mass, is more emphasized forconventional aircraft than for the cryoplane. Since liqhydrogen has a lower energy content per volume compwith kerosene (four times lower), it is thereby expectedhave a greater impact on the configuration for the saamount of extra energy required, this result might afirst sight appear to be slightly surprising. When the crualtitude is reduced and the amount of fuel required formission increases, larger tank volumes are demandeda consequence of the conventional aircraft storing itsin the wings, the wing size becomes the critical point,order to carry the additional fuel required for the missiThis implies that the wings need to be enlarged more trequired for performance needs (e.g. achieving the requtake-off performance).

The cryoplane, on the contrary, which stores its fuethe fuselage, is affected through a longer fuselage wmission fuel burn is increased. Under the circumstanstudied here, the results suggest that a larger performpenalty is indicated by storing the additional fuel in twings rather than in the fuselage. It is expected that thimpact on configuration and fuel burn for the conventioaircraft would be less if some of the additional fuel requiwhen lowering the cruise altitude is stored in the fuselathus avoiding the wing size being the critical point. Thhowever, would imply that the conventional aircraft wouno longer be conventional, but a novel aircraft configuratFurthermore, preceding investigations performed by thauthors, suggest that the impact on the aircraft configurabecomes larger for the cryoplane than for the conventioaircraft if part of the fuel is stored in the wings of thcryoplane. Obviously, the matter of where to store theseems to make a large difference on mission performawhen altering the cruise altitude.

According to Fig. 3, the fuelconsumption increases whethe cruise altitude is reduced, and since the emissionCO2 and H2O are proportional to the fuel consumptiothey also increase by the same percentage. The UHCand NOx emissions are not, as CO2 and H2O, proportionalto the fuel consumption, but are dependent on the speconditions in the engine’s combustion chamber, suchtemperature, pressure, equivalence ratio (a measure o

f

l

e

e

fuel flow related to the air flow in the combustor) athe physical design of the combustor. Thermal NOx isformed by the oxidation of atmospheric nitrogen in higtemperature regions of the flame and in the postflame gawhereas UHC and CO are formed in low temperatregions, due to incomplete combustion (e.g. [37]). Whencruise altitude is reduced, the power setting during crumust, if the cruising velocity should be kept constant,increased to overcome the increasing drag. An increasepower setting implies a higher combustion temperatwhich raises the emission index of NOx and diminishes theemission indices of UHC and CO. In addition, sincepower setting is increased, the fuel flow is also increaaffecting all emissions detrimentally. As for CO, the effeof the lowered emission index is larger than the effecincreased fuel flow (see Fig. 3), causing reduced emisswhen the cruise altitude is lowered. For UHC, the effof increased fuel flow dominates for the lower altitudGenerally, the emissions of NOx increase more than thfuel consumption, and the largest increase occurs forconventional aircraft.

5.2. Effect of reduced cruise altitude on global warmingpotential (GWP)

By employing the described GWP model, a measureGWP due to CO2, H2O, NOx and the sum of these cabe estimated for an aircraft performing a specific missConcerning the conventional aircraft, CO2 is the majorcontributor to global warming, H2O and NOx being ofsimilar magnitudes which are essentially lower than thaCO2. As the cruise altitude is reduced, the effect of Nxincreases slightly, whereas the effect of H2O decreases tbecome zero at FL 270. As the effect of CO2 on globalwarming is independent of the discharging altitude and sthe emissions of CO2 increase by decreasing altitude, tinfluence on global warming increases by decreasing cruialtitude. The net result, i.e. the sum of the contributions frCO2, H2O and NOx , is a GWP curve mostly influenceby the effect of CO2, which continuously increases as tcruising altitude is decreased. Hence, these results suggethat there seems to be no significant global warming benefifrom lowering the cruise altitude for conventional air traf(see Fig. 4).

For the cryoplane the situation is different. As the fucontains no carbon, the CO2 curve vanishes. The shapof the NOx curve is similar to that of the conventionaircraft, but the magnitude of the GWP values is smaas the cryoplane has been assumed to emit less NOx . As thecryoplane discharges significantly more water vapourtimes more if the same energy consumption is assumthe H2O effect of this aircraft is essentially greater thfor the conventional aircraft. In spite of increasing H2Oand NOx emissions when decreasing the flight altitude,contribution to global warming is continuously decreas


the

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Fig. 4. Effect of cruise altitude on GWP for both aircraft.

Fig. 5. GWP versus fuel burn for reduced cruise altitude.

with decreasing cruise altitude (Fig. 4) (at least forconsidered flight levels).

In total, the global warming effect of cryoplanesconsiderably less for all cruise altitudes, becoming smaas the cruise altitude is reduced. For the datum cruise alti(11.9 km), the contribution is slightly less than 40% of thof the conventional aircraft, and for flight level 270 (8.2 kthe contribution has decreased to a few percent compwith the conventional aircraft cruising at the datum level.

Finding the optimum cruise altitude from the enviromental point of view for the cryoplane is in this case a traoff between, on the one hand reduced global warming,on the other hand, increased operational costs due tcreased fuel consumption and increased investment dueheavier construction. The important issue is to find a crualtitude, and its corresponding configuration, that allows foa significant reduction in the GWP, with a reasonable penin fuel consumption and structure weight. In order to fithe optimum, the GWP is plotted versus the fuel burn, ascruise altitude is reduced (Fig. 5). As for the conventioaircraft, as far as this study is concerned, there is no oous reason for lowering the flight altitude, since it appe

a

that the fuel consumption would increase for no reductioglobal warming.

For the cryoplane, on the other hand, the possible retion in global warming is limited only by the amountincrease in take-off mass and fuel consumption that iscepted. At the end of the day, it becomes a question of mufacturing and operation cost of the aircraft, which are dementally affected by increased aircraft TOM and fuel csumption. If the flight level is reduced to FL 310 (9.4 kmthe contribution to global warming is reduced by about 6for a penalty of about 2% in TOM and 12% in total fuel cosumption. Furthermore, there is a tendency towards hipenalty in fuel consumption the lower the reduction isplied, i.e. the penalty in fuel consumption is higher whreducing from FL 310 to 270 than from FL 390 to 350.is thus questionable if the possible potential of reducingGWP further by lowering the flight altitude below FL 310justified by the increased cost for manufacturing and opeing the aircraft.

6. Conclusions and discussion

In this paper indicative results are presented, showhow changes in aircraft fuel technology and flight altitudemay influence the aircraft configuration as well as thevironmental impact. The paper is aimed at stimulatingther research work on flight altitude optimisation to reduaircraft environmental impact. From the results obtaiit is difficult to draw any confident conclusions regardiwhether there are any significant gains in terms of envirmental impact by lowering the cruise altitude. The mreason is that the results are highly dependent on thesumptions (e.g. the geographical location under considtion, and the type of model used to derive the figures) min the GWP model, which is derived by comprising thefluence of the numerous chemical, physical, and dynamiteractions in a parametric sense (see Appendix A). The relative influence of NOx emissions compared with CO2 and thealtitude where the impact of NOx emissions peaks are fators that highly influence the result. Conclusions concernthe effects on aircraft characteristics, fuel consumption anpollutant emissions when reducing the flight altitude, onother hand, are much more reliable.

When comparing two equally performing aircraft – opowered by kerosene and one powered by LH2 – the energyconsumption is inevitably higher (in this case by about 10for the cryoplane. This is due to the higher structural weiand the higher drag for the cryoplane configuration. Reducruise altitude has smaller impact on the cryoplane thanthe corresponding conventional aircraft, in terms of physsize, fuel burn and emissions. This means that, compwith the datum case, these quantities will increase min percentages for the conventional airplane than forcryoplane, when the cruise altitude is lowered. The mreason for this is a different fuel storage configuration.


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In spite of the uncertainty of the results concerningchange in environmental impact of lowering the cruisetitude, there seems to be a substantial qualitative differebetween the conventional aircraft and the cryoplane. Thesults suggest that the contribution to global warming is csiderably lower from the cryoplane than from conventioaircraft, particularly if the flight altitude for the cryoplane ireduced. That the effect upon the radiative balance of eis smaller from cryoplanes than from conventional aircraalso suggested by [42]. Ponater et al. [42] cover the effof changed CO2 emissions, NOx emissions and a changecontrail radiative impact of gradually introducing cryoplaninto the global air traffic according to different transition scnarios. According to their best estimate, a relative reducof aircraft induced radiative forcing in 2050 by between 1and 29% (depending on the speed of transition) couldachieved if cryoplanes were introduced. Due to inherententific uncertainties this range widens to between 14%40%.

Whereas the environmental impact of the conventioaircraft tends to increase, it seems to decrease essenfor the cryoplane when lowering the flight altitude. Providedthat an increase in fuel consumption in the order of 10%an increase in TOM of a few percent are accepted, the resuggest that cryoplanes should cruise at an altitude of a2–3 km below where conventional aircraft cruise today.this reduced flight level, the contribution to global warmifrom the cryoplane is slightly less than about 15% of tof the conventional aircraft cruising at the datum level.addition to the aspects considered here, reducing the flaltitude will help to avoid the formation of contrails [55Inevitably, this change in cruise altitude causes increaaircraft investments and operating costs.

In order to reduce the cruising altitude, air traffic maagement aspects need careful attention before such a measure may be realised. Williams et al. [55] studied the opof limiting the cruise altitude as a means to reduce themation of contrails. Their results indicate that this stratecould provide a net benefit to the climate, despite the assated increase in CO2 emission. According to their analysithe most likely operational obstacles to such a schemethe implications for controller workload, hence a reconuration of the air space would be required to mitigateimpacts.

Acknowledgements

This paper is a summary of a study that was carout during 2001 and 2002 within the European-sponsoproject CRYOPLANE. The authors would like to expretheir gratitude to Dr. Heinz-Günter Klug at Airbus Germaand Dr. Michael Ponater at DLR for helpful comments andadvice during the work and on the present paper.

y

t

Appendix A. Derivation of a GWP model

If aircraft produced particles and their effects upon cloare disregarded (see Section 4), the aircraft emissions athe radiative balance of the atmosphere directly throradiative forcing of CO2 and water vapour. Furthermoremissions of oxides of nitrogen affect the radiative forcindirectly through changes in concentrations of ozonemethane, which both are greenhouse gasses. The athis section is to derive a method for calculating the glowarming potential (GWP) of H2O and NOx emissionsemitted by aircraft. Rather than employing a sophisticate3D climate model, a simple parametric method is ufollowing the methodology proposed by [29,31], taking inaccount model results and recommendations from [22order to consider the nonlinearity and the large variabilitythe indirect effect of short-lived NOx emissions on radiativforcing, a plume model is employed.

The global warming potential of a trace gas is the ratiothe global warming through emission of one kilogram ofgas to the global warming through emission of one kilogrof CO2:

GWPi (T ) = Ei(T )

ECO2(T ), (A.1)

whereT is the time horizon andEi is the warming effecof the trace gas,i. The life time,τ , of the trace gas usualldiffers from the life time of CO2. Therefore, the warmingeffect of a trace gas is connected to a correspondinghorizon and the integrated radiative forcing, RFi :

Ei(T ) =T∫

0

RFi · e−t/τ dt = RFi · τ · (1− e−T/τ). (A.2)

Generally GWP varies with the time horizon chosen. Tically horizons of 20, 50, 100 or 500 years are used.the GWP calculations in this study, the time horizon of 1years is chosen. All data are intended to represent theation in summer atmosphere at mid-latitudes if such restion is possible.

The radiative forcing of a trace gas is related toclimate change expressed as change in surface temper�T0, through a climate sensitivity parameter,λ:

�T0 = λ · RF. (A.3)

The climate sensitivity parameter generally varies forferent species and altitudes. However, for the altitudequestion (0–15 km),λ differs within a factor of 2 (a value0.92 K W−1 m2 is recommended in [22]). Generally, RFseen as a parameter which islinearly related to climaticchange [22,23].

Increment of species,Y , at a given altitude,h, causes achange in radiative forcing. The change can be quantusing a sensitivity function,S(h), which determines thechange in RF at the tropopause level with stratosph


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RF(h) = �Y(h) · S(h). (A.4)

If Y is produced in the atmosphere from the emissionspeciesX and the lifetime of the emission is short in relatito the time horizon, the increase in the total columnY (DU) in the atmosphere over a certain area due toemission,Q (kg), is:

�Y(h) = Q ·τx∫

0

(Py(h)

)dt · C, (A.5)

where Py(h) is the production rate, i.e. the numbermolecules produced by one moleculeX released in theatmosphere within the lifetime ofX, τx . If Y is emitteddirectly, the integral expression in Eq. (A.5) vanishes. TconstantC has the unit DU kg−1 and recalculates the maof Y to a column expressed in DU for the size of the regexamined,A (m2):

C = 108 · Vmol

MWX · A. (A.6)

MWX is the molar weight ofX, Vmol = 22.414× 10−3

is molar volume at 273 K and 101.325× 103 Pa (comesfrom the definition of DU) and the numerical value comfrom the recalculation of m to 0.01 mm and of kg toFor example, taking the emission rate of NOx in kg andconsidering species being dispersed over the area bet30 and 60◦N, the parameterC gets a value of 5.24× 10−10

DU kg−1.The production ratePy(h) of secondarily formed green

house gases concerns the formation of ozone and thestruction of methane through reactions with oxides oftrogen. As argued in [22], the GWP concept is not wsuited for short-lived gases that are formed at producrates which are strongly dependent on atmospheric cotions. However, since there are no methods available thasuitable for our study, we approached the problem bysuming that the species are affected by oxides of nitrounder a rather short lifetime of NOx , using a fairly detailedchemistry description of the dispersing plume. The ozformation and methane destruction are evaluated undeferent conditions, and average values to describe the protion/destruction are used. Dispersion and the radiative fing of the ozone and methane are then estimated from reof global models.

In order to estimate the ozone production and methdestruction, a plume model of aircraft emissions, MAP [3is employed. The model simulates an entire flight missof an aircraft using aircraft emission data as descriin this work. In the model, the emissions in a dispersaircraft plume at engine outlet (approximately 10 s beh

2 One Dobson Unit (DU) is equal to a 0.01 mm thick column of gas273 K and 101.325× 103 Pa.

n

-

-

the engine) are followed for 10 days. The initial plumprocesses involving chemical and gas-to-particle converof emitted gases in the cooling plume were adopted fdetailed studies by [14,35,36,51]. The plume expansiothe vortex regime is describedusing parameterisation o[46]. After the turbulent dissipation of the vortex, i.e. aftwo minutes of simulation, the plume goes over todispersion regime when the plume expansion is induby atmospheric diffusion. The plume is then describedthe Gaussian approximation [33,47]. The chemical reacscheme of the expanding plume is based on work by [19,which has been further developed to consider the gas pand heterogeneous reactions possibly important for ozcycling in the tropopause region and in the stratosphere.chemical description considers 100 species in over 310phase and 15 heterogeneous reactions.

To consider the variability of ozone formation and effeon methane from oxides of nitrogen, a number of simutions involving different seasons, time of the day and ltudes are studied. Effects of both conventional and hydroaircraft are compared. As a result of this study it can be ccluded that production of ozone relative to emitted NOx islittle sensitive to the time of the day when emission taplace and is not sensitive at all to the aircraft type (whthe cruise phase is considered). There is large variabilitPO3(h) with altitude over the latitudes which coincide withe position of the tropopause and with the level of NOx pol-lution in this area. Table A.1 gives an overview of the resu

The GWP is a relative number where the RF of ospecies is related to the RF ofcarbon dioxide (CO2) (seeEq. (A.1)). It is important that the two forcings which acompared are based on the same assumptions. The RCO2

used here is based on the parameterisation of [16] whichagreement with [22]. The RF of additional CO2 is calculatedaccording to:

RFCO2 = f (c) − f (c0), (A.7)

where

f (c) = 4.996· ln(c + 0.0005· c2),

wherec andc0 are the CO2 mixing ratios after and before thaddition, respectively. The RF values do not increase linewith �CO2, and are dependent on the CO2 mixing ratiosinto which the additional amount of CO2 (ppm(V)) is added.However, for�CO2 bellow 10 ppm(V), the increase in Rcan be assumed to be linear, and thusSCO2 can be assumed tbe constant. For a 370 ppm(V) CO2 level, the dRFCO2/dCO2has a value of 0.0156 W m−2 ppm(V)−1, which, assuming1 ppm(V) throughout the whole atmosphere and the nber density profile of the US standard atmosphere, cosponds to anS value of 1.947× 10−5 W m−2 DU−1. Thelifetime of CO2 is assumed to be 150 years [23] andis homogeneously mixed over the same area as the eined species but throughout the entire vertical profile.suming a lifetime of CO2 of 150 years and a time horzon T of 100 years, the warming effect of CO2, ECO2, is


tedt

Table A.1Chemical lifetime of ozone, averageproduction/destruction of ozone P(O3) and methane P(CH4) in summer mid-latitudes due to oxides of nitrogen emitat different altitudes (for methane 2 different values for conventional and H2 aircraft are presented), and sensitivity function, S, for 1 DU ozone change [17] adifferent altitudes used in the model

Altitude O3 lifetime P(O3) S(O3) P(CH4) (g(CH4) g(NOx )−1)

(km) (d) (g(O3) g(NOx )−1) (W m−2 DU−1) Conv. H2

2 16 5.3 0.009158 −0.91 −1.054 24 7.2 0.0174 −0.72 −0.756 35 12.6 0.027052 −0.70 −0.728 52 21.4 0.032671 −0.46 −0.55

10 79 20.2 0.025889 −0.43 −0.5212 132 16.3 0.012964 −0.30 −0.3914 380 1.3 0.004449 0.10 0.0116 300 3.1 −0.0025 0.08 −0.01

d

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Table A.2Lifetime and sensitivity function versus altitude of water vapour

Altitude (km) τH2O (yr) SH2O (W m−2 DU−1)

0 0.040 0.01 0.041 “2 0.042 “3 0.044 “4 0.046 “5 0.050 “6 0.054 “7 0.058 “8 0.068 “9 0.110 “

10 0.250 5.55× 10−4

11 0.360 “12 0.450 “13 0.550 “14 0.650 “15 0.750 “

7.736×10−13 W yr m−2 kg−1 if the affected area is assumeto be between 30 and 60◦N, i.e. 9.36× 1013 m2. The area isa factor 5.5 smaller than the earth’s surface and is usedsequently for all species. As RF is considered to increlinearly with the concentration changes considered herearea is cancelled when the warming effects are relateeach other in the GWP calculation.

Radiative forcing of thewater vapour is coupled to largeuncertainties due to a huge variability of its concentrationin the troposphere where it is employed in the hydrologcycle. Furthermore, it is extremely sensitive to temperaturfeedbacks, such as sensitivity of cloudiness, evaporatiotensity and air water vapour content on surface tempera[22]. In this study RFH2O values obtained from a model eperiment [6] have been used. According to [6], the RF attropopause level (10 km) from rising H2O concentration by10% between 10 and 15 km altitude is 0.65 W m−2, result-ing in anS value (10–15 km) of 5.55× 10−4 W m−2 DU−1.This value does not include any stratospheric adjustmThe residence times used in Eq. (A.2) are listed in Table A.2The original lifetimes from [32] are adjusted around ttropopause region according to [29].

-

Radiative forcing ofozone is evaluated by combininplume modelling for ozone production calculations aestimates based on global models for radiative forcing ofozone formed from oxides of nitrogen.

• The ozone production rate,P(h), is highly non-linearand depends on the chemical composition of themosphere and on the radiation intensity. Productionozone integrated over the lifetime of NOx emitted at al-titude is calculated under a range of conditions (varyingtime of the day, time of the year and latitude). Seeble A.1.

• The chemical lifetime of nitrogen oxides is taken fro[11], who used a quasi-2D model. Values are varyfrom some hours at the ground to a few days in the lostratosphere.

• The sensitivity function,S, has been taken from [17who performed model experiments with the Wonderlaclimate model where they have added 100 DU ozonone model layer after the other and calculated the Rthe additional ozone for a period of 50–100 years athe addition. The values used at altitudes in questiondisplayed in Table A.1. From sensitivity tests of climaresponse to ozone changes at a certain altitude,concluded that the response could be linearly scalethe amount of ozone.

• The chemical lifetime of O3 needed in Eq. (A.2) islonger than the chemical lifetime of NOx . The lifetimespresented by [15] are used here (see Table A.1).

In order to consider the influence of mixing on tlifetime of ozone formed at a certain altitude, the followivery simple assumptions are made [31]:

• Since the lifetime of NOx is known to be much shortethan the lifetime of O3, only transport of O3 is included.The O3 is produced exclusively within a layer of one karound the emission height of NOx .

• The produced ozone is mixed equally down toground and one layer upwards. Right below the tro


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pause no upward mixing is possible. Above the tropause no mixing takes place at all.

• The tropopause is assumed to be located at the followaltitudes with the following probabilities: 10 km: 6%11 km: 22%, 12 km: 44%, 13 km: 22%, 14 km: 6%.

This mixing scheme only affects the ozone lifetimes, simixing is already included in the sensitivity functionozone,SO3 , from the model simulation.

Emissions of oxides of nitrogen affect mixing ratiosmethane through increased concentration of OH radicand hence decreased lifetime of methane. To estimateradiative effect of decreased methane concentrationmethod similar to that used for ozone is employed:

• The production rate,P , for methane is calculated usinthe MAP model. As the reduction of CH4 is mainlydriven by the increase of OH through the reactHO2 + NO → OH + NO2, τx in Eq. (A.5) isequal toτNOx . The production rates are summarisedTable A.1. There is a difference in methane betweffects of emissions from conventional and hydrogfuelled aircraft. This difference is, however, rather smawhen the effect of emissions on ozone is taken iaccount, and hence, the GWP values of NOx areassumed to be the same for both aircraft types.

• The chemical lifetime of NOx is assumed to be the samas assumed for the ozone calculations.

• The differential RF to mixing ratio of methane, dRF/

dCH4 (W m−2 ppm(V)−1) is taken from [16]:

RF= 0.0406· (√m − √m0

)− [

g(m,n0) − g(m0, n0)], (A.8)

where

g(m,n) = 0.5 · ln[1+ 2 · 10−5(m · n)0.75],

wherem0 andm are CH4 mixing ratios before and after the CH4 change, andn0 and n is the N2O mix-ing ratio before and after the change. The RF valdiffer for CH4 and N2O mixing ratios into which theadditional amount of CH4 (ppm(V)) is added, and dnot respond linearly to the increment of the CH4 mix-ing ratio. However, for a narrow range of�CH4, thechange in RF can be assumed to be linear. For C4levels corresponding to the year 2000, i.e. 1.7 ppmdRF/dCH4 is 0.38 W m−2 ppm(V)−1 when 1 ppm(V)change is considered, which is in agreement with thcorresponding value of 0.37 W m−2 ppm(V)−1 stated in[22]. For�CH4 smaller than 0.1 ppb(V), the dRF/dCH4is equal to 0.4297 W m−2 ppm(V)−1, which, assuming1 ppm(V) throughout the whole atmosphere and nuber density profile of the US standard atmosphere,responds toS equal to 5.362× 10−4 W m−2 DU−1.

• The global chemical lifetime of CH4 (used in Eq. (A.2))is assumed to be nine years [22], and independenaltitude.

Table A.3GWP figures for CO2, H2O and NOx versus altitude

Altitude (km) GWP (CO2) GWP (H2O) GWP (NOx )

0 1 0.00 −7.11 1 0.00 −7.12 1 0.00 −7.13 1 0.00 −4.34 1 0.00 −1.55 1 0.00 6.56 1 0.00 14.57 1 0.00 37.58 1 0.00 60.59 1 0.00 64.7

10 1 0.24 68.911 1 0.34 57.712 1 0.43 46.513 1 0.53 25.614 1 0.62 4.615 1 0.72 0.6

Using a methodology similar to that explained here aincluding vertical mixing, [31] calculated GWP numbedue to NOx emissions versus altitude. They compatheir result with 14 different earlier model simulationsvarious authors performed to estimate the change in oconcentration due to air traffic [3,5,27,28,56,57]. With 1NOx emission between 10 and 12 km altitude, the moruns yielded a GWP value ranging from 120 to 400, withaverage of approximately 220. Other published GWP vaof NOx cover a wide range of values between 210 [and 456 [26]. In order to obtain these values [28] assumthat the NOx emissions are emitted at 0–12 km altitudwhereas [26] assumed an emissions height of 8–10In the present study, values ranging from 1.7 to 6.6 timlower are derived. The deviation is due to using a differmethodology, mainly explained by the following factors:

• As oppose to the GWP model derived by [31] (wheris assumed that the warming function of CO2 is basedon the area of the whole earth), the warming functionCO2 is based on the same area as the warming funcof other species, resulting in a warming function of C2about 5.5 times higher than that based on the whearth’s surface area.

• In the large-scale models the emissions are dispedirectly over a large area which makes the ozonemation more effective, giving an overestimated valueozone production between 30 and 150%. The deviabetween plume models and global models was founding the MAP code. In addition, this deviation was fouby [34,41].

Having these factors in mind, the numbers obtained hare comparable to the values presented in the literatureglobal warming values of NOx and water vapour used in thpaper are presented in Table A.3 and in Fig. 1.


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References

[1] Air travel – Greener by Design – The Technology Challenge, Socof British Aerospace Companies Limited, London, 2001.

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