GEOPHYSICAL RESEARCHLETTERS, VOL. 25, NO.10, PAGES 1717-1720,MAY 15, 1998
DC-8-basedobservationsof aircraft CO, CH4,N:O, and HaO(g•
emissionindices during SUCCESS
S. A. Vay,•B. E. Anderson,
• G. W. Sachse,
2J. E. Collins,Jr.,3J. R. Podolske,
4
C. H. Twohy,-•B. Gandrud,
• K. R. Chan,4S. L. Baughcum,
6andH. A. Wallio•
Abstract. We report the first measurements
of CO, CH4, in quantityaccordingto the temperature,pressure,and other
N20, CO2,andH20(g)in theexhaust
trailsofT-39, B-757,and combustorconditions[Baughcumet al., 1996]. The emission
DC-8 aircraft at cruise conditions. Emissionindices(El) of nitrous oxide (N20) is likely small from turbine
derived from these in-situ measurementsare presented. combustors
[Brodericket al., 1975;Zhengeta!., 1994;Fahey
Resultsare in agreementwith ground-basedtestsindicating et al., 1995b]. SinceCO2,H20(g),
C1-14,
andN20 arenoted
aircraft act as a net sink for CH 4 and recent airbome in-situ greenhouse
gases,andthe oxidationof CO servesas a major
measurementsthat N20 is not an important exhaust sinkfor OH, quantifyingthe amountof thesespeciespresent
constituent.
Condensation
of H20(g)on exhaustparticles in aircraftexhaustis paramountto the understanding
of the
resulted
in EI(H20(g))
valueslessthanthoseexpected
fromthe effectof aviationuponlocal,regional,andglobalatmospheric
combustion of fuel alone. Observed apparent negative
processes.Presented
hereare observations
of CO2,H20(g),
EI(H20(g))valuessuggestthat aircraftaerosolemissions, CO, CH4, and N20 made within the exhaustplumesof three
underunique atmosphericconditions,seed cloud formation
different aircraft at cruise conditionsand the resulting
andleadto dehydrationof the exhaust-influenced
air parcel. emission indices calculated from these measurements.
Such conditionsmay induce the formationof cirrus clouds
from persistentcontrails.Comparisons
with the BoeingEMIT
Code show measurement-derived CO emission index values
consistent with model evaluations.
Experimental
The
Subsonic Aircraft:
Contrail
and Cloud
Effects
Special Study (SUCCESS) experiment was an airborne
Introduction
campaignbasedout of Salina,Kansasduringthe springof
1996. NASA's DC-8 aircraftservedasthe primarysampling
Engineexhaustemissions
of gasesandparticlesfrom fleets
platform while their T-39 and Boeing 757 (B-757) aircraft
of subsonic aircraft, operating primarily in the upper
providedthe emissionssourcefor the DC-8 target aircraft.
troposphereand lower stratosphere,are potentially in
Standard
JetA fuel wasburnedby all threeaircraftduringthe
sufficientamountsto affect atmosphericozone and climate
experiment.On a few flights,the B-757 switchedin-flight
[NASA, 1997]. Until recently, the only source-strength
betweenfuel tankscontainingJetA fuel havingeithera high
informationavailablefor assessing
the environmentalimpact
or low sulfurcontent(675 vs.75ppm S).
of these emissions has come from ground-basedtesting of
Measurements
of CO, CH4, and N20 were madeusinga
aircraftenginesin indoorfacilities. It is uncertain,however,
three-channel, mid-IR
diode laser-based differential
whetheremissionindices(El) derived from these data are
absorptioninstrument while CO2 was measuredwith a
representative
of aircraftoperatingat cruisealtitude(9-13km).
modifiedLi-Cor model6252 non-dispersive
infrareddetector.
Thus,with fuel usageby scheduled
airlinerandcargoaircraft
Samplingof thesetracespeciesoccurredthrougha common
projectedto triple by 2015 [Baughcumeta!., 1994],
inlet and associated
plumbing. Calibrationswere performed
increasingemissionsby approximately 220% from 1990
at 10 min. intervalswith measurement
accuracycloselytied to
levels[Stolarskiand Wesoky, 1993], thereis a growingneed
the
primary
calibration
standards
obtained
from the National
for in-situ measurements of aircraft effluents under actual
OceanicandAtmospheric
Administration/Climate
Monitoring
operatingconditions.
and DiagnosticsLaboratory. Data were recorded and
The primarygaseousemissionsfrom aircraftenginesare
archivedat 1Hz with precisions(1•) for CO, CH4, N20 and
carbondioxide(CO2)andwatervapor(H20(g))produced
by
CO2of 1 ppbv,1 ppbv,0.2 ppbvand50 ppbvrespectively.A
the combustionof jet fuel. Carbon monoxide(CO) and
detailed discussionof the operationof these instruments
methane(CHn) are alsoproducedin the combustors
andvary
duringpreviousaircraftmeasurement
campaigns
is givenby
Sachseet al. [ 1991] andAndersonet al. [ 1993].
Watervapor(H20(g))wasmeasured
utilizinga fastresponse
IAtmospheric
Sciences
Division
NASALangley
Research
Center
•Aerospace
Electronic
Systems
Div.NASALangley
Research
Center (-50 msec) near-IR diode laser (1.4gin) sensorrecently
3Science
andTechnology
Corporation
developed[Collins et al., 1995] and flown on two missions
hearthScienceDivisionNASA AmesResearchCenter
prior
to SUCCESS. This sensoris comprisedof a compact
5National
Center
forAtmospheric
Research
laser transceiver mountedto a DC-8 window plate and a
6Boeing
Commercial
Airplane
Group
sheetof high-graderetroreflecting
materialthat is appliedto
Copyright1998by theAmericanGeophysical
Union.
an outboard DC-8 engine housing. Using differential
Papernumber98GL00656.
0094-8534/98/98GL-00656505.00
absorption
detection
techniques,
H20(g)is sensedalongthis
28.5 m externalpath. An algorithmcalculates
1717
1718
VAY ET AL.: IN SITU OBSERVATIONS OF AIRCRAFT CO, CH4, N20, AND H20 EMISSIONS
Table 1. Flightparameters
for plumemeasurements
duringSUCCESS
Emissions
Source
Date(UTC)
(engine)
T-39
(JT-12A-8A)
DC-8
Pressure
Separation
Altitude
(km)
(km)
Mach#
FuelFlow*
(lbhr
4)
960418
960427
9.5
9.3- 10.7
2.4 -12.7
0.3- 10.4
0.66 - 0.76
0.60- 0.78
-1000
-1000
960429
12.5
self-exhaust
0.70 - 0.73
2500
960503
960504
960507
S Jet A**
9.0 - 11.6
11.5- 12.2
11.0 - 11.8
11.2
4.4 - 11.9
6.2- 18.7
9.5 - 44.1
17.0 - 32.0
0.59 - 0.71
0.71-0.72
0.70 - 0.74
0.70
2515 - 3510
2375-2713
2700 - 2869
2803
(CFM56-2-C1)
B-757
(RB211-535E4)
*Fuel flow rate is per engine.
**S JetA (Standard
JetA fuelwithno addedsulfur)caseis for a portionof theflighton960507UTC.
concentrationbased on the differential absorptionsignal
magnitude, ambient pressure and temperature, and
in operationtoday whereasDC-8 and B-757 (- 422 out of
748 B-757's have RB211 engines) aircraft are more
in the currentglobal fleet. Median valuesfor
spectroscopic
parameters
thataremeasured
in the laboratory. commonplace
The precisionof the instrumentis 2% of the mixing ratio. the B-757 are belowthoseof the DC-8 on all threedaysof inSUCCESSH20(g) datawererecordedat 20 Hz andarchived situ samplingand indicativeof the newerenginetechnology
at 1 Hz.
on that aircraft.Variability in measurements
behindthe same
Five flightswere conductedduringSUCCESSwith the aircraft can be attributedto the dependenceof EI(CO) on
primaryobjectiveof exhaustsampling.Measurements
of the ambient pressure, ambient temperature, ambient relative
DC-8's own exhaustwere also made (self-exhaust). During humidity,Mach numberand fuel flow rate. As shown in
themajorityof thetime,meteorological
conditions
weresuch Table 1, flight parameterswere quite variable during any
that visible contrails formed. Flight parametersduring givenflight. CO emissionsare highestat low powersettings
where the temperatureof the engineis low and incomplete
exhaustsamplingaregivenin Table 1.
combustionoccurs and they decreasewith increasingfuel
flow [Baughcumet al., 1994]. As an example,the wide range
Results and Discussion
of EI(CO) valuesfor the B-757 on 960503 UTC resultfrom a
The productionof CO2 from the combustion
of fuel is
directlyrelatedto fuel consumption.Measurements
made
simultaneously
with CO2 can thereforebe expressedas EI
whicharedefinedasgramsof speciesemittedperkilogramof
fuel burned. Analysis of the hydrogencontent of the
SUCCESSJet A fuel by the AerospaceFuelsLaboratoryat
KellyAFB yielded13.7% hydrogen
by weightandresultedin
change
in fuelflowrate(3523to 2815lbhr-l),Machnumber
(0.68to 0.59),andengine
gastemperature
(576øC
to 499ø).
Conversely,the compactnessof the data on 960507 UTC
(standardJet A only portion) resultsfrom the reasonably
constantMach number,fuel flow rateandflight level.
Changesin CH4 from backgroundwere observedon a
numberof T-39 and B-757 plume crossingsand found to be
a calculated
EI(CO2)
of 3159g (kgfuel)'xassuming
complete
anticorrelated
with CO2 in 81% of thosecasessuggesting
the
oxidation. For a speciesof interestsuchas CO, EI(CO) =
enginesof theseaircraft actuallyburn a fractionof the CH4
EI(CO2)* 28/44 * [ACO] / [ACO2]. Multiplicationby the
containedin backgroundair. The derivedvaluesfor EI(CH4)
ratioof atomicmassunitsis requiredto convertfrom volume
are noted in Table 2 where emission indices for a given
to weight fractions. Values for the ratio, ACO/ACO2,are
obtainedusinglinearregression
techniques
on time seriesof
individualplumecrossings.Emissionindiceswerecalculated
onlyfor time seriesproducinga linearcorrelation
coefficient Table 2. Emission indices and statisticsfor CO and CH4
of r_> 0.70. Error barsfor eachEI were determinedusingthe
errorof the slopeandaregivenasupperlimits. The errorsare
drivenby the numberof data pointscomprisingthe peak,
peaksizeandshape,andvariabilityof thebackground
level.
Table 2 presentsemissionindicesfor all three aircraft.
Dueto the changingflightparameters,
meanEI valuesarenot
indicated.The El(CO) valuesfor the T-39 are considerably
largerthanthosefor the DC-8 or B-757 reflectingthe older
combustor technology of that aircraft's engines.
For
comparison,recent in-situ emissionmeasurements
of
NASA's ER-2 aircraft,whichis poweredby an oldervintage
range
[Faheyet al., 1995b]. Despitethesecomparatively
largeCO
n
El _ % (1(•)
T-39
960418
960427
-1.08 to-0.37*
15.94- 18.67
15.60- 26.55
-0.52
17.10
20.98
10
10
72
20
10
5
DC-8
960429
2.06 - 3.43
2.53
6
20
B-757
960503
960504
960507
-1.60 to 1.19'
1.02 - 4.45
0.84- 3.86
0.77 - 1.86
-0.02
1.79
1.55
1.16
6
39
15
8
20
13
20
14
J75engine,
revealed
EI(CO)values
of 18.3to 20.2g kg'x
emissions,
it shouldbe notedthat only a few T-39 aircraft are
median
*EI(CH4) values.
n - samplesize.
1719
VAY ET AL.' IN SITU OBSERVATIONS OF AIRCRAFT CO, CH4, N20, AND H20 EMISSIONS
aircraft have been combined for different days due to the
small sample size.
The low number of EI(CH4)
determinationscan be attributedto fewer crossingshaving a
correlation coefficient r&0.70 for ACHdACO2. Since
EI(CH4) is small,plumedilutioncan lead to no detectable
changein CH4 [Zheng et al., 1994].
All detectable
500
.
'•'
• 450
400
'-"'---H20(g)
-X
•'•350•----•"•
o
'
-•i•
H20(i)
H20(s)
300
measurements
behindthe T-39 were found to be depletedin
c• 250
CH4 relativeto the backgroundconcentration. Peak ACH4 ß•- 200
valuesfor thesecasesrangedfrom -5.8 to -26 ppbv. These x
'g 15o
resultsare consistent
with the findingsof Spiceret al. [1992]
whoreportedground-based
testingof F101 andF110 engine
exhaustdepletedin CH4compared
with incomingair usedfor
combustion
at powerlevelsaboveidle. Similarly,Wiesenet
al. [1994] observeddecreasingCH4 concentrations
with
increasing
enginethrustin the ground-based
testingof the
PW 305 engine. For the B-757, CH4 was depletedin three
cases(ACH4 =-8.7 to -7.5 ppbv) and enhancedin three
(ACH4--7 to 19.8 ppbv). CH4 production
occurredduring
periodsof changing
enginethrustconditions.The potential
for someCH4 productionduringcombustion
was notedby
Spiceret al. [1992]. Altitudetestchambermeasurements
of
RB211 (e.g. B-757) engineexhaustshowedno changesin
lOO
60000
60200
60400
60600
UTC (sec)
Figure2. Mixing ratiosfor saturation
with respectto H20(l),
saturation
with respectto I-I20(s),and ambientH20(g)levels
duringperiodsof observed
anticorrelation
of H20(g)relativeto
CO2 between60326 - 60460 UTC on 960427.
formationof condensation
trails in the aircraftwakes.Regarding sampling error, aircraft exhaust plumes exhibit large
CH4concentrations
[Wiesenet al., 1994].
spatialvariabilityin speciesconcentrationso that singlepoint
N20 was foundnot to be an importantexhaustconstituent measurements
(e.g. CO2 data)will seldomcorrespondexactly
which is in agreementwith in-situ measurementresults to concentrations
averagedover 1O's of metersof path length
reportedby Zheng [1994] and Fahey[1995b]. In contrast, (e.g. the H20(g)data). In addition,the effectsof wingtip
altitude chamber tests of the RB211 engine showed N20
concentrations
tendingto increasewith increasingengine
power however,combustorinlet temperatures
were much
higherthan undernormalcruiseconditions[Wiesenet al.,
1994].
Basedon the hydrogencontentanalysisof the fuel and
assuming
100%combustion
efficiency,an emissionindexfor
vorticesand exhaustbuoyancycombineto displaceaircraft
plumesseveraltens of metersvertically from their original
depositionaltitude[Miake-Lye et al., 1993a]. BecauseEI are
calculated from the species enhancementsrelative to
backgroundair and water vapor concentrations
frequently
show steepvertical gradientsin the upper troposphere,this
effect can introducea significanterror in El determinations
waterof 1228g (kg fuel)-1is expected.However,
the forthisspecies.However,thelargedeviations
of EI(H20(.•))
apparentemissionindicesobserved
for H20(g) (EI(H20(g))) from the predicted value occurred when contrails were
duringSUCCESSwereoftensubstantially
differentfromthis formed.In thesecases,EI(H20(g))waslessthanpredicted
and
value. This was expected,due in part to the different indeed, under unique atmospheric conditions, exhibited
sampling
volumesviewedby the CO2and H20(g)instrumentsapparentnegative values, which indicates contrail growth
and to the vertical displacementof the plumes by took placeat the expenseof water vaporfrom the background
dynamical/thermodynamical
effects,but primarilyfrom the atmosphere.
Casesillustratingthis effectarediscussed
below.
Figure1 depictsEI(H20(g))asa functionof thechangein
cloud ice water content(AIWC) for a flight behindthe B-757
wherestandardJet A fuel was beingburnedand illustratesthe
4L
I .
inverserelationship
observed
betweenAIWC andEI(H20(g)).
r = 0.82
Times when sampling the plume, but with no contrail
formation,are reflectedby the valuesat or near 1228 g (kg
fuel)
-]. LowerthanunityEI(H20(g))
values
(i.e.EI(H20(g))
/1228) were realized for both B-757 low and higher sulfur
fuel cases.
No correlation
between the sulfur content of the
fuelandthedegreebelowunityfor EI(H20(g))
wasobserved.
Figure2 showsa time seriesof H20(g)measurements
recordedduringa periodwhenthis specieswas anticorrelated
with CO2 in crossingsof the T-39 plumeandthusproducing
[
400
I
600
I
I
800
I
i
I
1000
I
1200
-1
El(H20(g))
g kg
apparentnegativeEI(H20(g))values. Near-field(< 3 km
I
1400
separation) samples of the aircraft emissions recorded
between60326-60460 UTC, a time during which the T-39
was producinga persistentcontrail,all exhibiteda depletion
concentrations.Indeed
Figure 1. 960507 DC-8 chasingthe B-757 which is burning in H20(g)relativeto background
valuesfor EI(H20(g))rangedfrom-2391 to -4911 g
standard
JetA fuel. EI(H20(g))
atornear1228g kg-] arefor apparent
(kg fuel)'] for thethreecrossings
recorded. Duringthis
plumesamplingwithno contrailformation.
1720
VAY ET AL.' IN SITU OBSERVATIONS OF AIRCRAFT CO, CH4, N20, AND H20 EMISSIONS
period,
thestaticair temperature
was_<-44.5øC
andthe
References
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thansaturation
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Such intercomparisons
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Interim Assessmentof the AtmosphericEf•cts of Subsonic
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A comparison
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on 960429 UTC and the B-757 on 960507 UTC was made
with the Boeing EMIT code. Requiredmodel inputsfor
directcomparison
with flight resultswere ambientpressure,
ambienttemperature,
ambienthumidity,Mach number,fuel
flow rate, and enginetype. During bothperiodsof interest,
standardJetA fuel was beingburnedandreasonably
constant
flightconditions
wereexperienced.Emissioninformation
on
the T-39 JT-12A-8A enginewas not availablethereforean insitu vs. model comparisonwas not possible. From the
intercomparison,
the in-situmeasurements
for the B-757 were
lowerthanthosepredictedby the modeldifferingby 13-21%
while thosefor the DC-8 were higher by 14-30%. This is
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involved and it
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annually
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Usinga globalCO emission
rateof 2100Tg yr'• [Logan, P.O. Box 3000, Boulder, CO 80307.
kg'• at cruiseconditions,
thenthe amountof CO emitted
1994],the CO emissions
from the B-757 commercial
fleet in
2015wouldcomprise
1.74x 104% of theglobal
budget;
a
S. Baughcum,
BoeingCommercialAirplaneGroup,MS 6H-FC,
P.O. Box 3707, Seattle, WA 98124.
clearlyinsignificant
amountassuming
theglobalCO emission (ReceivedJune25, 1997; revisedFebruary5, 1998; accepted
rateis not vastlydifferentin 2015.
February19, 1998.)