NASA Technical Reports Server (NTRS) 20140011852: NF3: UV Absorption Spectrum Temperature Dependence and the Atmospheric and Climate Forcing Implications

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GEOPHYSICALRESEARCHLETTERS,VOL.40,440–445,doi:10.1002/grl.50120,2013
NF : UV absorption spectrum temperature dependence and
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the atmospheric and climate forcing implications
Vassileios C. Papadimitriou,1,2,3 Max R. McGillen,1,2 Eric L. Fleming,4,5
Charles H. Jackman,4 and James B. Burkholder1
Received9November2012;revised18December2012;accepted24December2012;published30January2013.
[1] Nitrogen trifluoride (NF3) is an atmospherically 2008; Zhao et al., 2010; Dillon et al., 2011; WMO, 2011],
persistent greenhouse gas that is primarily removed by UV withshort-wavelengthUVphotolysisbeingitspredominant
photolysis and reaction with O(1D) atoms. In this work, atmosphericlossprocess.The500-yeartimehorizonglobal
the NF gas-phase UV absorption spectrum, s(l,T), was warming potential (GWP) of NF is estimated to be 18,500
3 3
measured at 16 wavelengths between 184.95 and 250nm [WMO,2011],whichiscomparablewiththatofSF ,making
6
at temperatures between 212 and 296K. A significant NF oneofthemostpotentGHGintheatmosphere.NF is,
3 3
spectrum temperature dependence was observed in the therefore,consideredapersistentclimate-forcingagentwith
wavelength region most relevant to atmospheric photolysis animpactextendingoverthenextmillennium.Understand-
(200–220nm) with a decrease in s(210nm,T) of ~45% ingtheenvironmentalimpactofNF requiresathoroughun-
3
between 296 and 212K. Atmospheric photolysis rates and derstanding of not only its usage and emissions but also its
global annually averaged lifetimes of NF were calculated atmospheric loss processes to better define its atmospheric
3
using the Goddard Space Flight Center 2-D model and the lifetime, t,and, thus, its contribution to climate forcing. To
s(l,T) parameterization developed in this work. Including thebestofourknowledge,theNF UVabsorptionspectrum
3
the UV absorption spectrum temperature dependence temperature dependence has been neglected in all previous
increased the stratospheric photolysis lifetime from 610 to atmospheric lifetime calculations due to a lack of experi-
762years and the total global lifetime from 484 to mental data. A decrease in the NF absorption spectrum,
3
585years; the NF global warming potentials on the 20-, s(l,T), at the temperatures relevant to the stratosphere,
3
100-, and 500-year time horizons increased <0.3, 1.1, and whereNF isprimarilyremoved,wouldleadtoevenlonger
3
6.5% to 13,300, 17,700, and 19,700, respectively. calculated lifetimes andwarrants evaluation.
Citation: Papadimitriou, V. C., M.R. McGillen, E. L. Fleming, [4] In this work, the gas-phase UV absorption spectrum
C.H.Jackman,andJ.B.Burkholder(2013),NF :UVabsorption for NF was measured at 16 discrete wavelengths between
3 3
spectrum temperature dependence and the atmospheric and 184.9 and 250nm at temperatures between 212 and 296K.
climate forcing implications, Geophys. Res. Lett., 40, 440–445, The present results are compared with previously reported
doi:10.1002/grl.50120. room temperature spectra [Makeev et al., 1975; Molina
et al., 1995; Dillon et al., 2010] and discrepancies are
1. Introduction discussed. The wavelength and temperature dependence of
[2] Nitrogentrifluoride(NF3)isanatmosphericallypersis- tphaeraambesoterrpiztieodn sfoprecutrsuemi,ns(altm,To)s,pohbetraiicnemdoidnelthicsalwcuolraktiownass.
tent potent greenhouse gas (GHG), included in the Kyoto
TheNASAGoddardSpaceFlightCenter(GSFC)2-Dcoupled
protocol,whichisusedinthesemiconductorandelectronics
chemistry-radiation-dynamics model [Fleming et al., 2007,
industry. NF is emitted into the atmosphere and removed
3 2011]wasusedtoevaluatetheatmosphericphotolysis,local
primarily in the upper atmosphere (stratosphere and meso-
andglobalannuallyaveragedlifetimesofNF ,andthesignif-
sphere) by photolysis and reaction with electronically 3
excited oxygen atoms, O(1D). The atmospheric abundance icanceofincludingtheUVspectrumtemperaturedependence.
of NF in 2011 was 0.86ppt, with a growth rate of
3
~0.1ppt yr-1between 2008and2011[Arnoldetal.,2012].
2. Experimental Details
[3] TheatmosphericlifetimeofNF3hasbeenevaluatedin
several recent studies to be ~500years [Prather and Hsu, [5] UV absorption cross-sections, s(l,T), for NF3 were
measured at 212, 231, 253, 273, and 296K at 16 discrete
1Earth System Research Laboratory, Chemical Sciences Division, wavelengths between 184.9 and 250nm. The experimental
NationalOceanicandAtmosphericAdministration,Boulder,Colorado,USA. apparatus was similar to that used in a recent study from
2Cooperative Institute for Research in Environmental Sciences, our laboratory [Rontu Carlon et al., 2010] and is only de-
UniversityofColorado,Boulder,Colorado,USA. scribedbrieflyhere.Themajorcomponentsoftheapparatus
3LaboratoryofPhotochemistryandChemicalKinetics,Departmentof
are a 30W deuterium (D ) or atomic lamp light sources, a
Chemistry,UniversityofCrete,Heraklion,Crete,Greece. 2
4NASAGoddardSpaceFlightCenter,Greenbelt,Maryland,USA. jacketed absorption cell with a 90.4cm path length, a
5ScienceSystemsandApplications,Inc.,Lanham,Maryland,USA. 0.25mmonochromatorwithaphotomultipliertubedetector,
and a photodiode detector. The cell temperature was
Corresponding author: James B. Burkholder, Earth System Research
Laboratory,ChemicalSciencesDivision,NationalOceanicandAtmospheric adjustedbycirculatingatemperature-regulatedfluidthrough
Administration,325Broadway,Boulder,Colorado,80305,USA.(James.B. itsjacket.Thetemperaturewasmeasuredattheentranceand
[email protected]) exitofthecell,withanaccuracyof~0.5Kfortemperatures
>253 K and ~1K for the lower temperatures. The mono-
©2013.AmericanGeophysicalUnion.AllRightsReserved.
0094-8276/13/10.1002/grl.50120 chromator wavelength was calibrated using atomic lamps
440
PAPADIMITRIOUETAL.:ATMOSPHERICLIFETIMEOFNF
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to (cid:2)0.1nm and its resolution was ~1nm (full width at half Pressuresweremeasuredusing100and1000Torrcapacitance
maximum). For measurements at wavelengths <220 nm, a manometers((cid:2)0.2%).
193nm dielectric mirror was used to direct the light source
onto the entrance slit of the monochromator to minimize
3. Results and Discussion
scattered light detection. A 280nm long-pass filter was
insertedintothe beampathtorecordbackgroundsignals. [12] Gas-phase UV absorption cross-sections, s(l,T), for
[6] Absorption cross-sections were determined using NF3 were determined at 16 discrete wavelengths over the
Beer’slaw: range 184.95–250nm at 212, 231, 253, 273, and 296K.
TheresultsaresummarizedinTableS1intheSupplementary
AðlÞ¼(cid:3)ln½ΙðlÞ=Ι ðlÞ(cid:4)¼σðl;TÞ(cid:5)L(cid:5)½NF (cid:4); (1)
0 3 Material and plotted in Figure 1. The s(l,T) values are
whereAistheabsorbanceatwavelengthl;I(l)andI (l)are averages of all the measurements performed over a range
0
the measured light intensity with and without the sample of conditions at each wavelength. Measurements were per-
presentintheabsorptioncell,respectively;Listheabsorption formed under different experimental conditions, including
pathlength(90.4(cid:2)0.3cm);and[NF ]istheconcentrationof variationsintherangeofabsorbance,lampintensity,aswell
3
NF . NF concentrations were determined using absolute as using pure and mixtures of NF in He. In all cases, the
3 3 3
pressure measurements and the ideal gas law. Absorbance measured absorption signals varied linearly with [NF ] at
3
wasmeasuredforarangeofconcentrationsands(l,T)deter- all wavelengths, that is, obeyed Beer’s law. In most cases,
minedfromalinearleast-squaresfitofAversus[NF ]. three to four sets of measurements were performed at each
3
[7] Measurements were performed by first flushing the wavelength and temperature. The precision of the measure-
absorption cell with He bath gas, evacuating the cell, and ments was high, particularly in the wavelength range most
then recording I (l). The cell was then flushed with NF , relevant for atmospheric photolysis (200–220nm), whereas
0 3
filled to a known pressure, and I(l) was measured. The the uncertainty increased slightly at the longer wavelengths
absorptioncellwasthenevacuatedandrefilledtoadifferent dueprimarilytotheweakerNF absorptionsignals.Thedata
3
NF concentration, and I(l) was recorded. This procedure reproducibility was tested extensively, including variations
3
was repeated at least eight times to obtain A(l) values in oftheopticalsetup;differentopticalfiltering,lampintensity,
therange0.02and0.5;atthelongerwavelengths,therange andmonochromatorresolution.Themeasureds(l,T)values
inA(l)valueswasless. under these conditions agreed to within the measurement
[8] s(l,T) was also measured, in several experiments, precision. In addition, the experimental methods were
usingneutraldensityfilters(OD:0.2–0.8)intheopticalpath further validated via measurements of the well-established
between the light source and the absorption cell to test for N O absorption spectrum at room temperature at wave-
2
possibleNF photolysis,aswellasthelinearityofthedetec- lengthsbetween185and230nm.Thesetestmeasurements
3
tion system. The s(l,T) values measured with and without usedasimilarrangeofpressuresandthesameproceduresused
thefiltersagreedtobetterthan1%,thatis,withinthepreci- intheNF measurements.TheresultsforN Owerefoundto
3 2
sionofthemeasurements. be in excellent agreement, to within 1% or better, with the
[9] Measurements of s(l,T) at the 184.95nm atomic Hg
line were madeusinganHg Pen-Ray lightsource and pho-
todiodedetector.Themeasurementprocedureswereidentical
to those described above. Band-pass filters mounted in front
of the Hg lamp and detector were used to isolate the
184.95nmHglineandminimizeexposureoftheNF sample
3
tootherlinesfromthelamp.Experimentsperformedwithout
thesourcefilteryieldedidenticalresultstowithintheprecision
ofthemeasurements,thatis,NF photolysiswasnegligible.
3
[10] I(l), I0(l), and the cell pressure were recorded at a
1kHz sampling rate for at least ~20s, an average value
was used in the data analysis. The signals were stable to
better than 0.5% and I (l) values were measured at the
0
beginning and end of an experiment, which had a typical
durationof 15min,agreedtowithin0.5%;thiscorresponds
toanabsorbance uncertainty of lessthan~0.005.
[11] NF3 (electronic grade, 99.99%) and He (ultrahigh
purity, 99.999%) were usedas supplied. Pure NF was used
3
inthemajorityofthemeasurements,whereasseveraltestmea-
surementswereperformedusing1.0%and10.1%mixturesof
NF in He. Measurements performed with the gas mixtures
3
yieldedresultsinexcellentagreement,towithin1%orbetter,
withs(l,T)measuredusingpureNF samples.Gasmixtures
3
were prepared manometrically in 12L Pyrex bulbs. N O Figure1. NF UV spectrum. (top) Data measured in this
2 3
(ultrahighpurity,99.997%)usedinseveraltestmeasurements work and other studies (see legend). The lines are calcu-
wasdegasedduringmixturepreparationviafreeze-pump-thaw lated using the parameterization given in Table 2. Dashed
cycles.N OinHemixtures,0.1%and1.0%,wereusedinthe lines are an extrapolation. (bottom) Residual of the
2
short-wavelength(185–210nm)measurements,whereaspure experimentally measured cross-section data. Residual
N O was used for measurements in the 215–230nm range. (%)=100 (cid:5) (Exp - Par) / Par.
2
441
PAPADIMITRIOUETAL.:ATMOSPHERICLIFETIMEOFNF
3
values recommended by Sander et al. [2011] and recently respectively. However, the differences between the spectra
reportedfromthislaboratory[RontuCarlonetal.,2010]. reportedbyDillonetal.andinthisworkcannotbeexplained
[13] The 2s uncertainties reported in Table S1 are from bythepresenceofanNOimpurityalone.Basedonours(l,T)
the measurement precision and encompass the extremes measurementsat225and226nm,onandoffaNOabsorption
from all of the measurements. The overall 2s uncertainty peak,weestablishedanupperlimittotheNOimpurityinour
in s(l,T) including estimated systematic errors was ~6% NF sampleof~1ppm.A1ppmNOimpuritywouldleadtoa
3
for wavelengths <205 nm, ~8% between 205 and 225nm, ~2.5%overestimateins(225nm,296K)forNF andsignifi-
3
and10–15%atwavelengths >225nm. cantly less error at shorter wavelengths. Therefore, an NO
[14] The NF3 spectrum decreases monotonically between impuritywasfoundnottobeasignificantsourceofsystematic
185 and 250 nm, whereas the spectrum maximum is at a errorinourmeasurements.
wavelength shorter than included in the present work. The
majority of the spectrum in this region is most likely due 3.2. NF3SpectrumParameterization
to a s* s electronic transition. The spectrum also shows [18] The NF3 absorption spectra reported in this work
a long-wavelength tail (Figure 1); this is also observed in reduces the overall uncertainty in the wavelength region
the spectrum reported previously by Molina et al. [1995]. mostcriticallyrelevanttoatmosphericphotolysisandprovides
s(l,T) measurements at wavelengths >250 nm were thedataneededformodelcalculationsofatmosphericphotolysis
attempted in this work and indicated further leveling off of rates. The wavelength- and temperature-dependent UV
theNF spectrum atlongerwavelengths.However,thepre- absorption cross-sections for NF were parameterized using
3 3
cisionofthemeasurementswaslow,approximately >25%, the empirical formula given in Table 1. The fit parameters
and the s(l,T) values are not reported here. The long- are given in Table 1 and calculated spectra are included in
wavelength behavior may imply the presence of a weaker Figure 1 for comparison with the experimental data. The
electronic transition(s) with a peak near 235 nm, which parameterizationwasoptimizedtoreproducetheexperimental
maycorrespondtoap* nors* nelectronictransition. datainthecritical200–220nmregion(Figure1,bottom),with
[15] The NF3 spectrum was found to have a strong somewhat larger deviations obtained at the longer wave-
dependence on temperature, particularly in the 200–220 lengths.Theparameterizationisvalidovertherangeoftheex-
nm wavelength region. A decrease in s(l,T) with decreas- perimentaldata,whereasextrapolationtolowertemperatures
ing temperature was observed at all wavelengths included yields reasonable s(l,T) values. Using the parameterization
in this study, for example, the change in s(210 nm,T) be- toextrapolatetolongerorshorterwavelengthsthanincluded
tween 296 and 212 K is ~45%. The temperature-dependent intheexperimentdatamaybelessreliable.
cross-section data are included in Figure 1. The tail in the
long-wavelength portion of the spectrum was observed in
4. Atmospheric Implications
the lower temperature spectra as well and was even more
pronounced. The observed decrease in absorption cross- [19] In this section, the GSFC 2-D model was used to
sectioninthe“tail”ofacontinuousabsorptionspectrumwith quantify the atmospheric loss processes of NF (photolysis
3
decreasingtemperaturehasbeenobservedfornumerousother andO(1D)reaction)andcalculateitslocalandglobalannu-
moleculesandisqualitativelyconsistentwithchanges inthe ally averaged lifetimes for present-day conditions. Steady-
groundstateBoltzmanndistributionwithtemperature. state calculations were performed with and without the
temperature dependence of the UV absorption spectrum
3.1. ComparisonWithPrevious Studies determined in this work to evaluate its overall significance.
[16] There are three studies of the NF3 UV absorption The possible range (uncertainty) in the calculated lifetime
spectrum (all performed at room temperature) published was evaluated using the 2s max/min in the model kinetic
before the present work [Makeev et al., 1975; Molina and photochemical input parameters. The model results are
et al., 1995; Dillon et al., 2010]. The spectra are included used to report revised GWPs for NF and also provide a
3
in Figure 1 for comparison with the results from this work. realistic estimate intherangeofpossible lifetime values.
The spectrum reported by Makeev et al. [1975] seems to [20] Tropospheric loss processes for NF3, for example,
be subject to measurement error and is not considered reaction with the OH radical [Dillon et al., 2011] and wet
further.ThepresentresultsareconsistentwiththeUVabsorp- and dry deposition, are expected to be negligible and were
tionspectrumreportedbyMolinaetal.,agreeingtobetterthan not included in our 2-D model calculations. The contribu-
10%overthecommonwavelengthrangeof185–250nm.In tionsofvariouswavelengthregionstotheoverallphotolysis
thewavelengthrangemostcriticalforatmosphericphotolysis of NF were evaluated by breaking its spectrum into four
3
(200–220nm),theagreementisbetterthan5%.
[17] Thespectrumreported byDillon etal.[2010]isalso
inagreementwiththepresentworkovertheatmospherically Table 1. Absorption Cross-section, s(l,T), Parameterization for
important wavelength range, agreeing to within 5–10%. At NF3(SeeText)
X X
shorter and longer wavelengths, the Dillon et al. spectrum log ðsðl;TÞÞ¼ Aliþð296(cid:3)TÞ Bli
10 i i
is systematically greater than our results by as much as i i
~20%. The reason for the discrepancies of the Dillon et al. i A B
i i
results with the present results and those of Molina et al. is
0 (cid:3)218.67 0.9261
unknown. Dillon et al. corrected their cross-section mea- 1 4.03743 (cid:3)0.0130187
surements for the presence of an NO sample impurity on 2 (cid:3)0.0295605 6.096(cid:5)10-5
the order of ~10 ppm. The NO spectrum has discrete band 3 9.596(cid:5)10-5 (cid:3)9.75(cid:5)10-8
structure and a 10 ppm impurity would contribute ~5% 4 (cid:3)1.3171(cid:5)10-7 9.76(cid:5)10-12
5 4.929(cid:5)10-11 —
and ~50% to the measured absorbance at 215 and 226 nm,
442
PAPADIMITRIOUETAL.:ATMOSPHERICLIFETIMEOFNF
3
wavelength regions: Lyman-a (121.567 nm), 169–190 nm,
190–230 nm, and 230–286 nm. Dillon et al. [2010, 2011]
reported a unit photolysis quantum yield for NF at 193 nm
3
and evidence for NF dissociation at 248 nm. Our model
3
calculationsassumeaunitquantumforNF intheVUVand
3
UV wavelength regions at all temperatures and pressures; a
quantumyieldlessthanunitywouldresultinlongerphotolytic
lifetimes and greater GWPs than reported in this work.
Presently, there are no experimental measurements of the
NF Lyman-a cross-section available in the literature. A
3
Lyman-a cross-section of 4.8 (cid:5) 10-18 cm2 molecule-1 was
used in our model calculations, which was obtained
from an extrapolation of the NF VUV spectrum reported
3
by La Paglia and Duncan [1961] between 126.6 and
178.6nm. For the 169–185 nm region where no experi-
mental data are available, an extrapolation of the param-
eterization given in Table 1 was used. For the longer
UV wavelengths, the s(l,T) parameterization from the
present work was used.
[21] The O(1D)+NF3 reaction is an important strato-
spheric loss process for NF . The total rate coefficient for
3
thisreaction, that is, O(1D) loss, as well as the reactive rate Figure 2. GSFC 2-D model global annually averaged
coefficient, NF loss, for this reaction has been reported in atmospheric vertical profile results (see legend). The model
3
several studies [Sorokin et al., 1998; Zhao et al., 2010; resultsarefrom steady-state simulations for thepresent-day
Dillonetal.,2011],includingrecentworkfromthislabora- conditions.
tory [Baasandorj et al., 2012]. Sander et al. [2011] recom-
mend the total rate coefficient value from the Zhao et al.
study (2.4 (cid:5) 10-11 cm3 molecule-1 s-1). The more recent significantly to NF loss throughout the stratosphere. The
3
values from the Dillon et al. and Baasandorj et al. studies fractionalcontributionsofthevariouslossprocessesaregiven
are 16% less and 6% greater than this value, respectively. in Table 2. Photolysis is the dominant loss process and
MoreimportantlyforNF atmosphericlifetimecalculations, accounts for >70% of the NF loss. Given the decrease in
3 3
however, is the reactive rate coefficient, which leads to the cross-section at the colder temperatures of the stratosphere,
loss of NF . Zhao et al. reported an O(1D) quenching yield includingthespectrumtemperaturedependencedecreasesthe
3
for this reaction, which translates into a reactive branching relativeimportanceofphotolysisby~10%.
ratioofbetween0.95and1.0,thatis,thereactionismostly [23] TheNF3mixingratioandmolecularlossratevertical
reactive. The Baasandorj et al. study used a relative profiles are shown in Figure 3, assuming a surface mixing
rate technique to measure a reactive rate coefficient of ratio boundary condition of 1 pptv for present-day condi-
(2.21 (cid:2) 0.33) (cid:5) 10-11 cm3 molecule-1 s-1; this corresponds tions [Arnold et al., 2012]. The maximum molecular loss
toareactivebranchingratioof0.87(cid:2)0.13(2suncertainty) rateoccursat38.5km,whichis~1kmhigherthanthemaximum
based on the total rate coefficient measured in their study. calculated using only the room temperature absorption
The 2-D model used the Arrhenius total rate coefficient spectrum. Other differences in the loss rate vertical profile
expression k(T)=2.0 (cid:5) 10-11 exp(44/T) cm3 molecule-1 s-1 with and without taking into account the UV spectrum
and a temperature-independent reactive branching ratio of temperature dependence are also evident in Figure 3. The
0.93+0.07/-0.21. calculated tropospheric,stratospheric,mesospheric,and total
[22] The global annually averaged vertical profiles for the global annually averaged lifetimes with and without the UV
photolysis and O(1D) reactive loss terms are shown in spectrum temperature dependence included are given in
Figure 2. Tropospheric loss is negligible. Photolytic loss, in Table 2. The total lifetime using only the room temperature
all wavelength regions, was reduced when the absorption spectrum was calculated to be 484 years and is comparable
spectrum temperature dependence was included in the model with the 500-year lifetime cited in WMO [2011]. Lyman-a
calculation.Photolysisinthe190–230nmregionisimportant photolysis, which was shown here to account for ~5% of
throughout the upper troposphere and stratosphere with theatmosphericlossofNF ,wasnotincludedintheprevious
3
>95% of the photolysis occuring at wavelengths between lifetime calculations. Including the temperature dependence
200 and 220 nm. Therefore, the present measurements cover of the NF UV absorption spectrum in the calculation leads
3
themostimportantUVwavelengths.Thenextmostimportant toa longer atmosphericlifetimeof585 years, an increaseof
wavelengthregionis 169–190nm,whichis ~1% of thetotal ~20%. That is, including the UV spectrum temperature
loss at 30 km, but makes a nearly equal contribution to the dependence had a significant effect on the global annually
190–230 region near 60 km. In this wavelength region, averagedlifetimeofNF .
3
>90%ofthephotolysisoccursatwavelengths>180nmand [24] Basedontheestimated2suncertaintiesofthemodel
>80% occurs at wavelengths >185 nm. The 230–286 nm inputkineticandphotolysisparameters,therangeinthetotal
region makes a negligible contribution to the total photolysis NF lifetimewasdeterminedtobe500–712years(Table2).
3
throughout the entire atmosphere. Photolysis at Lyman-a The uncertainty factors for the O(1D) reaction were taken
becomes important above ~60 km and is the dominant loss from Sander et al. [2011] where the 2s uncertainty in k
process at altitudes >65 km. O(1D) reactive loss contributes (298 K) was 44%. The 2s uncertainty in the estimated
443
PAPADIMITRIOUETAL.:ATMOSPHERICLIFETIMEOFNF
3
Table2. 2-DAtmosphericModelResultsoftheLossProcessesforNF andtheGlobalAnnuallyAveragedLifetimes
3
FractionalContributiontoLoss
ModelInput 121.56nm 169–190nm 190–230nm 230–286nm Total O(1D)Reaction
T-dep 0.063 0.100 0.549 0.002 0.713 0.287
296K 0.044 0.096 0.634 0.002 0.776 0.224
Lifetime(yr)
Trop. Strat. Meso. Total Rangea
T-dep >50,000 762 2600 585 500–712
296K >50,000 610 2410 484 426–563
aRangeoflifetimecalculatedusingthe2smax/minuncertaintiesinthekineticandphotochemicalparameters(seetext).
5. Conclusions
[26] ThisstudyhasprovidedtheUVabsorptionspectrum
dataandanalysisneededtocriticallyevaluateandrefinethe
contribution of the persistent GHG NF to climate forcing.
3
IncludingthetemperaturedependenceoftheUVabsorption
spectrum in the photolysis rate calculations, which was
neglectedpreviously,wasdemonstratedtohaveasignificant
impact on the calculated NF atmospheric lifetime and thus
3
estimates of its climate forcing. It was shown that strato-
spheric UV photolysis is the dominant atmospheric loss
processforNF ,accountingfor71%ofitsatmosphericloss
3
(Table2).ReactionwithO(1D)andLyman-aphotolysisare
alsoimportantlossprocessesthataccountfor~25and~5%
of the total loss, respectively. Including the NF UV spec-
3
trumtemperaturedependenceincreaseditstotalglobalannu-
allyaveragedlifetimeby20%.Inaddition,thelifetimerange
obtained using the 2s estimated uncertainty in the NF
3
kinetic and photolytic parameters was determined to be
500–712years,thatis,aspreadof (cid:2)20%.
[27] Atmosphericlossrateandlifetimemodelcalculations
rely on accurate laboratory kinetic and photolytic data
Figure 3. Steady-state GSFC 2-D global annually aver-
obtained under representative atmospheric temperature and
aged NF molecular loss rate vertical profile model results
3 pressureconditions.IncludingtheNF UVabsorptionspec-
(see legend). The calculated NF mixing ratio assumed a 3
3 trum temperature dependence measured in this work and
present-day surface mixing ratio boundary condition Lyman-a photolysis was shown to affect its atmospheric
of1pptv. lifetime significantly. The modeling methods used in this
work can be applied to other molecules to obtain or refine
Lyman-across-sectionwastakentobe50%andthe2sun- estimates of ozone depletion substance and GHG atmo-
sphericlossprocessesandtheassociated uncertainties.
certaintyintheNF UVabsorptioncross-sectionswastaken
3
tobe10%foralltemperatures.Theobtainedlifetimerange,
approximately(cid:2)100years((cid:2)20%)atthe2slevelofuncer- [28] Acknowledgments. This work was supported in part by the
tainty,reflects the current overall level of uncertainty inthe National Oceanic and Atmospheric Administration Climate Goal Program
andtheNASAAtmosphericCompositionProgram.
NF lossprocesses.
3
[25] IncludingthetemperaturedependenceoftheNF3UV
absorption spectrum in the 2-D model leads to a longer
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