JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 99, NO. D9, PAGES 18,839-18,846, SEPTEMBER
20, 1994
Kinetics
ofreactions
ofground
statenitrogen
atoms
(4S3/2)
with NO and NO2
Paul O. Wennbergand JamesG. Anderson
Department
of ChemistryandtheDepartment
of EarthandPlanetarySciences,
HarvardUniversity,
Cambridge,Massachusetts
Debra K. Weisenstein
Atmospheric
andEnvironmental
Research,Incorporated,
Cambridge,Massachusetts
Abstract. The dischargeflow techniquehasbeenusedwith resonancefluorescencedetectionof
N atoms
tostudy
thefastradical-radical
reaction
ofground
state
nitrogen
atoms
(4S3/2)
withNO
andNO2. Therateconstants
obtained
are(inunits
ofcm3molecule
-•s-j)k• = (2.2+ 0.2)x
10-•1exp[(160
+ 50)/T]inthetemperature
range
213K _<T _<369K forN + NO-->N2 q-O and
k2 = (5.8+ 0.5)x 10-12exp[(220+ 50)/T]inthetemperature
range
223K < T _<366K forN
q- NO2 --> N20 q- O. The reportederror limits are at the 95% confidencelevel. The reaction
kineticsare consistent
with otherradical-radicalreactions,essentially
no enthalpicbarrier is
observed. Substitution
of the measuredrate of R1 for the valuerecommended
in the latestJet
Propulsion
Laboratorycompendium
[DeMoreet al., 1992]resultsin a smallchangein the
concentration
of ozonepredictedin a two-dimensional
photochemical
model. Modeledozone
concentrations
are higher(approximately
1%) in thehigh-latitude
upperstratosphere
as a result
of a 3-10%reduction
in thecalculated
concentrations
of NOy.
Introduction
by knowledgeof the temperaturedependence
of the reaction
Thereactions
of ground
statenitrogen
atoms
(4S3/2)
with
nitric oxideandnitrogendioxide,
N + NO
N + NO2
-->
-+
rates. This work reportsthe temperaturedependence
of the title
reactions
over the rangeof 213-369K, includingthe first such
studyof (R2).
N2 + O
N20 + O,
(R1)
(R2)
Althoughthesetwo reactions
havebeenstudiedextensively
(seeTables2 and4), thereis still largeuncertaintyin the rates.
There is generalagreementon the roomtemperature
rate constantfor (R1), but the two direct studiesof the temperature
dependence
below300 K disagree[ClyneandMcDermid, 1975;
are of importancein atmosphericchemistry[Brasseurand
Solomon,1986] as well as in attemptsto determinethe relationLee et al., 1978]. There is little consensusabout the value of
shipbetweenelectronicstructureand reactivity[Howardand
Smith, 1983]. Understandingthese processes,however, has
been hinderedby the large uncertaintyin the measuredrate
constants;despite numerousstudiesof these reactionsthe
kineticsare poorly defined.
Reaction(R1) is the dominantremovalmechanism
for odd
the room temperaturerate constantfor (R2). A recentevaluation givesan uncertaintyof _+300% [DeMoreet al., 1992]. The
resultsof the latestdirectstudiesof thisreactiondiffer by more
than an order of magnitude[Clyneand Ono, 1982; Husain and
Slater, 1980].
Previousattemptsto measurethesereactionrateshave been
nitrogenin the upperstratosphere
and mesosphere.It is this
hinderedby poor detectionefficiencyand complications
due to
reactionthat preventsthe upper atmospherefrom being a.
impuritiesin dischargeandphotolyticN-atomsources. In these
significant
sourceof NOyfor the stratosphere.
Because
the studieswe have employeda gas filter schemethat allows for
atmospheric
sourceof nitrogenatomsis the photolysis
of NO,
selectiveresonance
fluorescence
detectionwith very high sensi(R1) hasbeendubbed"cannibalistic"
as it resultsin the lossof
tivity. The low wall reactivityof N atomswasexploitedto protwo odd nitrogenmolecules[Brasseurand Solomon,1986].
duce a dischargesource free of radical impurities. This
Reaction(R2) playsa far lesssignificantrole in stratospheric
combination of a clean N-atom source and low detection limits
chemistrybecauseof the small concentration
of NO2 in the allowed us to use initial radical concentrations more than 1
upperstratosphere.
The reactionsof radicalswith NO and NO2 constitutethe
most studied series of radical-radical reactions [Howard and
Smith,1983]. By examininga seriesof homologous
reactions,
insightcan oftenbe gainedaboutthe reactionmechanism
and
the potentialenergysurface. This analysisis greatlyenhanced
Copyright1994by theAmericanGeophysical
Union.
Papernumber94JD01823.
0148-0227/94/94 JD-01823505.00
order of magnitudelower than previousstudieswithout compromisingon precision. The 2c• confidencelimits (basedon
precisionalone)for all the experiments
waslessthan3 %.
Experiment
The dischargeflow systemused in these studieshas been
describedin detail elsewhere[Bruneet al., 1983]. Briefly, the
reactor is composedof two sectionsof Pyrex tubing separated
by a lasermagneticresonance
(LMR) axisnotusedin thiswork.
18,839
18,840
WENNBERGET AL.' KINETICSOF N(4S3n)
WITH NO AND NO2
The kineticswerecarriedout in a 70-cm-long,25-mm-IDflow
mixtures;He (99.9999%)and N2 (UHP, 99.999%) for the
flowinglamp. All gases
weresupplied
by Matheson
except
the
circulated for thermal control. After the LMR axis the radicals
HP andUHP He andthe UHP N2, whichwere supplied
by
passinto the resonance
fluorescence
(RF) block, whichhouses Northeast
Airgas. NO (CP, 99.0%) andNO2 (99.5%) were
three detectionaxes. For theseexperiments
the gapsin the providedby Mathesonandpurifiedbeforeusedas follows: NO
LMR and first RF axis were sealedwith Pyrex tubesto mini- waspassedthrougha sodium-hydroxide
coatedsilica(Ascarite)
tube that is jacketedto allow for chilled or heatedfluid to be
mize the radical losses. All reactor surfaces are coated with
Teflon.
trap and then over potassiumhydroxide,which was cooledto
-50øC;NO2 waspassedovera trap at -40øCandthencollected
In thedetermination
of therateconstants,
NO andNO2 were at -196øC. The condensatewas then warmed to -60øC to allow
anyNO to be pumpedoff. The NO2 wasthensublimed
intothe
addedin excess([NO] or [NO2] >> IN]0) at the back of the
reactionzone. The nitrogenatomswere producedin a microwave dischargeof trace N2 in He and addedto the flow tube
througha movablePyrex7-mm-OD injector. The reactiontime
was varied by changingthe locationof the additionof the N
atoms.Becauseof the very low wall reactivityof N atomsthe
interiorwall of theprobewasnotcoatedwithTeflon. Thisproduceda puresourceof groundstateN atoms;in theprobe,there
was nearlycompleteremovalof the H and O impuritiesproducedin the discharge.The longresidence
time in the probe
(approximately300 ms) also ensuredthat excited statesof N
were quenchedby wall collisionsbeforebeingaddedinto the
bulk flow.
ResonanceFluorescence
Lamps
Nitrogenatomdensities
weremeasured
by resonance
fluores-
cence. The RF lampconsisted
of a quartzbodywitha MgF2
reservoir. NO and NO2 concentrations
in the flow tubewere
determined
by monitoring
therateof pressure
dropin reservoirs
of knownvolumes. The NO2 concentration
was correctedfor
N204 formationusingJANAF thermochemical
data [Chaseet
al., 1985]. Because
of the low pressures
of NO2 usedto make
up the reservoirs (<10
(<10%).
torr), this correction was small
Data Analysis
Thepseudo
firstorderdecayrates(ko•bs
(x))wereobtained
froma weightedleastsquares
fit of thelogarithmof thedetected
signalminusthe background
chamberscatterversusinjector
distance.TypicalN atomdecaysare shownin Figure1. The
initialN concentrations
forthissetof decays
was5 x 10•øatoms
cm
-3,which
wastypical
ofalltheexperiments.
Using
theplug
flow approximation,
thereactiondistance
is directlyconvertible
window. The resonanceradiationwas producedin a microwave to reactiontime and so,
dischargeof trace quantitiesof N2 in about2 torr of He. A
pump was usedto maintaina slow gas flow throughthe lamp.
kj•s
(t)= Vx ko•
•(x).
The1200• 4p_ nS (3s-2p)
resonant
emission
fromthelamp
was isolatedfrom linesdueto O andotherexcitedstatesof N by
flowing CO2 at a pressureof 1000 torr thougha 3-mm gasfilter
cell locatedin front of the lamp. Hydrogenimpuritiesin the
I
I
I
I
lampproduced
Ly-otradiation
at 1216• thatisnotabsorbed
by
the CO2. By replacingthe CO2 filter gas with 02, which
o.oo
1E4
strongly
absorbs
all butLy-ot,theratioof the1216-to 1200-•
0.62
-1.66
light could be measured. In no casedid this ratio exceedone
third. With O2 in the gas filter cell the lamp couldbe usedto
diagnoseatomichydrogenproductionby the N source. The H
2.00
2.77
atom concentrations
in the flow tube were lessthan 1 x l0 s
atomscm-3. Resonance
fluorescence
wasdetectedwith a KBr
3.75
1E3
photomultipliertube (EMR 541J) operatedin photoncounting
4.32
o
mode.
--
Oxygen atoms were detectedwith a sealedcombinationO
and H lamp, which hasbeendescribedelsewhere[Bruneet al.,
6.14
1983]. To isolate
theoxygen
tripletat 1304• fromthehydro-
7.01
gen Ly-ot, a CaF2 filter was insertedin front of the lamp. A
flow of dry N2 was maintainedin the gas filter cell to prevent
damageto the MgF2 windows. The lamp was calibratedby
titrating a known densityof NO with excessN atoms. Sensi-
1E2
cally3 x 10-7(counts
s-1/ (atomcm-3))
witha scattered-light
count
of < 100counts
s-• resulting
in a detection
limitof less
The followinggaseswere usedwith their statedpurities' He
--
--
1E1
measuringthe N-atom sensitivityrelative to the O lamp using
the NO titration. The responsivityof this lamp for N was typi-
Materials
8.27
9.92
tivityof thelampwas8 x 10-9(counts
s-I / (atom
cm-3))
with
background
signals
typically
< 100counts
s-1giving
a detection
limitof3 x 109atoms
cm-3. TheN lampwasthencalibrated
by
than1 x 108atomscm-3.
_
10
20
30
40
50
60
Disfnce (cm)
Figure1. Pseudo-first-order
N atomdecays
in excess
NO2.
T - 298 K; P = 1.7 tort. Distances
aredirectlyconverted
to
reaction
time(usingtheplug-flowapproximation)
fromthebulk
velocity,
F = 9.03x 102cms-•. Given
attheright
areNO2
(HP, 99.99%) for bulk flow; He (UHP, 99.999%) and N2 concentrations
inunits
of 10•2molecules
cm
-3. Thequality
of
(UHP, 99.999%) for the dischargesourceand excessreagent thedatashown
is typicalof all theexperiments
reported.
WENNBEROET AL.: KINETICSOF N(4S3n)
WITH NO AND NO2
Tablel.
18,841
N+NO•N:+O
Condition
Result
Observedrate constantk I
(2.2+ 0.2)x 10-11exp[(160
+ 50)/•
Numberof experiments
11at369K
13at334K
40 at298K
12at273K
12at253K
10at237K
12at228K
12at213K
for 213 K < T < 369 K
(3.30+ 0.05)x 10'll cm3molecule'Is
'l
(3.52+ 0.05)x 10'll
(3.65+ 0.08)x 10'll
(3.68+ 0.14)x 10'll
(3.94+ 0.15)x 10'll
(4.19+ 0.05)x 10-ll
(4.38+ 0.06)x 10-ll
(4.56+ 0.04)x 10'll
Pressurerange
1.0 - 4.0 torr
Flow velocityrange
6.8- 11.1 m s'l
.Carriergas
He
Diffusion coefficient
N- He,0.158T3a/P(< 8%correction)
Speciesdetected
N (RF)
0 (RF)
H (RF)
Excess reactant
Initial N concentration
[NO]- (0.5- 5.0)x l0n molecules
cm-3
[N]o- (0.3- 1.0)x l0II atoms
cm-3
Stoichiometric
4-
ratio
Observed first-order wall removal rates
150
< ls -I
The decay rates were then corrected for axial diffusion
[Howard, 1979]:
k• = ko•bsd
(t)[1+ ko•bsd(t)D
/ •2]
where V is the meanbulk flow velocityandD is the radicaldif-
fusion
coefficient
(D = 4.1 x 102cm2s-• forN atoms
inhelium
at 298 K and 2.0 torr [Clyne and Ono, 1982]). The diffusion
correctionwas small and did not changethe calculatedbimolecularrate constantby morethana few percent. The correction
did, however, significantlyincreasethe precision. After subtractionof the chamber-scatter
componentof the signal, pureexponentialdecayswere observedfor over 5 e-foldingsof the N
atom concentrations.
The temperaturedependence
of the bimolecularrate constant
(k[•) wasdetermined
between
213and369K andwasfound
to
be slightly negative, i.e., k• increases with decreasing
temperature.The Arrheniusplot is shownin Figure5. From a
weightedleastsquaresfit of all the data, includingan estimate
of systematicerrors, we find
k[• = (2.2+ 0.2)x 10-11exp[(160
+ 50)/T]cm
3molecules
'• S-1.
Table 2 gives previousmeasurements
of this rate constant
near and below room temperature. The measuredroom temperaturerate is consistent
with mostpreviousmeasurements.
In
general,the rate constants
obtainedwith massspectroscopy
determination
are lower, and the flash photolysisresultsare
higher than studies employing discharge flow/resonance
fluorescence.
160
Results and Discussion
N + NO --• Nz + O
120
Table 1 summarizesthe experimentalconditionsand results
forthisreaction.Theplotof ff versus
NO for all theroom
temperaturedata is shown in Figure 2.
squaresfit of the datayieldsa valueof
A weighted least '--
80
40
k•298= (3.63
+ 0.08)x 10-• cm3molecules
-• s-1
for the bimolecularrate constantfor (R1). The precisionof the
measured
pseudo-first-order
rateconstant,
if, givesthe 2c•
experimentaluncertaintyquoted. With the addition of other
uncertainties
and includingan estimateof systematic
errors(due
to pressureand flow measurement
errors)we report
0
1
2
3
4
[NO] (102 molecules
crn-z)
Figure
2. Plotof/dversus
NOconcentration
atroom
temperature. Data are for experimentsdoneat pressures
between1 and
k1298
= (3.6+ 0.4)x 10-• cm3molecules
-• s-•.
4 torr.
18,842
WENNBERGET AL.: KINETICS OF N(4S3n)
WITH NO AND NO2
Table 2. LiteratureComparisonfor N + NO --> N2 q- O (200 K < T < 400 K)
Reference
This work
ClyneandThrush[1961]
Herron[1961]
Lin et al. [1970]
ClyneandMcDermid[1975]
Lee et al. [1978]
Lee et al. [1978]c
Sugawara
et al. [1980]
Cheahand Clyne[1980]
Husainand Slater[ 1980]
ClyneandOno[1982]
Brunning
andClyne[1984]
DeMoreet al. [1992]
k(298)
a
k(T)a
3.6 + 0.4
Method
b
(2.2 + 0.2) exp[(160+ 50)/7]
1.7 _+0.8
3 _+1
2.2 + 0.2
4.0 _+0.5
3.0 _+0.2
1.9 _+0.2
3.4 + 0.6
4.5 _+0.2
2.8 + 0.1
2.0 + 0.3
3.4 + 1.0
DF-RF
5.0 exp[(-100
_+350)/T1
relative
(8.2 + 1.4) exp[(-410+ 120)/7]
(3.2 _+0.2) exp[(65_+20)/7]
(1.7 + 0.4) exp[(150+ 50)/7]
(3.4 _+1.0) exp[(0+ 100)/7]
DF-MS
KS
DF-MS
FP-RF
DF-RF
PR-RA
DF-RF
FP-RF
DF-RF
DF-MS
review
X 10-• cm3 molecule
4 s4.
FP, flashphotolysis;
DF, discharge
flow; KS, kineticspectroscopy;
PR, pulsed
radiolysis;
RF, resonance
fluorescence;
RA, resonance
absorption;
MS, massspectroscopy.
Data correctedfor axialdiffusionandfit by weightedleastsquares
analysis.
Lee et al. [1978] measuredthis reactionby both discharge
flow and flash photolysis. The rate determinedin their flash
photolysisstudywas 50% higherthan the rate from discharge
flow. In their dischargeflow experimentthe calculatedrate
constantwas seen to increaseslightly with pressure,and this
pressuredependence
was suggested
to explainthe differencein
the measuredrate constants. However, no axial diffusion correction was used in their data analysisof the dischargeflow
experiments. Correctingtheir data increasesthe rate constant
and removesthe observedpressuredependence.In this work,
no statisticallysignificantdifference in the calculatedbimolecularrate constantwas seenfor the differentpressuresoncethe
diffusion correctionwas applied. The data shownin Figure 2
includeexperimentsdoneat 1, 2, and4 torr.
The rate constantsmeasuredby flash photolysisare systematicallyhigherthanstudiesemployingdischarge
flow. BothLee
et al. [1978] and Husain and Slater [1980] used N20 as the
sourcefor N atoms. This sourceproducesonly excitedstatesof
N. In addition, the primary photoproductis excited state
oxygenatoms[Okabe,1978]. It is likely that the presence
of
theseexcitedstatespeciescomplicates
the kinetics,enhancing
the determined rate constants.
The temperaturedependence
measuredin this work is in
agreement
withLee et al. [1978]whoobserved
a slightincrease
of the rate with decreasingtemperaturein both dischargeflow
and flashphotolysisexperiments.A weightedleastsquaresfit
of the diffusion-corrected
dataof Lee et al. yieldsan Arrhenius
expression
of (1.7+ 0.4)x 10-11exp[(150
+ 50)/T]forthedis-
Table 3. N + NO•. --> N•.O + O
Condition
Result
!
Reportedrate constant
kl
(5.8+ 0.5)x 10-1:exp[(220
+ 50)/7]
for 223 K _<T _<366 K
Numberof experiments
14at366K
13at328K
50at298K
14at270K
14at248K
14at223K
(1.04_+0.01)x 10-• cm3molecule4s
4
(1.13+ 0.01)x 104•
(1.20+ 0.02)x 10-11
(1.31+ 0.02)x 10-l•
(1.42_+0.02)x 10-ll
(1.52_+0.02)x 10-l•
Pressurerange
1.0 - 3.0 tort
Flow velocity range
6.9 - 11.0 m s-I
Carrier gas
He
Diffusion coefficient
N- He, 0.158T3a/P(< 8% correction)
Speciesdetected
N (RF)
O (RF)
H (RF)
Excess reactant
Initial N concentration
[NO2]= (0.05- 1.20)x 10•3molecules
cm-3
[N] = (2.5- 4.0)x 10•2atoms
cm-3
[N]o= (0.2- 1.0)x 10• atoms
cm-3
Stoichiometric ratio
4 - 400; 0.1 - 0.05
Observed first-order wall removal rates
< ls -•
WENNBERGET AL.' KINETICSOF N(4S3n)
WITH NO AND NO2
18,843
120
?
8o
o
-
4o
•
4
o
2
•
.?
m
0
2
4
6
8
10
0
0
12
2
4
6
8
[NO], [NO2] (10TMmoleculescm-•)
[NO2] (10•2 moleculescrn-s)
Figure3. Plotofk/ versus
NO2concentration
atroomtempera-Figure 4.
ture. Data are for experimentsdoneat pressuresbetween1 and
4 torr.
charge
flowexperiment
and(3.2+ 0.2)x 10-•l exp[(65
+ 30)/T]
for the flashphotolysisexperiment. The temperature
dependence doesnot agree with the work of Clyne and McDerrnid
[1975]. They reporteda positivetemperature
dependence
with
an activationenergyof 0.8 Kcal / mol. In that experiment,N
atomsweredetected
by massspectroscopy
withpoorsensitivity.
Threehightemperature
studies
of thisreactionhaverecently
been published[Koshi et al., 1990; Davidson and Hanson,
1990'MichaelandLirn, 1992]. Thesewereshocktubeexperimentsemployingresonance
absorption
detectionof N. In excellent agreementwith the resultsreportedhere, Koshi et al.
obtained
a rateconstant
of (2.2+ 0.5)x 10-l• cm3molecule
4 s-•
over the temperaturerange 1600 to 2300 K. Michael and Lim
Yield of oxygenatomsfrom the reactionof N with
NO2 (trianglesand circles)versusthe reactionwith NO (solid
squares)in excessN. Reaction (R1) is known to have unit
quantumyield. Two different concentrationsof N were used:
squares,
circles,
[N] = 2.4x 10•2-triangles,
[N] = 3.6x 10•2.
Theyieldof atomic
oxygen
(O 3p)wasmeasured
at room
temperaturein excessN. Figure 4 showsthe O atom signal
producedin the titrationof NO and NO2 with initial N concen-
trationsof 2.4 and3.6 x 10•2molecules
cm-3. Theseconcentrationswere not quitelargeenoughto drive (R2) to completionin
the 55-ms reactiontime. In Figure 4, the data have beencorrected for the incompleteconversionusing the rate constants
determined in this work.
This correction is less than 20% for
the lowestN-atomconcentration.(Attemptsto furtherincrease
the N-atom concentrations
resultedin a large increasein H and
by the source).The oxygenatomyields
report
a temperature
independent
rateof (3.7+ 0.8)x 104•cm3 O impurityproduction
molecules
-1s-• overthetemperature
range1540to 2500K.
are equalfor the two reactions.
The productanalysisis consistent
with (R2a) beingthe only
Davidson
andHanson
report
k• = 7.13x 10-• exp(-787/T)for
by Clyneand McDerrnid[1975]. Reaction
1400K <_T <_3500 K. Becauseof the very differentreaction channel,as suggested
(i.e.,
energiesthe disagreement
with the Arrheniusexpression
deter- (R2d) is unlikelyto be important;to proceedadiabatically
to conservespinmultiplicity),it wouldrequirea quintettransiminedin thiswork maynotbe significant.
tion state,whichis likelyto havea largebarrier. Thisexperiment cannot completely rule out (R2b) and (R2c).
N + NO2 --> N20 + O
Table3 gives
a summary
oftheresults
for(R2). A plotofk/
versusNO2 for all the datatakenat roomtemperature
is shown
in Figure3. Includedare experiments
with initialN concentra-
If the
branchingratiosof thesetwo channels
wereequal,themeasured
1E-10
I
I
I
I
T
tions
aslowas2 x l0watoms
cm
-3. Nopressure
dependence
wasobserved
between1 and3 torr. A weightedleastsquares
fit
E
of the datayieldsa valuefor the bimolecularrate constantof
N
+
NO
T
k2298
= (1.22+.02)
x 10-l•cm
3molecules
-1s-•
wherethe error quotedis 20 and represents
the precisionin the
measurement
of k•. With the additionof uncertainties
dueto
o
E
1E-11
flow andpressuremeasurements,
I
I
I
I
3.0
3.5
4.0
4.5
k•98-- (1.2+ 0.1)x 10-ll cm3molecules
-1S-1.
2.5
Reaction(R2) hasfourthermodynamically
accessible
channels:
N
N
N
N
+
+
+
+
NO 2
NO 2
NO2
NO2
--->
--->
--->
--->
N20 + O
NO + NO
N2 + 02
N2 + O + O
(R2a)
(R2b)
(R2c)
(R2d)
5.0
1000 * T-• (K-•)
Figure 5.
Arrheniusplot of the reactionsN + NO and
N q- NO2. Linesarethereported
ratesderivedfroma weighted
leastsquares
fit to all thedata. The2o uncertainties
(precision)
for the individualdatapointsare listedin Tables1 and3.
WENNBERGET AL.: KINETICSOF N(4S3a)
WITH NO AND NO:
18,844
Table 4. LiteratureComparisonfor N + NO2 --->Products
Reference
k(298)a
k(T)b
Method c
This work
1.2 + 0.1
(5.8 _+0.5) exp[(220+ 50)/T]
Verbekeand Winkler [ 1960]
-
8 5:1 at T = 500 K
Phillipsand Schiff [1965]
ClyneandMcDerrnid [1975]
1.8 _+0.2
0.14 + 0.02
3.8 + 0.1
0.30 + 0.07
-
DF-MS
DF-MS
FP-RF
DF-RF
o•.O.•
.0-0.2
-
review
Husain and Slater [ 1980]
Clyneand Ono [1982]
DeMore et al. [1992]
DF-RF
DF-VA
a x 10'• cm3 molecule
-• s-•
b x 10'12cm3 molecule
4 s'l
c FP, flash photolysis;RF, resonancefluorescence;
DF, dischargeflow; MS, mass
spectroscopy;
VA, volumetricanalysis
The authors argued that this was the true bimolecularrate
becauseat very high stoichiometric
ratiosthe reactivityof N
will be dominatedby lossto NO2.
The methodusedin this studydiffersfrom the work of Clyne
223 K to 369 K and is shownin Figure 5. From a weighted
leastsquaresfit of all the data, includingan estimateof the and Ono only in that our N-atom detectionthresholdwas lower
andthe initialN concentrations•could
be muchsmaller,thereby
systematic
errors,we find
reducingthe interferencefrom the productof (R2a) to a neglitheH andO impurities
k•• = (5.8+ 0_5)
x 10-•2exp[(220
+ 50)/T]cm3molecules
'• s-• giblelevel. Furthermore,by measuring
in the flow tube we coulddirectlyinsurethat the kineticswould
not be subjectto the interferencesnotedabove. No curvature
Table 4 givesthe previousmeasurements
of the roomtemperawas noted in any of the experiments,even at stoichiometric
ture rate constant.The reportedrate constants
vary by over an ratios far less than 80.
orderof magnitude.Our valueis 4 timeslargerthanthe value
The lack of impuritiesand secondary
chemistryis alsoeviof Clyneand Ono [1982] whichhas beenacceptedin recent
dentin the low reactivityof N atomswith CIOCI and C12measreviews.
uredwith the sameapparatus.An upperlimit for the reactionof
It has been noted that the measurement of this rate constant is
O atom yields in excessN would still be one as each NO
produced
by (R2b)wouldreactwithN to giveoneO atom.
The temperature
dependence
of (R2) was determined
from
N + ClOCl at 373 K was determined
to be 3 x 10'•6 cm3
subjectto catalyticinterferences
frombothhydrogen
andoxygen
molecules
-t s4 [Stevens
andAnderson,
1992].ForN + CI2 at
[ClyneandMcDerrnid,1975;Clyneand Ono, 1982]. Bothcan
387K anupper
limitof 5 x 10-•6wasmeasured.
Boththese
be produced
as impuritiesin discharge
N-atomsources,
andin
measurements
are very sensitiveto radicalimpurities.
addition, oxygen atoms are producedin (R2a). Hydrogen
It is not obvioushow to explainthe resultsof ClyneandOno.
impurities
reactveryquicklywithNO2producing
NO andOH,
It is possiblethat theyhad an unidentifiedNO2-dependent
back2
2
2
which in turn are very reactivetowardN. This schemeis
groundsignalfrom excitedstates.of N( D, P, or S) or from
catalyticin hydrogen:
reactions
ofN2(A3E)
withNO2ortheN20product.
H + NO2
-->
NO + OH
N
-->
NO
-->
-->
Nz + O)
2N2 + 20
+
OH
2(N + NO
Net: 3N + NO2
+
H
Oxygenatom dischargeimpuritiesand productfrom (R2a)
canalsoreactcatalyticallywith NO2:
O + NO2
-->
NO + O2
N + NO
-->
Nz + O
-->
N2 + O2
Net: N + NO2
The rate reportedhere is 3 times slowerthan that measured
by ttusain and Slater [1980] usingflash photolysis. As mentioned earlier, this N-atom sourceproducescopiousconcentra-
tionsof O(tD). Ground
stateN atoms
areonlyproduced
after
quenchingof the excitedstateN photolysis
products.The N20
photolysissourceseemsinappropriatefor the studyof ground
stateN-atom kinetics,especiallyfor reactionsthat are knownto
be sensitiveto radicalimpurities.
Conclusion
Reaction
Mechanisms
As demonstrated
by the fast reactionratesand slightlynegative activationenergies,it is clear that both (R1) and (R2) prorate constant.Clyneand Ono [1982]and ClyneandMcDerrnid ceed through "loose" early transition states. The dominant
[1975] notedthat the measuredsecond-order
rate constantde- orbital interactions at the transition state will be between one of
creasedas the stoichiometricratio, ([NO2]/[N]0) increased. the lonep-orbitalelectronson the N with the radicalelectronof
Theyattributed
thelargeobserved
curvature
intheirk• versusNO (•*) or NO2 (a0. Polanyi and Wong[1969] have shown
concentration
plot to the interferences
notedabove. At very that for this type of potentialenergysurface,a large fractionof
high stoichiometricratios (> 80), the apparentrate constant the exothermicitywill be disposedof as vibrationalexcitationof
reachedan asymptote,
whichClyneandMcDermidmeasured
to the products.This is exactlywhathasbeenobservedfor bothof
be1.4x 10-12,andClyne
andOnoreported
tobe3.0x 10'n. thesereactions[Cloughand Thrush,1969;Blacket al., 1973].
These interferences lead to an enhancement of the observed
WENNBERGET AL.' KINETICSOF N(nS3n)
WITH NO AND NO2
18,845
and mesosphere.Althoughthe discrepTo show the generalityof the electronicstructureof the the upperstratosphere
islargest
fork•• , it isofminor
importance
because
ofthe
transitionstate,it is instructive
to comparethe observed
reaction ancy
kineticsof N atomswith the high-pressure
limits for the asso- smallmixingratiosof NO2 at thesealtitudes. The slightnegadependence
of (R1) hasa largerimpact:at 220
ciationreactionsof otherradicalswith NO and NO2. Although tive temperature
K
the
rate
is
35
%
faster
than
the acceptedvalue. A two-dimenthesereactionswill proceedto form stableadducts,we expect
sional model from Atmosphericand EnvironmentalResearch,
the radical-radicalinteractionsat the transitionstatesto be very
Incorporated[Ko et al., 1993] wasusedto investigate
the effect
similar. The high-pressure
rate constants
for the reactionsof
of this changeto the chemicalkineticdatabase.Figure 6 shows
OH, O, CIO, BrO, F, FO, CI, Br, I with NO and NO2 [DeMore
the calculatedpercentchange(comparedto the Jet Propulsion
et al., 1992; Howard and Smith, 1983] are all within a factor of
5 of the rates of (R1) and (R2). Most of thesereactionsalso Laboratory
recommended
rate)in ozoneandNOyfor June15.
As expected,the largestchangesoccurin the high-latitude
upper
displaylittle or no temperature
dependence.
stratosphere
and lower mesosphere.The calculatedchangesare,
however,smallandresultin little changeto the calculatedozone
AtmosphericImplications
Comparedwith the Jet Propulsion
Laboratoryrecommended
column.
The temperaturedependence
of (R1) has recentlybeen the
values
of k[z and k•z, these
newmeasurements
willdecreasesubject
of speculation.In a modelof theMartianthermosphere,
the modeledlifetime andthusthe modeledconcentration
NOx in
60
55
50
45
40 E
35 •
30
•
25
•
20 <
lOO
15
10
'NOy
1000 '
90øS
.1
60øS
5
30øS EQ 30øN 60øN 90øN
Latitude
Fox [1993] speculated
that this reactionsloweddown at very
cold temperatures
(130-160K); theseresultsshowthatjust the
oppositeoccurs,at least down to temperaturesof 220 K. To
explain anomalouslylow concentrationsof N atoms in the
Earth's thermosphere
(wheretemperatures
are as high as 1000
K), Siskindand Rusch [1992] also speculatedthat (R1) had a
positivetemperaturedependence.The measurements
described
here certainlydo not precludethe possibilitythat the temperature dependencecould exhibit non-Arrheniusbehavior. Indeed,
somerecentmeasurements
at high temperatures
suggestthat the
rate may increaseslightlywith increasingtemperature[Michael
and Lim, 1992; Davidsonand Hanson, 1990].
Acknowledgment. We wish to thank P.S. Stevensand D.W.
Toohey for experimental assistance. Discussionswith J.P.D.
Abbatt and SusanSolomonare gratefullyacknowledged.This work
was supported by NSF grant ATM-8601126.
P. Wennberg
acknowledges
the supportof the NSF GraduateStudentFellowship
Program.
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Jun 15
60
55
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(Received March 7, 1994; revisedJuly 8, 1994;
acceptedJuly 8, 1994.)