JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. D7, PAGES 10,235-10,247,JUNE 20, 1990
OBSERVED AND MODEL-CALCULATED NO2/NO RATIOS IN TROPOSPHERIC AIR
SAMPLED
DURING
THE
NASA
GTE/CITE-2
FIELD
STUDY
W.L.Chameides,
1D.D.Davis,
1J.Bradshaw,
• S.Sandholm,
• M. Rodgers,
• B.Baum,
•
B.Ridley,
2S.Madronich,
2M.A.Carroll,
3G.Gregory,
4H.I.Schiff,
5
D.R.Hastie,
5A. Torres,
6 andE. Condon
7
Abstract. Data gatheredduringthe NASA GTE/CITE 2
airbornefield campaignwere analyzedand comparedwith
whichconverts
NO and0 3 to NO2 and
NO2 + hv-• NO + O
diagnostically
derived parametersto study the NOx
(R2)
photostationary
statein the troposphere
andtheprocesses
that
controlthisphotostationary
state.Our analysisfocussed
on two
(R3)
O + 0 2 + M-• 0 3 + M
setsof NO2/NO ratiosderivedfrom the data;thesewere
whichreformNO and 0 3 from NO2' Duringthe daylight
based
onoverlapping
NO andNO2 measurements
madebytwo
ß
.
hours,(R1), (R2), and (R3) can typicallycycleodd nitrogen
independent
techniques;
i.e.,a chemiluminescent
technique
and
a techniquebasedon two-photon,
laser-induced-fluorescence.betweenNO and NO2 on a time scaleof a few minutes.
Becausethis time scaleis considerably
shorterthan the time
While for any given 6- to 10-rain time interval the two
anddestruction,
these
observedNO2/NO ratios often exhibited significant scalefor NOx (NO + NO2)production
discrepancies,
these discrepancies
appearedto be mostly
randomrather than systematic,
and as a result,the average
differencefor all time intervalswith overlapping
NOx
measurements
wasonly12%. Onenotableexception,
however,
wasthe blockof data gatheredduringthe last three CITE 2
missions;
duringthesethree missions
the ratiosobservedby
thechemiluminescent
technique
weresystematically
largerthan
thoseobserved
bythelaser-induced
fluorescence
technique
by
reactions
tendto establish
a photostationary
statein whichthe
rateof destruction
(production)
of NO2 (NO) from(R2) is
balanced
by NO2 (NO) production
(destruction)
from(R1)
[Cadle and Johnston,1952; Leighton, 1961]. Under these
conditions,
therelativelevelsof NO2 andNO aredetermined
{[NO2]/[NO]}pss
= [03]kl / J2
a factor of 1.6. When the data from these three missions were
omittedfrom the analysis,the averagesof the observedratios
agreedto within 1%. In contrastto a numberof previous
studies,the ratios predicted from photochemicalmodel
calculations
were foundto be reasonably
consistent
with the
observed
ratios,althoughon averagetheytendedto fall about
20 - 25% belowthe observations.This agreementbetween
observations
andtheoryprovides
strongevidence
in supportof
the importanceof peroxyradicalsin the fast photochemical
cycling
of NO. (andtheconcomitant
photochemical
production
of03)inboth
themarine
andcontinental
troposphere.
Introduction
(1)
wherethe squarebracketsdenotea species'
concentration
and
k andj denotethe rate constantfor the appropriatereaction
or photolysis,
respectively.
A morecomplete
description
of the NOx photostationary
statemustaccountfor the additionaleffectof peroxyradical
reactionswith NO; for example,
(R4)
(RS)
(R6)
NO + HO2 -• NO2 + OH
NO + CH302 -• NO2 + CH30
NO + RO2 -• NO2 + RO
(In (R6), R is usedto denotean organicradicalinvolvingtwo
or moreC atoms,suchas CH3CH2.
) Thesereactions,
like
A fundamental tenet of present-day tropospheric
photochemical
theoryis that therelativelevelsof tropospheric
NO and NO2 are determined
by a rapidcycleof reactions
which establish a photostationarystate. The simplest
description of this photostationarystate involves three
(R1), convertNO to NO2 and lead to a modifiedexpression
for theNOx photostationary
state;i.e.,
{NO2/NO}pss,
=
([O3]k
1+ [HO2]k
4+ [CH302]k
5+ [RO2]k6)/j
2 (2)
reactions:
(R1)
Note thatunlike(R1), theperoxyradicalreactions(R4), (R5),
NO + 0 3 -• NO2 + 0 2
and (R6) convertNO to NO2 withoutconsuming
an 0 3
1School
of EarthandAtmospheric
Sciences,
Georgia
Instituteof Technology,
Atlanta.
2National
Centerfor Atmospheric
Research,
Boulder,
moleculeandasa result,whenfollowedby (R2) and(R3), act
asa netphotochemical
sourceof tropospheric
0 3. Whilethe
differencein the NO/NO
ratiospredictedby equations(1)
2
and(2) appearsto be smallfor conditions
typicalof the urban
atmosphere,
it canbe quitepronouncedin cleanair wherethe
Colorado.
ratioof{[HO2]+ [CH302]
+ [RO?.]}
to0 3tends
tobehigher
3National
Oceanographic
andAtmospheric
Administration,[Calvertand Stockwell,1983;Parrishet al., 1986].
Boulder, Colorado.
Simultaneous
observations
of NO, NO2, 0 3, andsolarflux
in Detroit, Michiganappearto verifythe accuracy
of equation
4NASA
Langley
Research
Center,
Hampton,
Virginia.
5Department
of Chemistry,
YorkUniversity,
Downsview,(1) for conditionstypicalof urbanlocations[Stedmanand
Ontario, Canda.
6NASA
Wallops
Flight
Center,
Wallops
Island,
Virginia.
7Atmospheric
Experiments
Branch,
NASAAmes
Research
Center, Moffett Field, California.
Copyright1990by the AmericanGeophysical
Union.
Paper number90JD00249.
0148-0277/90
/ 90JD-00249505.00
Jackson,1975]. However,the samehas not been true of
observations
madein lesspollutedenvironments
[McFarland
et al., 1978;Ritter et al., 1979;Fehsenfeldet al., 1983;Parrish
et al., 1986; Trainer et al., 1987]. The NO/NO
ratios
2
observed
in nonurbanlocationshavetendedto be largerthan
that predictedby equation (1). Furthermore,with the
exceptionof Trainer et al., theseinvestigators
havefoundthat
in orderto reproducetheirobserved
ratioswith equation(2),
it wasnecessary
to invokeperoxyradicallevelssystematically
10,235
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,236
GTE/CITE-2
larger than those predicted by simulations using a
by dividingthe NOCAR observed
NO2 by the NOCAR
photochemical
model;moreover,
therateof 0 3photochemical observed
NO and{NO/NO}G
formedfrom
2
rr wassimilarly
productionimplied by these peroxy radical levels were
the GIT NO2 andNO observations.
It shouldbe notedthat
generallyfound to be unrealisticallylarge and were not
for conditions
typicalof the daytimeCITE 2 flights(i.e.,[NO]
reflected
in a commensurate
increase
in local0 3levels.There ~ 10 - 15 pptvand [NO2]~ 20 - 30 pptv),the (l-a) total
are at least three possibleexplanationsfor the apparent
inconsistency
betweentheo observedNO/NO
ratios in non
ß
2
urban air and photochemical
theory:(1) the measurements
were subjectto systematicerrors,(2) our understanding
of
NO_ chemistryand/or the processesthat control peroxy
radicalsand O photochemistry
•s incomplete,or (3) the
atmospherecontranss•gnificantlevels of oxidants,like IO
radicals[Chameidesand Davis, 1980],whichare capableof
converting
NO to NO2 withoutproducing
0 3.
uncertainty(combinedprecisionandaccuracy)in the NO and
NO2 measurements
were20%for NOCARNO, 27%for GIT
NO, 30% for NOCAR NO2, and40% for GIT NO2 [Gregory
et al., thisissue(a, b)]. (Theseuncertainties
are basedon 1min and 6-min integrationtimes for the NOCAR and GIT
instruments,
respectively,
and are appropriatefor the typical
NO andNO2concentrations
indicated.Largeruncertainties
applyforsmaller
NOxconcentrations,
andsmaller
uncertainties
apply for larger concentrations.) If these uncertainties
propagated
quadratically
in theratioformedfromtheNO and
In thiswork,datacollectedduringthe NASA GTE/CITE 2
field operation[Hoell et al., this issue]are analyzedto shed
furtherlight on the problemof the NO 2/NO photostationary
statein cleanair. Our analysishastwo main facets.To obtain
an indicationof the accuracy
withwhichcurrenttechnology
can
wouldtypicallybe about35% for the NOCAR ratio and47%
measureNO2/NO ratiosin cleanair, an intercomparison
is
individual
NO andNO2 measurements
maycancelwhenthe
made of ratios measured during the field exercisewith
differentinstruments
usingdifferenttechniques.To assess
the
accuracyof present-daytroposphericphotochemical
theory,
each of the measuredratios is also comparedwith ratios
calculatedusinga photochemical
model.(For an additional
analysisof NO2/NO ratiosmeasuredduringthe NASA
GTE/CITE fieldstudiesthereaderis referredto M.A. Carroll
NO2 measurements,
the uncertainty
in the observed
ratios
for the GIT ratio. However, the actual uncertainties in the
ratio may be smaller,as someof the systematicerrorsin the
ratio is formed, since both the NOCAR systemand GIT
system
measured
ambientNO andphotolyrically
converted
NO
(from ambientNO2) usingthe sameprocedures
(i.e.,
chemiluminescent in the case of NOCAR
and TP-LIF
in the
caseof GIT).
The datafromthe NOCAR NO andNO2 instrumentation
weregenerallyrecordedin 1-mintimeintervals,whiletheGIT
etal.,(Observed
ratiosofNO2/NOcontrasted
withtheoretical NO data had resolutionvaryingfrom 2 to 6 min and the GIT
calculations:
NASA/GTE Cite 1 andGTE/CITE 2, submitted NO2 datawererecorded
with6- to 10-minresolution.Over
to Journalof GeophysicalResearch,1990).
the entire 8 missions,74 time intervals were identified with
overlapping
measurements
ofNO andNO2frombothNOCAR
Observed
NO2/NORatios
Our intercomparisonis based on data gathered during
missions
7, 8, 10, 11, 12, 14, 15, and 16 of the GTE/CITE 2
airbornefield campaign.(For a completedescription
of the
flight tracks and instrumentconfigurationthe reader is
referredto Hoell et al., [thisissue]and the referencescited
therein.) We focus primarily on two sets of NOx
measurements,
one beingthe simultaneous
NO and NO2
measurements
obtained
with
the
chemiluminescent
and GIT and it is the data gatheredduring these74 time
intervalsuponwhich our analysisis based. A listingof the
missionnumber,samplingperiod,andbarometricpressureof
the air sampledfor eachtime intervalis presentedin Table 1.
Note that the duration of each time interval varied from 6 to
10 min and was chosento approximately
coincidewith the
sampling
periodof the lowest-resolution
NOx measurement;
iße,ß theGIT NO measurement
ratiosobtained
ß The NO/NO
2
from
theNO•J•R
andGITmeasurements
along
withthe
observed
valuesfor0 3, CO,dewpointtemperature
(Td),and
instrumentationof the National Center for Atmospheric
Research and National Oceanic and Atmospheric
Administrationgroup, hereinafter referred to as NOCAR
NO (from NOCAR and GIT) for each time interval are
depictedin Figs. la throughlg. Each of the ratiosindicated
in the figureswasdeterminedby first averaging
the individual
[Ridleyet al.,thisissue],
andtheotherbeingtheNO andNO2
NO andNO2 measurements
overthetimeintervalandthen
dividing
theaveraged
NO2bytheaveraged
NO; thisapproach
measurementsobtained with the photofragmentation
twophotonlaser-induced-fluorescence
(TP-LIF) instrumentation
of the GeorgiaInstituteof Technology,
hereinafterreferredto
as GIT [Sandholmet al., this issue].
It should be noted that in addition to the NOCAR
and GIT
NOxobservations,
NO_measurements
weremadebytwoother
groups
duringthe •ITE 2 campaign.
Theseincluded
chemiluminescent
measurements
of NO and NO2 using
hasbeenfoundto yieldresultsthat are in closeagreement
with
thoseobtainedfrom the mathematicallyexactprocedureof
first forming the ratio and then averagingover the time
interval[Chameides
et al., 1987].Thevaluesfor 0 3,CO, and
Tdindicated
in thefigures
wereobtained
byaveraging
the30-s
observations
recordedfor theseparameters.
It shouldbe noted that while there was alwaysexcellent
instrumentation
from the WallopsFlightFacility,hereinafter
overlapbetweenNOCAR NO andNO2 measurements
and
referredto as WFF [Torres,1985]andNO2 measurements betweenthe GIT NO and NO2 measurements,
the overlap
usinga tunablediodelaserabsorption
spectrometer
fromYork
University,hereinafterreferred to as YU [Schiffet al., this
issue].
TheWFF NO2 measurements
werenotincluded
in our
between the NOCAR
and GIT
NO_ measurements
was more
variable.
Because
ofthisfact,and•ecause
thelength
ofeach
time interval was chosen to conform to the GIT
6- to 10-min
analysisfor the samereasonsas thosecitedby Gregoryet al.,
[this issue (a)]. The WFF NO measurementswere not
included because of the very limited overlap these
samplingtime, many of the time intervalsincludedin our
analysis had only partial coverage by the NOCAR
measurements
had with the NOCAR and GIT NOx
the entire samplingperiod of any giventime intervalshould
not havecausedsignificant
errorsin our analysis
aslongasthe
time intervalhad little or no chemicalvariability,sincethe
averageconcentrationover a portion of the time interval
shouldbe reasonablyrepresentativeof the averagefor the
entire interval. On the other hand, the lack of complete
NOCAR coveragemayhavegivenriseto significant
errorsin
our analysisfor thoseintervalswhichhad significantchemical
variability.To assess
thepossible
magnitudeof thiseffect,two
measurements.
Finally,because
ß the NO2 levelsfor mostof the
daytimeCITE 2 flightsremainedcloseto the 25 pptv (parts
pertrillionbyvolume)detection
limitof theYU NO2 system
[Schiffet al., thisissue],the YU datawerenot includedin our
analysis
exceptfor thosedatagatheredduringmission14,when
theNO2 levelswereunusually
high.
Twosetsof NO2/NOratioswereformedfromtheNOCAR
andGIT NOx observations:
{NO2/NO}NoCAR
wasobtained
measurements.
Thelackof NOCARNOxmeasurements
over
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
GTE/CITE-2
TABLE 1. (continued)
TABLE 1. Listingof Time Intervalsand Their Mission
Numbers,SamplingTimes,andAveragePressures
Time
Time
Interval
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
Mission
Time (UT)
P, mbar
10,237
Mission
Interval
Time (UT)
P, mbar
No.
No.
7
7
8
8
8
10
10
10
10
10
10
10
10
10
10
10
10
10
10
11
11
11
11
11
11
11
11
11
11
11
11
11
12
12
12
12
12
12
12
12
12
12
12
12
13
13
13
13
13
13
13
13
13
14
14
14
14
14
14
14
14
14
14
14
14
2056:51-2104:04
2107:05-2114:18
1824:53-1831:06
1850:37-1857:22
1900:34-1906:37
1822:53-1829:13
1828:53-1834:53
1836:37-1843:36
1843:36-1850:15
1917:29-1923:48
1923:29-1929:29
1929:29-1936:26
1938:10-1944:10
1944:10-1950:10
2000:33-2006:33
2006:33-2013:18
2014:33-2020:33
2105:49-2111:53
2122:18-2128:18
1833:19-1839:38
1839:38-1845:57
1847:38-1854:22
1909:26-1916:22
1917:26-1923:47
1923:47-1929:50
1936:24-1942:30
1942:30-1949:02
1950:30-1957:27
2044:33-2051:31
2055:59-2101:59
2101:59-2108:21
2123:10-2129:25
1817:44-1823:44
1823:32-1829:44
1829:44-1836:10
1842:52-1849:48
1850:52-1857:12
2015:00-2021:15
2025:15-2031:50
2034:24-204:36
2043:49-2050:24
2121:41-2128:5
2131:07-2137:2
2137:28-2143:45
1822:45-1829:4
1829:41-1835:44
1837:41-1844:3
1848:22-1855:00
1857:00-1903:00
1955:27-2001:27
2000:58-2007:27
2022:53-2028:53
2030:53-2036:53
1755:10-1802:01
1826:05-1832:49
1834:05-1840:38
1843:39-1849:39
1849: 27-1856:18
1857:18-1903:39
1912:56-1919:36
1919:36-1925:47
1927:36-1933:36
1938:43-1944:43
1944:43-1952:15
1959:05-2003:08
1002
1002
552
553
553
550
550
550
550
679
770
870
987
999
1000
1000
1000
930
930
738
738
738
718
709
632
564
562
561
631
633
633
632
541
540
540
540
541
770
769
769
769
826
688
612
561
561
561
561
562
566
566
544
540
769
773
770
770
772
772
804
834
778
630
549
465
66
67
68
69
70
71
72
73
74
15
15
15
15
15
15
15
16
16
1522:37-1533:20
1540:10-1550:00
1550:00-1600:25
1633:58-1644:47
1710:34-1721:22
1819:12-1829:12
1829:12-1839:12
2126:12-2136:12
2301:37-2312:27
531
530
531
531
535
530
531
529
529
analysesof the observedratios will be carried out and
discussed later:
one which
includes
all time
intervals
and
anotherwhichexcludes
all intervalsthatexperienced
morethan
a 5 K variationin dewpointtemperature.Thoseintervalsthat
had more than a 5 K variationin dew point temperatureare
identifiedin Figs.la throughlg by an asterisk.Duringmost
of these intervals,the aircraft was undergoingan ascentor
descentduringat leasta portionof the samplingperiod.
Beforecomparing
the observed
NO2,/NOratios,it is
interesting
to comparethe 74 overlapping
NO and NO2
measurements
made by the NOCAR and GIT instruments.
Scatter
plotsof NO andNO2dataareillustrated
in Figs.2 and
3 alongwith the Pearson-Rcorrelationcoefficients
calculated
from the data sets and the slopesobtained from linear
regressionof the data.The NO data setswere foundto be in
excellentagreement. The Pearson-Rcorrelationcoefficients
of 0.88 for all 74 time intervals and 0.85 for all intervals with
NO levelsbelow30 pptvindicatea highdegreeof correlation
betweenthe two techniques.The slopesobtainedfrom linear
regression
of the NO data (i.e., 0.79for all 74 time intervals
and 1.02for time intervalswith [NO] lessthan 30 pptv) also
indicate good qualitativeagreementin the absolutelevels
recordedby the two techniques.However,the interceptof
about5 pptv in both casesindicatesthat the GIT NO tended
to be systematically
higherthan NOCAR NO when [NO] was
belowabout20 pptv.
Giventhat the uncertainty
in the NO2 measurements
was
generallya factor of 1.5 larger than that of NO, it is not
surprisingthat the agreementbetweenthe NOCAR and GIT
NO2 measurements
was not as goodas that of the NO
measurements.The relativelyhigh degreeof scatterin the
NO2 measurements
is apparent
froma comparison
of Figs.2
and 3, and this is reflectedin the relativelysmallcorrelation
coefficient
of 0.45calculated
for [NO2]whenintervals
with
levelsgreater than 50 pptv were omittedfrom the analysis.
Like theNO scattergram,
linearregression
of theNO2 data
yieldeda slopelessthanunityß However
in the NO2 case,
.
.
.the
slopewassmallerand more consistently
lessthan umty0.e.,
0.62for theentiredatasetand0.46forall intervals
with[NO2]
lessthan 50 pptv). Togetherwith the interceptof about 10
pptv,theseplotsindicate
thatfor[NO2]< 20pptv,GIT tended
to be higherthan NOCAR, while for [NO2] > 40 pptv,
NOCAR was systematically
higher. While it •; possiblethat
the small bias in the NO data sets noted above and the more
substantial
bias in the NO 2 data sets noted here are only
ß
artifactscausedby the dataselectionandaveraging
techniques
adopted in our work and are not a reflectionof a real
systematicdifferencein the two techniques,they should
nevertheless be borne in mind in the discussion that follows.
Comparisonof ObservedRatios
The ratios from the first and second time intervals are
depictedin Fig. la. The datafor theseintervalswerecollected
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,238
•4
0d
o
GTE/CITE-2
•
•o
I
Mission
7 + 8
Mission
o
10
4
5
Time
6
7
8
9
10
11
Interval
12
Run
Fig. la
13
14
15
16
17
18
19
/•
Fig. lb
*
, , , , , ?i ?
Mission
20
i
21
!
22
'
11
i
23
i
24
i
25
Time
•
26
•
27
Interval
i
28
i
29
i
30
i
31
i
32
I
i
/t
Fig. l c
i
Mission
i
33
i
34
i
35
36
37
Time
38
39
40
41
42
43
44
Interval
Fig. l d
Fig.' 1ß Bar graphsdepicting
NO/NO
ratiosasa functionof time interval:openbar, GIT observed
ratio;
2
solidbar,calculated
ratiousingequation(2) withGIT NO; downward
hatchedbar,NOCAR observed
ratio;
andupwardhatchedbar: calculated
ratio usingequation(2) with NOCAR NO. Listedalongthe top in
orderof appearance
are[03] (ppbv),[CO](ppbv),Ta (øC),GIT [NO](pptv),andNOCAR[NO](pptv).
Asterisks
areusedto denote
timeintervals
witha vaiiation
greaterthana 5øCin Td duringthesampling
period. Quotationsmarksare usedto denoteparametersthat were extrapolated.(a) Intervals1-5,
missions
7 and8; (b) intervals6-19,mission10;(c) intervals20-32,mission11;(d) intervals33-44,mission
12;(e) intervals45-53,mission13;(f) intervals54-65,mission14;and (g) intervals66-74,missions
15 and
16.
duringmission7 while the aircraftwassampling
boundary
layerair overthe easternNorth PacificOcean. The NO levels
measured
byNOCAR andGIT duringthesetwotimeintervals
werein the 5 - 10pptvrange;theyagreedin onecasenearly
exactlyand in the other to within a factor of 2. However,
because
of a muchlargerdiscrepancy
in themeasured
NO2
levelsfor thesetwo time intervals,
the observed
NO2/NO
ratiosdifferedby a factorof 6, the largestdiscrepancy
in the
observed
ratiosof the entiredataset. By contrast,
the ratios
observedduringtime intervals3, 4, and 5, whichhad similar
levelsof NO, arein excellent
agreement
(seeFig. la); thedata
for theselater three time intervalswere obtainedduring
mission
8 whiletheaircraftwassampling
mid-tropospheric
air
over the eastern North Pacific Ocean.
Time intervals6 through19 (Fig. lb) are for datacollected
duringmission10,anotherflightovertheeasternNorthPacific
Ocean; as is indicated in Table 1, these data include air
sampledin the mid-troposphere
andthe boundarylayer. The
NO levelsrecordedduringthesetime intervalsvariedfrom
below 5 pptv for the boundary layer data, to values
approaching
20 pptvfor the free troposphere
data. A cursory
comparison
of the observed
ratiosdoesnot revealanyobvious
patternwithgoodagreement
for someintervalsandsignificant
disagreement
for others.
Time intervals20 through32 coverthe data analyzedhere
from mission11, a flight over the San JoaquinValley of
California;the dataare all from the mid-troposphere,
andNO
levelsin somecaseswere observedto exceed30 pptv. Note in
Chameides
et al.:NO2/NO Ratiosin Tropospheric
Air
GTE/CITE-2
10,239
.
3
c•
•
58
59
o
•
o
•
•-
60
61
62
63
64
$
Mission
13
M•ssion
14
4
2
o
z
2
1
1
O,
,
45
46
47
48
Time
49
50
51
52
0
53
54
55
56
57
Interval
Time
Fig. le
65
Intervol
Fig. If
.
I
I
Mission
I
15 +
I
I
I
I
?
I
70
71
72
73
74
16
o
66
67
68
69
Time
Fig. 1, (continued)
Fig. lg
Fig. lc that when the NO levelswere the highest(i.e., time
intervals24, 27, and 32), the GIT NO was lower than the
NOCAR
NO.
In contrast to data from later missions with
relatively
highNOxlevels,
theNO2/NOratioobserved
byGIT
was larger than that observedby NOCAR for two of these
three time intervals.
The dataanalyzedfrom mission12,anotherflightoverthe
easternNorth PacificOcean,are coveredby time intervals33
through44 and are depictedin Fig. l d. While thesedatawere
all obtainedfrom the mid-troposphere
(seeTable 1), the NO
levels remainedbelow 20 pptv. The largest discrepancy
betweenthe two observed
NO2/NO ratiosfor thesedata
occurredduringtime interval42 whenthe NOCAR ratio was
foundto be a factorof 3 largerthanthat of GIT; thisinterval
wascharacterized
by largevariationsin dewpointaswell as
NO andNO2,andit is possible
thatthe discrepancy
is more
a reflectionof samplingdifferences
in the twotechniques
than
in an actualsystematicerror in either of the measurements.
Timeintervals45 through53 depictedin Figurele coverthe
data analyzedhere from mission13, a continentalflight over
the southwestern
United
Interval
States.
All the data for these time
intervalswereobtainedfromthemid-troposphere,
andthe NO
levelsaveragedabout 20 pptv. With the exceptionof time
interval51,whenthe GIT NO2/NOratiowasa factorof 3
lower than the NOCAR ratio, the observedratios from this
missionagree quite well. While the data analyzedfrom
mission14 (i.e., time intervals54 through65) were also
obtainedfromthemid-troposphere
overlying
the southwestern
UnitedStates,theresultscontrastsharplywiththoseof mission
13. The NO levelsrecordedby both GIT and NOCAR were
significantly
higherthanthoserecordedduringmission13 and
exceeded30 pptv for all but the last three time intervals.
FUrthermore,
the GIT NO2/NOratiowasfoundto be less
thanthe NOC^R ratio for all the time intervalsanalyzedfrom
thismission(seeFig. If).
Time intervals66 through74 coverthe data obtainedfrom
missions
15and16,whenthe aircraftwasreturningto Wallops
Island,Virginia from Moffett Field, California. The NO levels
duringthesemissionsrangedfrom 15 to 30 pptv and, like
mission 14, the GIT observed ratios were less than the
NOC^R ratiosfor all but one of the time intervalsanalyzed
from the two missions.
A scatter plot of the entire set of NOCAR and GIT
observedratiosis presentedin Fig. 4. The figureindicatesa
large amountof scatterbetweenthe two ratios,with a fairly
low correlationcoefficientof only 0.18. (Note that the
correlationcoefficientimprovedto 0.44 when the two time
intervalsfrom mission7 were omittedfrom the analysis;the
largest discrepanciesbetween the observed ratios were
obtainedfromthesetwotimeintervals.)Thepoorcorrelation
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,240
GTE/CITE-2
80
I
I
I
I
I
I
0
I0
I
,,,,,,,,I
70 R:0.88
(0.85) ,,,,,,,"
o
_
60
b=0.79 (1.
R-0.18
b-0.27
_
o
R'-0.44
z
_
i•1
¸z
o
b'-0.59
.......
.......
-
4o
i
0
oo
"
2 ooO
o...?""
........
0
2O
0
10
1
0
0
10
20
30
40
50
60
70
80
o
NOCARNO [pptv]
Fig. 2. Scatterplot of GIT NO versusNOCAR NO. The solid
line showslinear regressionfor all data. The dashedline
showslinear regression
for datawith [NO] < 30 pptv. Values
for R, the correlationcoefficient,andb, the slope,are shown
for the entiredata set and for the datawith [NO] < 30 pptv
by the numbersoutsideand insidethe parentheses.
0
1
2
3
4
5
6
7
NOCARNO2/NO
Fig. 4. Scatterplot of GIT observedratio versusNOCAR
observed ratio.
The solid line, R, and b indicate linear
regressionline, correlationcoefficientand slope of linear
regressionline, respectively,for the entire data set. The
dashed line, R', and b' indicate linear regressionline,
in the observedratios is perhapsnot surprisinggiven the
correlationcoefficient,and slope of linear regressionline
sizeableuncertainties
in the NO and NO2 measurements respectively,for the data withouttime intervals1 and 2 from
mission 7.
themselves
andthe additiveeffectof theseuncertainties
upon
the
random
error
in
the
ratios
formed
from
these
measurements.
In order to further investigate the source of the
discrepancies
in theobserved
ratiosconsider
theobserved
ratio
difference(ORD), definedby
180
i
i
OR.D= {NO2//NO}NoCAR'
{NO2/NO}Grr
(3)
MAX[
{NO2/NO
}NOCAR,
{NO2/NO
}GIT]
wherethe functionMAX[A,B] is equalto the maximumof the
two quantitiesA and B. (Note that becausethe maximum
observedratio is usedin the denominatorof equation(3),
ORD is constrained
to varybetween+ 1 and -1. ORD values
i
160
of +0.25, +0.5, and +0.75 occur when the NOCAR ratio is
14-0
100
o
•
R-0.91 (0.4-5)
b-0.62
120--
larger than the GIT ratio by factorsof 1.33, 2.0, and 4.0,
respectively.
O RD valuesof-0.25, -0.5, and -0.75 occurwhen
the NOCAR ratio is smallerthan the GIT ratio by factorsof
0.75,0.5,and 0.25,respectively.)The ORD valuefor anygiven
time intervalrepresents
the relativeor normalizeddifference
in the two ratios for that time interval and is causedby a
combinationof random and systematicerrors in the two
(0 4-6)
'
__
80--
measurements. However, because random errors tend to
cancelwhenindividualobservations
are averaged,
the average
ORD for the entiredatasetshouldreflectonlythe systematic
60--
bias in the two measurements.
Thus if the two sets of
measurements
had no systematicbias, we would obtain an
averageORD of zero with a standarddeviationcharacteristic
of the random
o
o
0
20
4-0
60
80
100 120 140 160 180
NOCARNO2 [pptv]
Fig.3. Scatterplotof GIT NO2 versus
NOCARNO2. The
solidline showslinearregression
for all data. The dashedline
shows
linearregression
fordatawith[NO2]< 50pptv.Values
for R, the correlationcoefficient,andb, the slope,are shown
for theentiredatasetandfor thedatawith[NO2]< 50 pptv
by the numbersoutsideand insidethe parentheses.
differences.
In Fig. 5, O RD is plottedas a functionof time interval;the
meanvalue(+ 0.12)and1-a standarderrorof the mean(0.04)
for all 74 time intervalsare alsoplottedon the far right-hand
side of the figure. While a good deal of randomscatteris
apparentin Fig. 5 with ORD varyingfrom a minimumof-0.85
and -0.82 for the first two time intervals, to a maximum of
+0.7 for intervals31 and69, the meanORD is considerably
closerto zero. Thisresultsuggests
that a largefractionof the
non zero O RD valuesin Fig. 5 were causedby the random
errorsin the instrumentation
and samplingrather than by
systematic
errorsand thusthat at leaston average,the two
techniques
yieldedreasonably
consistent
NO2/NOratios.It is
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
GTE/CITE-2
1.000
I
I
I
I
co
o o
o
o
0.500
o o
o
o
o
0.000
4.O
ooo
o
o
o
10,241
o
o
2.0
o
o
o Ooøøo
ø
c•øo
oo
o
z
,1.33
o
o
•
o
o
o
z
ß
o
•
o
o
o0
o
o
O
o
0.75
o
o
o
-0.500
o
o
z
o
o0
o
z
0.5
o
o
o
•
o
0.25
- 1.000
0
15
30
45
Time
z
60
Interval
z
#
Fig.5. Observed
ratiodifference,
ORD,(withcorresponding
ratioof {NO2/NO}NOCAR
to {NO2/NO}orr
indicated
onrighthand
verticalaxis)asa function
of timeinterval.Meanratiodifferences
and1-astandard
error of the meansare indicatedon the far right for all time intervals(circle),time intervalswithout
missions
14,15,and16(triangle),
onlytimeintervals
frommission
14,15,and16(square).Notethatan
ORD value of +0.5 is indicative of a factor of 2 difference in the two ratios.
NO levelsexceeded
20 pptv;for thesehigherNO levelsthe
ORD valuesarepredominantly
positive,
indicating
a tendency
forNOCARratiosto begreaterthanGIT ratios.Interestingly,
alsointerestingto note that our resultsdo not appearto be
dominated
by errorsrelatedto chemical
variabilityduringthe
individualtime intervals;for instance,if we eliminateall the
intervalswhich had more than a 5 K changein dew point
closerexaminationof thesetime intervalsrevealsthat virtually
temperature
overtheirsampling
periodsandareindicated
by
an asteriskin Fig. 1, the meanORD improvesslightlyto 0.09.
Someinterestingcharacteristics
of the ratio differences
are
revealedfrom an inspectionof Fig. 6, where the ORD is
plottedas a functionof NOCAR [NO]. We find that the
largestpositiveandnegativeORD valuesare associated
with
NO levelsof about20 pptvor less,as onemightexpectsince
in thisrangethe NO_levelsare closeto theNOCAR andGIT
all of the largepositiveORD valuesobtainedwhen[NO] was
greaterthan20 pptvwerefrommissions
14, 15,and 16 (see
Fig.6). Whileit is notdearwhatfactoror factorsuniqueto
these missionsmight have given rise to a systematic
discrepancy
between
thetworatios,it isinteresting
to notethat
byeliminating
allthedatafromthesethreemissions,
themean
ORD decreases
from 12% to only1%. In contrast,the mean
betweenthe two ratios for low NO levelsappearsto be
randomin nature,a more systematic
trend is apparentwhen
NOCAR and GIT observed
ratiosfor theseflights.(SeeFig.
5 and Table 2).
ORD for the time intervalsfrom missions14, 15, and 16 is
detection
limits.•owever,whilemostof thediscrepancy+0.38, indicatinga factor of 1.6 differencebetweenthe
1.000
I
I
0.500
ß
cP
ß
o
o
-0.500
o
oo
OOoo o
-1.000
0
10
20
30
40
50
60
70
80
NOCARNO [pptv]
Fig.6. Observed
ratiodifference
asa functionof NOCAR NO. Opencirclesarefrommissions
7, 8, 10,
11, 12,and 13,andsolidcirclesare from missions
14, 15,and 16. The solidline indicateslinearregression
line.
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,242
GTE/CITE-2
TABLE 2. Mean Ratio Differences,StandardDeviations,and StandardErrors of Means
Ratio Difference
Data Set Without
Full Data Set
Missions
Data SetWith OnlyMissions
14, 15, and 16
14, 15, and 16
ORD
+0.12 +0.38(0.04)
+0.01+ 0.36(0.05)
+0.38+ 0.19(0.04)
(CRD)GIT
+ 0.38+0.32(0.04)
+ 0.49+ 0.27(0.04)
+0.01+ 0.27(0.06)
(CRD')GIT
+ 0.09+0.36(0.04)
+ 0.20+ 0.30(0.05)
-0.20+0.25(0.05)
(CRD)NOCAR
+0.50 +0.23(0.03)
+0.50 +0.25(0.04)
+0.50+ 0.18(0.04)
(CRD')NOCAR
+0.23 +0.26(0.03)
+0.23 +0.30(0.05)
+0.23+ 0.16(0.04)
Valuesshownare the meanratiodifference 1-rstandarddeviations
(1-r standarderrorof the means).
Could the poor agreementbetweenthe observedratios
during missions14, 15, and 16 have been causedby a
systematic
error in either the NOCAR or GIT NO2
theobserved
NO2/NOratiosexhibited
muchlessvariability,
rangingfrom a minimumof about0.5 to a maximumof about
6ß The fact. that. the
ratio variabilityis muchless
. NO/NO
.2
measurementsmade during these flights? One way of
investigating
this possibilityis to comparethe NOCAR and
thanthevariability
ane•ttierNO or NO2 suggests
thatthese
GIT NO2measurements
fromtheseflights
withthoseobtained
would expectif NO and NO2 were controlledby a
from the YU tunable diode laser absorptionspectrometer.
(A uniqueaspectof the the YU systemis that it measures
photostationary
staterelationshipof the form of equation(1)
or (2). In order to carry out a more quantitativetest of
two speciesare positivelycorrelatedwith eachother,as one
theory,NO2/NOratiosbasedonequations
(1)
NO2 directlywhereas
boththe NOCAR andGIT systems photochemical
andcompared
withtheratiosobtained
measureNO2 throughdetectionof photolytically
converted and(2) werecalculated
from the GIT and NOCAR measurements,as describedbelow.
NO.) In Table3 the NO2 concentrations
recorded
fromthe
threedifferenttechniques
arelistedfor ninetimeintervals(5462) from mission14. We limited the comparisonto these
intervalssincethey had high NO. levelsand were therefore
mostsintablefor an totercomparison
•nvolvlngthe YU data.
Unfortunately,no firm conclusions
can be reachedfrom the
datain Table3. Whilethe YU NO2 measurements
are in
better agreementwith the GIT measurements
during some
time intervals, they agree better with the NOCAR
measurements
duringothers.Thusno apparentreasonfor the
anomalously
poor agreementbetweenthe NOCAR and GIT
In the caseof the simplephotostationary
staterelationship
(iße,' equation(1)), valuesfor {NO•,/NO}
,z
p sswerecalculated
for eachof the74 timeintervals
usingtheaveraged
[03] and
temperatureobserved
duringeachinterval.(The temperature
duringeachtimeinterval
wasusedto determine
kl, using
the
Arrheniusexpression
recommended
byDeMore et al., [1987].)
Values
fromtheUV-Eppleyphotometer
ß for J2 werecalculated
readingsrecordedduringthe flightsusingthe semi-empirical
formulaof Madronich[1987];i.e.,
ratios from missions14, 15, and 16 can be identified at this
J2=
time.
1.35
Eu
+ 2(1.14)E
a
(0.56 + 0.03z)cos0 + 0.21- 0.015z
Calculated
NO2/NORatios
While the NO and NO2 levelswere observed
to varyby
almost 2 orders of magnitudeover the 74 time intervals
considered,
examinationof Figuresla throughlg revealsthat
wherez is altitude(in kilometers),
0 is zenithangle,andEu
andEd are the upward-looking
anddownward-looking
Eppley
readings(in megawatts
per squarecentimeter),respectively.
TABLE 3. Comparisonof NO2 Measurements
for Time IntervalsWith HighestNOx Concentrations
Time Interval
NO2 Observed
NOCAR
54
55
56
57
58
59
60
61
62
110
76
66
122
127
140
130
175
128
(4)
Concentrations,
pptv
GIT
YU
102
38
46
90
124
105
91
103
67
66
63
50
125
100
99
71
66
95
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
GTE/CITE-2
The reader shouldnote, however,that there is someindication
10,243
duringthe flights. Sincethe model-calculated
peroxyradical
concentrations
are dependenton [NO], twosetsof calculations
that the useof the Eppleyreadingsfromthe CITE 2 flightsto
infervaluesfor
a systematic
errorinto
ß J2 mayhaveintroduced
our calculations. While the Eppley readingsupon which
Madronich based his formula typicallypeaked at about 6
were carried
out for each time interval
in order to avoid
biasing the calculationsin favor of either set of NO_
observations;
onesetofcalculations
usedNOlevels
observec']
mWatts
cm-2, thepeakreadings
fromthe upward-lookingby GIT and the otherthe NOCAR NO levels.Concentrations
Eppleydurin•g
someof theCITE 2 flightswerein excess
of 7
of CH4 andH2 wereassumed
to be 1.7and0.55ppmv,
mWatts cm-'. While radiative transfer calculations indicate
respectively.
Photolyticrate constantshave a strong impact on our
that theselargepeakEppleyreadings
couldhavebeencaused
bycloudalbedoeffectsfromclouds
positioned
overthehorizon
but out of the line of sightof the sun,it is alsopossiblethat
the largereadingsreflectinstrumental
problems
with the UVEppleyphotometers
usedduringthe flights.If thislater case
is
ß true, thenthe
, J2 valuesinferredfrom equation(4) andused
tn our calculations
are overestimated
by 15-20%.
calculated
NO2/NOratios.Theserateconstants
include
theJ valuefor thephotolysis
of NO2,whichappears
explicitly
in equations(1) and (2); and a hostof otherswhichare input
into the photochemicalbox model and affect the levels
calculatedfor peroxy radicals. The J values used in our
calculationsof the modified steady state equation were
calculatedfor eachtime intervalusinga two-streamradiative
Valuesfor {NO../NO} ss'(i e, the modifiedsteadystate
relationship,
equation
(2)•,reqmre
allofthedatadescribedtransfer
resultspresented
here,we usedconcentrations
of HO2 and
CH302 radicalsobtained
froma photochemical
boxmodel
similarto that describedby Chameideset al., [1987, 1989].
The model is based on a chemical mechanism which included
ox/dation
sequence.
•a•culations
were
also
carried
outthat
estimated
from
a
more
detailed
photochemical
modelcapableof simulatingthe oxidationof
nonmethanehydrocarbons
[Chameides,1984; Richardson,
1988]whichwasconstrained
toyieldnonmethane
hydrocarbon
and peroxyacetyl
nitrate (PAN) levelsconsistent
with those
observedduringthe CITE 2 flights[Singhet al., thisissue].
data set was 1.1.
The resulting
NO2/NO ratioscalculated
for eachtime
The effectof the calculated
RO2 radicalswasfoundto be
small;in general,
inclusion
of RO2 radicals
resulted
in onlya
10%enhancement
in thecalculated
NO2/NOratiosoverthose
calculated
withonlyHO2 andCH302 radicals.)
In thephotochemical
boxmodel,thelevelsof 03, NO, H20,
CO. CH..
- ß
-_-q,,
and H•. as well as teml•erature
•
the solar flux for cloud-free
two-streammodelandthatcalculatedusingequation(4). The
minimumscalingfactorobtainedfor the datasetwas0.7;this
valuewas calculatedfor a boundarylayer time intervaland
probablyreflectsthe influenceof overlyingclouds. The
maximumscalingfactor,on the otherhand,was1.6;thisvalue
wascalculated
for a mid-tropospheric
time intervalandcould
havebeencausedbyunderlying
cloudsreflectingradiationback
up to the aircraft. The averagescalingfactorfor the entire
included
theeffectof longerchainperoxyradicals
(i.e.,RO2)
on the NO2/NO ratios. The RO2 levelsneededfor these
were
to determine
equalto the ratiobetweenthe J2valuecalculated
usingthe
the reactions
of the HO -NXOy -O•.• system
andthe
CH4-CO
.
calculations
model
conditionsas a functionof altitude,solar zenith angle,and
wavelength
[Dickersonet al., 1979;]andan empiricallyderived
scalingfactorto includethe effectof cloudsandothervariables
not simulatedin the radiative transfermodel. The scaling
factor was assumedto be wavelengthindependentand was
aboveas well as concentrations
of peroxyradicals. For the
intervalusingGIT and NOCAR NO levelsas input to the
photochemical
boxmodelare indicatedin Figuresla through
lg. The effectof the calculatedperoxyradicalconcentrations
ontheNO2/NOratiosisillustrated
in Figures
7 and8,where
NO2/NOratioscalculated
fromequation
(1) andequation
(2)
and r)ressure are
specified,
andtheconcentrations
o•short-lived
species
such
as
are plotted as function of time interval. The modified
stateequationwasfoundto be largerthanthat
HO2 and CH302 are calculated
assuming
photochemical photostationary
equilibrium.For the calculations
presented
here,the values
of the simplephotostationary
stateby a factorof 1.3to 2. The
largestdifferenceswere calculatedfor time intervalssuchas 1
and 2 with NO levelsof 10 pptv or less,relativelyhigh dew
usedfor [O3], [CO], [NO], dew point temperature,
and
pressurefor eachtime intervalwere equalto thoseobserved
o
z
o
5
10
15
20
25
30
Time
35
40
Interval
45
50
55
60
65
70
75
•t
Fig.7. Calculated
NO2/NOratiousing
NOCARNO asa function
of timeinterval.Opencircles
denote
ratioscalculated
with equation(1), andsolidcirclesdenoteratioscalculated
with equation(2).
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,244
GTE/CITE-2
0
z
ßß
ß
ß
o
oo
ooo
ß
ooo
•oo•
o
o
--
o
ßß
•
ß
ßß
o
•
ß
ß
ß
o
o
ß
oo
o
0
5
10
15
20
25
30
Time
35
40
45
50
55
•0
•5
o
70
75
Intervol
Figß 8ø Calculated
NO/NO
ratiousingGIT NO asa function
of timeinterval.Opencirclesdenoteratios
2
calculated
with equation(1), andsolidcirclesdenoteratioscalculated
with equation(2).
pointtemperatures,
and0 3 levelsof 30 ppbvor less;thelow
NO andhighH20 concentrations
favorlargerperoxyradical
o
200
levels,and the combinationof this effect and the low [O ]
causes
relatively
large
rates
ofconversion
ofNOtoNO
2!•y o<
reactions(R4) and(R5) comparedwith (R1). Conversely,
the
smaller differences were obtained for time intervals such as 70
characterized
by higherlevelsof NO and0 3 andlowerdew
point temperatures.An exception
to this generaltrendwas
found for time interval 74, which had the smallestdifference
betweenthe twocalculated
ratiosof the entiredatasetin spite
of a relativelymodestNO level;however,in thisparticularcase
z
Ro-097
'"
b -0 63
150
o
ß
,,,,'
'•
R'-0.98
•
b'-0.95,,•
•> 100
-
,,,,
thesolarzenithanglewasexceptionally
large(i.e.,80ø) andas
a result,radicalformationwas suppressed.
z
g 50
Comparison
of Observedand CalculatedRatios
In Figures9 and 10 we presentscatterplotsof calculated
andobserved
NOs concentrations
fromtheNOCAR andGIT
datasets,respectFvely.
While linearregression
of the datain
theseplotsyieldsslopesreasonably
dose to unity and high
Pearson-r correlation coefficients,some caution should be
0
o
•"'• o
50
100
150
200
NOCARNO2 [pptv]
exercisedin interpretingthese results. The slopesare
dominated
by the datapointswithhighNO2 concentrations Fig.9. Scatterplot of calculated
[NO• (usingNOCAR NO
andare not representative
of the entiredataset. In the case
obse•ations)
andNOCARobsemed
[NO•. •e circlesare
of the correlation coefficient it should be borne in mind that
for [NO• calculated
uskgequation(1) andthetrian•esare
the calculated
NO2 is obtainedfrom the productof the
for [NO-1 calculatedusingequation(2) •e sold line, R,
observed
NO concentration
andthecalculated
NO2/NOratio. andbo •dlcate•ear recession
•ne, correlatmn
coefficient,
SincetheNO2/NOratiois onlya weakfunction
of [NO],the
and slope of •ear recession,respectively,
for [NO•
high correlationcoefficientobtainedbetweenthe calculated
and observed
NO2 is more an indicationof the strong
correlation
betweenambient[NO] and[NO2]ratherthana
quantitative
validationof photochemical
theory.Nevertheless,
the fact that ambient[NO] and [NO2] are found to be
correlated is consistentwith the photostationarystate
equationsand as such representsat least qualitative
confirmationof photochemical
theory.
As an alternate approachto comparingtheory and
observations,
we define the followingfour calculatedratio
difference(CRD) functions:
calculateduskg equation(1). •e dashedline, R', and b'
•dicate linearregessionline,correlationcoefficient,
andslope
of lkear recessionline, respectively,
for [NO• calculated
uskg equation(2).
(CRD')Grr =
{NO2/NO}
GIT'{NO2/NO
}pss',GIT
(Sb)
MAX[
{NO2/NO
}GIT,
{NO2/NO
}pss',GIT]
(CRD)NoCAR
=
(CRD)Grr=
{NO2/NO}Gr
r - {NO2/NO}pss
MAX[
{NO2/NO
}orP
{NO2/NO
}pss]
(5a)
{NO2/NO}NoCAR
' {NO2/NO}pss
MAX[
{NO2/NO
}NOCAR,
{NO2/NO
}pss]
(5c)
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
GTE/C1TE-2
photostationary
stateequation,(CRD')GiTis the fractional
0
difference
Z
•
10,245
140
between
the GIT
observed
ratio
and the ratio
calculatedwith the modifiedphotostationary
state equation
,,"
usingthe GIT observed
NO, (CRD)NoC^R
is the fractional
•120
•
difference between the NOCAR
•,,,,"
observed ratio and the ratio
calculatedfrom the simplephotostationary
stateequation,and
(CRD')NOCARis the fractionaldifferencebetweenthe
NOCAR
,_• 100
observed ratio and the ratio calculated with
the
modifiedphotostationary
state equationusingthe NOCAR
8o
•
z
o
•
60
,"
•,'"
,,,,,"•
o
observed NO.
Plots of the four CRD
bo=0.85
/•o
functions versus time interval
are
presentedin Figures11 and 12.Similarto our comparison
of
the observedratios in Fig. 5, we found a large amountof
variabilityin all four CRD functionsfrom onetime intervalto
another,withmanyintervalshavingCRD magnitudes
in excess
of 0.5 (i.e., a factor of 2 discrepancy).However,a large
fraction of the discrepancybetween the calculated and
observedratiosappearsto be randomratherthan systematic.
As indicatedon the far righthandsideof Figures11 and 12
b'=1.5
and in Table 2, the mean calculatedratio differencesare within
0
20
40
60
80
100
120
140
20-25%of 0 whenthe modifiedphotostationary
stateequation
wasused,implyingat leastreasonably
goodagreement
between
observations and present-day photochemical theory.
Furthermore,whilethe averageratio differences
betweenthe
160
GITNO2 •-pptv]
Fig. 10. Scatterplot of calculated
[NO2] (usingGIT NO
observations)
andGIT observed
[NO2]. The circlesare for
[NO2]calculated
usingequation
(1), andthetriangles
arefor
[NO2]calculated
usingequation
(2). Thesolidline,Ro,andbo
GIT
observations and the calculations are somewhat smaller
indicatelinearregression
line,correlationcoefficient,
andslope
thanthoseinvolvingtheNOCAR observations
whenthe entire
data set is used, the agreementbetweenobservationsand
theoryisvirtuallyidenticalfor bothsetsof ratio measurements
of linearregression,
respectively,
for [NO2]calculated
using
when the time intervals from missions 14, 15, and 16 are
omittedfrom the analysis.(Interestingly,the NOCAR CRD
functionsare not affectedby the inclusionor exclusionof the
equation(1). The dashedline, R', and b' indicatelinear
regressionline, correlationcoefficient,and slope of linear
data from the last three missions, while the GIT CRD
regression
line, respectively,
for [NO2] calculatedusing
functionsincreasesignificantly
whenthe datafrommissions14,
15, and 16 are omitted. In fact, the ratios predictedby
equation(2) are actuallylargerthanthe ratiosobserved
by the
GIT instrumentation
duringthesemissions.)
equation(2).
(CRD')NOCAR
=
{NO2/NO
}NO'CAR
- {NO2/NO
}pss',NOCARk-,u)
Conclusions
MAX[
{NO2/NO
}NOCAR'
{NO2/NO
}pss',NOCAR] An analxm;cnf
Wan [nclopgnclgnt
qgtqnf airhnrne.NO
where(CRD)Grristhefractional
difference
between
theGIT
measurements
obtained
during
theNASA
GTE/CITE
2 fiei•
observed ratio and ratio calculated from the simple
samplingcampaignindicatesreasonableagreementbetween
-o
o
1.000
I
-o
o
c.a
0
•
I
I
o o o:p m o
oo
o o oo oo ooo
oO
ø øaD o øooee
ß o
ßß o,ß ß * o'ßo*ß .oo.oo ßo ß•mOoOd•
,o.
4.0
n,'
• 0.500
.o''cøø''ø
o
.o.
o.
o
q.øoøm.
}...
2.0
o ß ß ß
ß
.'
!!•. 1.33
Q)
o
z
ße
0.000
o
ß
o .'
.•
ß
eeee e
Oe
e ee
ß
1.0
ee
.
0.75
.•-0.500
ß
0
o
I0.5
•
O
•
0.25
o
•0
n,'
-1
'
000
0
i
i
i
15
30
45
60
z
Time Interval #
Fig.11. Calculated
ratiodifference
(withcorresponding
ratioofobserved
tocalculated
NO2/NOindicated
onright-hand
verticalaxis)usingNOCARobservations
asa function
of timeinterval.Opeficircles
denote
(CRD)NoCAR,the NOCAR calculatedratio differenceusingequation(1). Solid circlesdenote
(CRD')NOCAR , the NOCAR calculated
ratio differenceusingequation(2)ß Mean valuesandstandard
errors6f themeanare indicatedon the rightfor all timeintervals(circles),all timeintervalsexceptthose
frommissions
14,15,and16(triangle),andonlythetimeintervals
frommissions
14,15,and16(square).
Chameides
et al.:NO2/NORatiosin Tropospheric
Air
10,246
'• 1.000
I
t
•
•
co
o...o"
o i•,o
oC•
E
o
•
0.000
ß ß.
•o
.
4.0
o
ooOcr
p
*
•Oo.
•
•
o .. o ooo
o
• 0.500
, o **
•
•
ß
oo
eee
GTE/CITE-2
o
o
. 2.0
• .o •
oe
ß
. Oo
ß
o o
o •
• -0.500
1.0
ß
'o
0.75
e•
0.5
•
••
0.25
-•
. 000
0
•
•
•
•
•5
30
•5
•0
z
7ime Interval
Fig.•2. Calculated
milodiff•nc• (wi•hcorresponding
miloof obs•d m calculated
NO•/NO •d•ca•d
on •gh•-handv•fical •is) usingOTT obs•afions as a functionof fim• ••al.
(C•)o•
Open c•cl•s
•h• OTTcalculated
milodiff•nc• using
•quafion(1). Solidc•cl•sd•no• (CRD')o•,
calculated•afio d•f•nc• using•quafion(2). •an valuesand s•anda•d•o•s of •h• m•an a• •dica•d
on •h• •
fo• • •
••s
(c•d•s)• aH fim• in••s
•xc•p• •hos• •om missionsl•, l•,
(•Jan•)• and o•y •h• fim• incomesf•om missions• • and •6 (squa•).
theNO2/NOratiosinferred
fromtheobservations.
Whilethe
two ratioswere found to differ by a factor of 2 or larger for
some 6 to 10 min time intervals with overlapping
measurements,
the averagedifferencefor all time intervals
analyzedwas only about 10%, with a standarddeviationof
40% and a standard error of the mean of 4%. Elimination
of
anomalousdata from three missions,
yieldedan averageratio
differenceof only 1% with a similar standarddeviationand
standarderror of the mean. Two importantpointscan be
inferred from theseresults:(1) large randomvariationsare
overestimation
ofthe
oJ
• values
from
equation
(4).Thus
a
model-predicted
peroxy
radical
more stringent test
conversion
of NO to NO2 will likelyrequireaccurate
NO2actinometric
measurements
aswellasNOx measurements.
Acknowledgments.This work was supportedin part by
fundsfromtheNationalAeronautics
andSpaceAdministration
(NASA) undergrantsNAG-1-385andNAG-1-50andfromthe
National
Science
Foundation
under
8208828
and
ATM-
8600888. We thank RogerNavarro,the flight crew,and the
associated
withthe NOx measurements,
makingcomparisons maintenance
personnelof the NASA Wallops Island Flight
of quantities
suchastheNO2/NO ratiofromindividual
time
Facilityresponsible
for the Electraaircraft. We alsothankthe
intervalsstatistically
meaningless;
and (2) with the useof the
personnelat the NASA Ames ResearchCenter for providing
entire data set, a statisticallymeaningfulcomparisoncan be
facilitiesfor ground-based
operationsaswell as flightsupport.
made, and this comparisonindicatesreasonableagreement
Partial supportwas provided by the NASA Tropospheric
within the stated standard error of the mean.
Chemistry Program to M.A.C. and B.A.R. The National
A comparisonof the observedratios and thosecalculated
Centerfor Atmospheric
Researchis sponsored
bytheNational
basedon photochemical
theoryalsoindicatesreasonably
good
Science Foundation.
agreement. In contrastto a numberof previousstudiesof
NO2/NO ratiosin nonurban
air, therewasno evidence
of a
large systematic
trend with model-calculated
ratiosfallingfar
below the observedratios. The better agreementbetween
observations
and theoryin the caseof the CITE 2 data base
is likelydue,at leastin part,to the techniques
usedby the GIT
and NOCAR instruments
to convertNO2 to NO when
measuring
NO2 levels;thesetechniques
appearto be more
specific
toNO2thantechniques
usedbyotherinvestigators
and
as a resultthe GIT and NOCAR NO2 measurements
are
probably
lessinfluenced
byartifacts
arising
fromotherNOy
compounds
[cf.,Gregoryet al., this issue(b)].
However,whileour analysis
appearsto qualitatively
confirm
present-day
photochemical
theory,thereis stillevidencefor a
small,but statistically
significantdiscrepancy
betweenmodel-
calculated
andobserved
NO2/NOratios.On average,
modelcalculatedratios using the modified photostationarystate
equationare about25% smallerthan the observedratios;the
standarddeviationof the meanfor bothdatasetsis onlyabout
5%. While this discrepancy
couldeasilyhavebeencausedby
an error eitherin the NOx measurements
or in the modelcalculatedperoxyradicalconcentrations
or somecombination
of the two, it couldjust as easilyhave been causedby a 25%
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