JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D23, PAGES 30,275-30,307, DECEMBER 20, 1999
Photochemistry and budget of ozone during the Mauna Loa Observatory Photochemistry Experiment
(MLOPEX 2) D. A. Hauglustaine, • S. Madronich,B. A. Ridley,S. J. Flocke,C. A. Cantrell, F. L. Eisele,R. E. Shetter,D. J. Tanner,P. Ginoux,2 and E. L. Atlas National Center for Atmospheric Research, Boulder, Colorado
Abstract.
During the Mauna Loa Observatory Photochemistry Experiment
(MLOPEX 2), simultaneous measurements of a large numberof photochemical specieswere measuredduring different seasonsat Mauna Loa Observatory
(MLO), Hawaii. In this study,thesemeasurements are usedto constraina detailed photochemicalbox model and evaluate our understandingof the tropospheric photochemistryin this region of the Pacific. The simulationsgenerally reproduce satisfactorilythe NO/NO• photostationarystate, which controlsthe ozoneproduction rate. However, the model fails in simulating the concentrationof peroxy
radicals(PO2) duringall seasons and of hydroxylradical(OH) duringsummer. Severalhypothesesare consideredto assessthis discrepancy,includingthe removal of radicalsby unidentifiedmechanisms and the potentialimpact of biogenicorganic compounds.None of the testedhypothesesgivesatisfactorilyresultsin terms of OH, PO2 and NO/NO2 simultaneously. Althoughexperimentaluncertaintiesare large for radicals,this issueconstitutesa major inconsistencybetweenmeasurementsand model resultsduring MLOPEX. Another disagreementarisesfrom the simulationof peroxidesfor free tropospheric conditions. The model tends to overestimate H202
and CH3OOH by a factor of 1.5-2.5. On the other hand, a fair agreementis achieved in simulatingformaldehydewhen CH3OOH is constrainedin the model. Finally, we find that the grossozoneproduction and destructionrates are nearly in balance in this regionof the Pacific troposphere.The net productionis slightly negative, rangingfrom nearly0 in winter to about-1.4 ppbv/d duringsummer.In contrast, the NOx budget showsa severeimbalance. Our resultsindicate that an additional
sourceof NOx rangingfrom 18 to 48 pptv/d (in winterand summer,respectively) would be requiredto sustainthe 30 pptv of NO• measuredon averageat the site during free troposphericconditions.Acetone has little effecton the budget of HO•
at the altitudeof MLO (3.4 km). However,includingthis speciesin the model inducesan even larger imbalancein the NO• budget through the productionof peroxyacetylnitrate. 1. Introduction
In orderto providea better understandingof the photochemicalprocesses that control the oxidative properties of the remote troposphere,a seriesof experiments
wasconductedat the Mauna Loa Observatory(MLO) on the islandof Hawaii (19ø32•-155ø36•).The first •Presently at Serviced'A•ronomie du Centre National de la RechercheScientifique,Paris.
•Presently at NASA Goddard Space Flight Center,
MaunaLoa Photochemistry Experiment(MLOPEX 1) was carriedout in spring1988 [Ridley and Robinson, 1992]and the secondMauna Loa PhotochemistryExperiment (MLOPEX 2) was conductedin fall 1991 (MLOPEX 2a), winter 1992 (MLOPEX 2b), spring 1992(MLOPEX 2c), andsummer1992(MLOPEX 2d) [Atlasand Ridley, 1996]. During theseexperiments,a largenumberof photochemicalspeciesand related physical quantities were simultaneouslymeasured,which offered the opportunity to constrain and evaluate photochemical
Greenbelt, Maryland.
Copyright 1999 by the American GeophysicalUnion. Paper number 1999JD900441.
0148-0227/99/1999JD900441509.00
models.
The trade
wind inversion
near the Hawaiian
islands
is almost alwayslocated at a height below the MLO researchstation. However,from midmorning to evening, an upslopeflow generallyadvectsair from the boundary layer to MLO. During night to midmorning, a downs30,275
30,276 HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
AND BUDGET
lope flow is usually observed,and when the trade wind inversion is located below the observatory, concentrations of speciesat the site are expected to be representative of the free troposphereand not influencedby
localsources[Mendonca,1969;Hahn et al., 1992;Hess et al., 1996]. Dependingon the time of day, samples collected at MLO can be representative of the remote
free troposphere at the site elevation(3.4 km abovesea levelor about680mbar) impactedby large-scale transport of anthropognenicemissions,or air massescan be a mixture of remote marine boundary layer, free troposphere, and emissionsfrom the island. The free troposphericfiltering usedhere was madefor eachday of the
experimentas describedby Atlas and Ridley [1996].
OF OZONE DURING
MLOPEX
2
pointed out several shortcomingsin our understanding of the NO-NO•. photostationarystate and budget of radicals during MLOPEX 2. No definitive conclusionson the presenceand nature of NO to NO2 oxidants could be proposedin this previous work for MLOPEX conditions. Furthermore, this previouswork considered the daytime variation at MLO which includesdifferent types of air masses.The main objective of the present study is to assessthe ability of the model to simulate time-varying conditionsand the day-to-day variability of the measurementscollectedduring MLOPEX 2 during the four seasons.We emphasizethe simultaneous simulationof abundances and partitioningof the odd nitrogen and odd hydrogenspecieswhen constrainedby
During MLOPEX 1, inconsistenciesbetween mea- additional measurements. The model has been modified surements and photochemical models were identified to include NMHCs and their potential role on radicals [Liu et al., 1992], and suggestedthe needfor reduc- and NO to NO2 conversion. The model is driven by ing experimentaluncertaintiesand for conductingmea- observedtime-varyingNO2 and Oa photodissociation surementsof radicals. During MLOPEX 2, new tech- rates and considersthe effect of clouds on photolysis niques were adopted to provide measurementsof ozone rates. In section 3 the focus is on a subset of the obphotolysisrate [Shetteret al., 1996],and of hydroxyl servationsmade over the four seasons,namely, thoseof [Eiseleet al., 1996]and peroxyradicals[Canttellet al., the free tropospherewhich are expected to be repre1996a],and more effort was placedon comparingdif- sentativeof a much larger scaleover the North Pacific ferent measurement techniquesfor nitric acid, perox- Ocean. Here the budgetof ozone,odd nitrogen,and odd ides[Staffelbach et al., 1996],andformaldehyde [Heikes hydrogenis presentedas inferred from the model and et al., 1996a]. Furthermore,MLOPEX 2 took place observations.Section4 concludeswith a summary emduring four intensivesto provide a picture of the sea- phasizing unresolvedissuesbetween observationsand sonal variations in transport and photochemical pro- the model simulations. cesses.Interpretation of someof the photochemistryat MLO during MLOPEX 2 can be found in Journal of
2. Variability of the Photochemistry
Geophysical Research (volume101(D9), 14,531-14,813, 1996)and in the workby Canttell et al. [1997a],Ridley During Upslope and Downslope Periods et al. [1997a],Ridleyet al. [1998],and Hauglustaine et at Mauna Loa Observatory al. [1996a].Severalinconsistencies and contradictions between
measurements
and models
have been identi-
2.1. Modeling Approach
fied. In particular, assessments of our understanding The photochemicalbox model used in this study is of radical concentrations [Eiseleet al., 1996; Canttell conceptually similarto that usedby Ridley et al. [1992] et al., 1996a,b; Hauglustaine et al., 1996b; Canttell et and Hauglustaineet al. [1996b].The chemicalmecha-
al., 1997a],formaldehyde andperoxides photochemistrynismdevelopedby Madronichand Calvert[1989,1990] [Zhouet al., 1996;Heikeset al., 1996a;Brasseuret al., has been adopted and includesabout 1600 reactionsin 1996],or the budgetof nitrogenspecies[Brasseuret al., its currentversion.NMHCs (i.e., C2H6, C2H4, C2H2, 1996;Hauglustaineet al., 1996a]havebeendiscussed. C3Hs, C3H6, nC4H10,iC4H10, C5H12, C5Hs, C6H6) In this paper, section 2 usesthe MLOPEX measurements made during both upslope and downslopeflow conditions in combination with a detailed photochemical box model to investigate the simultaneoustime evolution and day-to-day variability of both radicals (OH, peroxyradicals)and the NO/NO2 photostationary state observedduring the four seasons.The model
and SO• photochemistry have been consideredin order to accountfor potential effectsduring upslopeconditions.
The
reaction
rate
constants
included
in the
mode] have been updated accordingto De/1//oreet •l.
[1997].The chemicalrate equationsare integratedforward in time with Gear's predictor-correctornumerical
technique[G•r, 1971]. Photodissociation coefficients includesnonmethanehydrocarbon(NMHC) chemistry. are calculatedevery 15 min usingreeva]uatedcrosssecPreviously,Canttell et al. [1996b]and Eisele et al. tions for peroxyacety]nitrate (PAN) [T•lu}•r ½t •l., [1996]useda steadystate modelto assess the budgetof 1995]and acetone[McKeenet al., 1997a],and a disradicalsduring spring and summer seasonsand to in- crete ordinatesradiative transfer scheme[Stamneset vestigatethe potentialimportanceof heterogeneous loss al., 1988]. Photolysisratesare calculatedfor October of radicalsthrough reaction on aerosols.Hauglustaine 1, 1991; February 1, 1992;May 1, 1992; and August 1, et al. [1996b]studiedthe Oa-NO-NO2photostationary 1992for autumn(2a), winter(2b), spring(2c) andsumstate at MLO on the basis of mean diurnal cycle and mer (2d) intensives, respectively, with the surfaceat 3.4 clear-skyconditions.Hauglustaineet al. [1996b]have km, the elevationof MLO, and a surfacealbedo of 3%
HAUGLUSTAINEET AL.' PHOTOCHEMISTRYAND BUDGET OF OZONEDURING MLOPEX 2 30,277 Table 1. VariablesMeasuredDuring MLOPEX 2 and Usedto Constrainthe Model for TimeDependent Simulations Time Resolution H20 Os NOx = NO + NO2 + NOs
1 min 1 min 1 min
CO
30 min
HNOs H•O• CHsOOH CH•O C•H6 C2H4 C•H• C•Hs C•H6 nC4H•o iC4H•o C•H•0 C•H•
20 min 1 min 1 min 20 rain 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours
Isoprene
4 hours
SO•
1 min
Temperature
1 min
JNO•. Jos
1 min 1 min
appropriatefor blacklava [Shetteret al., 1992]. The
Technique
Reference
hygrometer Atlas and Ridley[1996] UV absorption Atlas and Ridley[1996] chemiluminescence Atlas and Ridley[1996] gaschromatography Greenberg et al. [1996] ion chromatography Atlas and Ridley[1996] liquid chromatography Staffelbach et al. [1996] liquid chromatography Staffelbach et al. [1996] liquid chromatography Zhou et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] gaschromatography Greenberg et al. [1996] fluorescence Atlas and Ridley[1996] psychrometer Atlas and Ridley[1996] actinometer Lantz et al. [1996] actinometer Shetteret al. [1996]
model results obtained withandwithout JNO2scaling).
aerosol extinction altitude profile was determined us- The computed time dependentratio betweencalculated ing non-Rayleighbackscattercoefficients obtainedfrom and observed(scaled]j. •c). is applied to the theoretithe NOAA Climate Modeling and DiagnosticsLabora- cal clear-skyrate coeificientsof other photodissociation tory (CMDL) lidar systemusingextinctionto backscat- reactions,on the assumptionthat any deviationsfrom perturbations ter ratiosgivenby Russellet al. [1993]. Mean ozone unity are due to wavelength-independent columns of 265+7, 247+12, 284+8, and 259+8 Dob- of the radiationfield (e.g., local clouds,haze, ground son units were used for intensives2a, 2b, 2c, and 2d, reflections) [Ridleyet al., 1992].Simulations werecon-
respectively [Lantzet al., 1996].
ductedovereachintensive(September15, 1991to Oc-
For time dependent simulations, observedtempera- tober 23, 1991 for MLOPEX 2a; January 15, 1992 to ture, concentrations of H20, O3, NO• (- NO + NO2 + February 15, 1992 for MLOPEX 2b; April 15, 1992 to NOa), CO, HNOa, H202, CH3OOH, CH20, NMHCs, May 15, 1992 for MLOPEX 2c and July 15, 1992 to
SO2andphotolysis ratesJNO2,. andJo3 wereused August15, 1992for MLOPEX 2d). to constrain
the model.
The
data
were introduced
in the model on the basis of 1-hour-averagedvalues 2.2. NO-NO2 Photostationary State or interpolated in time for each model time step for Nitricoxide(NO), nitrogendioxide(NO•.),andozone speciesmeasuredwith a longertime resolution.The qb-
weightingprocedureintroducedby Ridley et al. [1992] (0•) are coupledthroughthe cycling: and Hauglustaine et al. [1996b]to accountfor different (R1) NO + O• ---• NO2 + O2 measurement uncertainties at various NO• levels is not
applied over the 1-hour-averagingperiod consideredin (R2) this study. Table i givesa list of speciesand physical quantities usedto drive the model, the correspond- (R3) ing time resolution, and the measurementtechnique.
NO2 + hv
> NO + O
O+O2 +M
>Oa+M
takesplaceon a Thereisanunresolved discrepancy between JNO2mea- Duringdaytime,thisreactionsequence
of a few minutes,andphotostationary stateis surements and modelcalculations[Lantzet al., 1996]. timescale On average, the actinometer values were about 1.31.5 times higher than the model-predictedvalues. In
thisstudythe observed JNO2values werescaled to
match the calculationson averageover each intensive
(seeHauglustaine et al. [1996b]for a comparison of the
rapidly establishedbetweenNO and NO2. Under these conditions,the NO• partitioningis expressed as
[NO]
-
[Oa]
[NO] JNO2
(1)
30,278HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY ANDBUDGETOF OZONEDURINGMLOPEX2 Thisphotostationary stateisusuallyrepresented by the (R5)
NO + CHaOs >NOs+ CHaO
photostationary stateparameter • defined as[Ridleyet al., 1992;Daviset al., 1993;Hauglustaine et al., 1996b] (R6)
NO q-ROs,i ) NOsq-ROi
(R7)
JNO2[NO2]
[o1[NO]
NO + XO --+ NOs+ X
In this case,•bis larger than unity:
If only (R1)-(R3) are considered, ½ = 1. A more completeNO-NOs photostationary statemustconsider the additionaloxidationof NO by hydroperoxy(HOs),
methylperoxy(CHaOs),or otherperoxy(Res,i)radicals,andpossibly otherspecies suchas halogens (denotedhereafterX):
(R4)
NO + HOs >NOs+ OH
- +
k4[HOs] + k5[CHaOs] + Ei ki[R02,i]+ k7[XO] (3)
Figures la-ld compare the day-to-day evolution of the model-calculatedNO/NOs ratio with I-houraveragedobservedvaluesfor each intensive. The relative instrumentalerrorfor the measuredNO/NOs ra-
1.0 0.8
0.6 0.4 0.2 0.0
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Date 1•0 0.8 0.6 0.4 0.2 0.0
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0
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0.8
.Z. 0.6 •
0.4-
0
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z
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•':•½½', '
'""'":i:':':':':' ,1.
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.,•z.:.:.:: .... .,.
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............. .
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::1
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2/12
2/13
2/14
,.,
2/05
,
2/06
1.0 0.8
0.6 0.4
0.2 0.0
Date
Figure 1. One-hour-averaged measured (dots)andmodel-calculated (solidline) NO/NOs concentration ratio as a function of time for MLOPEX 2a, 2b, 2c, and 2d. Regionsof shading indicate periodsof free troposphericconditions.
HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGETOF OZONEDURINGMLOPEX2 30,279 1.0 0.8 0.6 0.4
0,2 0.0
4/15
0
o.8
•
•
04
O
0.2
•
4/16
4/17
4/18
4/19
4/20
4/21
4/22
4/23
4/24
............................ :........:,,.:....:.:.,.:.:.:.:.:.:.:.:.:.:.: .:.:.:...:.:.:.:.:.:.:.:.:,:.:..: ß:':':':':':':':':':':':':':':':':'" :::::::::::::::::::::: :::::::::::::::::::::::::::: ::::::::::::::::::::: :::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::
ß
.Z. 0,0
4/25
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1.0 0.8
0.6 0.4 0.2 0.0
Date
1.0 0.8 0.6 0,4 0.2 0.0
7/16
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o.8
0.6 0.4
0.2 o.o
L:
,
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8/13
1.0 0.8
0.6 0.4
0.2 0,0
Date
Figure 1. (continued) tio was about 15%. As obtainedby Hauglustaineet al. [1996b]for clear-skyconditionsand overa meandiurnal cycle,the observedratio and the shapeof its diurnal variation are generallywell reproducedby the model for the day-to-day simulationsreported here. The agree-
ploratoryMission-West A [Crawfordet al., 1996].The reasonfor this disagreementhas been tentatively attributed by Crawfordet al. [1996]to interferences in the NO2 measurement duringthe PEM-Westcampaign. Maximum ratios for midday conditionsare generally
ment obtained for MLOPEX
stable over the different seasonsand reach values of 0.5-
conditions contrasts with
the systematic underestimateby the model of the mea- 0.8. As illustratedby theseresults,a largepart of the
suredNO/NO2 ratio by morethan a factor of 3-4 dur- variabilityin themeasured NO/NOa ratioisreproduced ing the western Pacific aircraft flights of Pacific Ex- by the modeland associated with the variabilityof the
30,280
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY MLOPEX
AND BUDGET
2a
MLOPEX ,
1.2•
½
:]/
0.0 :•
0.2
fo.... 4
o
00 o2,,••o 0.4
0.6
MLOPEX
o.s
1.0
0.0 'ø
,.2
0.0
0.2
0.4
0.6
2c
....ß'--•: R•=0'96
MLOPEX
' ///
•
:,
."' / /
, ......'• •
o,,•/
•• o.•
•
? •o .•;•Ooo
½o.4"•o o 0.0
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øo
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•,,,,,,,
0•2
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•.2
o.••
,'"
Ooo øø..,?/?
[
f [[
2d
•O
1.0
oo
•
o.s
NO/NO, (m•os)
•.2 "','",'",'","',.,• •.2• 1.0-
'
o o..•o
NO/N% (m•os)
0.2 •
2b
1.0 _NFT: R•=0.86 / '" //
,% 0.0
2
,
}
oo••
[
,
MLOPEX
1.2 I ' ' ' , ' ' ' I ' ' ' , ' ' ' , 1/'•/•
1.0 }
OF OZONE DURING
o o •/
o
/
•_•'Z •o o
o•••
[ ••• o •%o
02•o •• •,,,
o.s
ß
• , , .
•.0
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.2
NO/N% (meos)
0.0
0.2
0.4
0.6
o.s
•.0
.2
NO/NO2 (meos)
•igure 2. Oorrelationbetweenmodel-calculated and measuredNO/NO2 ra•io for the different MLOPEX 2 intensivesfor free tropospheric(FT) conditions(solidcircles)and non-freetropospheric (NFT) conditions(opencircles).The unity line is represented by the solidline, and the dashed lines represen• linear figs.
measurements usedto constrainthe model (Oa, NOs,
(HST)) conditions.In this case,40-50% of the day-to-
temperature, H20,JNO2)'During summer (MLOPEX day variability is generally reproduced by the model, 2d), the NO/NO2 ratio is morevariable,and observed exceptduringwinter (MLOPEX 2b), whena lowercormidday maximum values range from 0.3 to 0.9, due to cloudy conditionsprevailing during much of the measurement period. A more detailed comparisonbetween
relation of 25% is obtained. Despite this smallercorrela-
tion coefficient,the measuredNO/NO2 is still fairly well reproducedby the simulations(excepton January18,
measuredand calculatedNO/NO2 is given in Figure 22, 2,5 when the model underestimates the NO oxida2, which shows the correlation between observed and model-calculatedvalues. During all seasons,more than
tion to NO2, and on February 12 when an overestimate
ratio is captured by the model when the full diurnal cycle is considered. It should be noted that a large part
0.23-0.7,5, 0.27-0.7,5, and 0.22-0.76 during MLOPEX 2a, 2b, 2c, and 2d respectively. The calculated range is 0.30-0.7,5, 0.21-0.73, 0.36-0.77, and 0.11-1.06 during MLOPEX 2a, 2b, 2c, and 2d respectively. The day-to-day variation of the photostationary state
of this conversionis calculated). The rangeof measured NO/NO2 filtered over 10-14 HST is 0.22-0.70, 80% of the variability(r2) in the measured NO/NO2
of the correlationbetweenthe NO/NO2 measuredduring MLOPEX and the model calculationsis driven by the diurnal variation of the NO/NO2 ratio. In order
parameter• is shownin Figure 4 for spring(2c) and tion betweencalculatedand measuredNO/NO2 ratios summer(2d) conditions.Note that near sunriseand filtered over high-Sun(10-14 Hawa'ianstandardtime sunset, measuredvaluesare less reliable, sincegreater to illustrate this feature, Figure 3 shows the correla-
HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 30,281 MLOPEX 20,
10-14
HST
MLOPEX 2b, 10-14
1.;2
HST
1.2 1.0
1.0--e--NFT: r•=0.48 / / /
0.8
0.61
o 0.4
z
0.2
0.0•,,,
0.0
0.2
•,,,
0.2
• •,
0.4
i,,,
0.6
•,,
0.8
, • ,.
,
1.0
1.2
0o0
0.2
0.4
NO/NO2 (meos)
MLOPEX 2c, 10-14
HST
0.8
1.0
MLOPEX 2d, 10-14
1.2
1.0
0.6
1.2
NO/NO2 (meos) 1.2
'
'
'
I
'
'
'
I
'
'
'
I
'
'
'
I
HST '
'
'
I
'
'
'''1'''1'''1'''1'''1• 1.0
0.8
0.8
o
(•0.6
•0.6
z
%.o•'
OOoø
z
o
o
z
z
0.4
0.4
0.2
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0.4
0.6
0.8
1.0
1.2
0.0
0
, , , i , , , I • • • I • • • I • , • I , • ,
0.2
NO/NO2 (meos)
0.4
0.6
0.8
1.0
1.2
NO/N% (me•s)
Figure 3. Correlationbetweenmodel-calculated andmeasured NO/NO2 ratiofor the different MLOPEX 2 intensives for high-Sunconditions only (10-14 HST). The unityline is represented
by the solidline, and the dashedlines representlinear fits.
errors at small NO mixing ratios are expected. Fur- 5-9). Hauglustaine et al. [1996b]haveshownthat durthermore,duringthesetwoperiodsof the day,achieve- ing these specificdays the photostationary state ratio ment of photostationary state is no longerexpected cannot be reproducedby a simple radical budget fordue to slowerNO2 photolysis.The experimentalde- mulation. The present results indicate that, even when termination of • hasfairly largeuncertainties (reach- a detailed simulation of the photochemistryincluding
ing about15-20%for noonconditions), mainlydominatedby the uncertainty on the NO/NO2 ratio [Ridtey et at., 1992;Haugtustaine et at., 1996b]. During fall and winter(not shown),and spring,measured and
the role of various hydrocarbonsis considered,the comparisonwith observationsis not significantlyimproved. This suggeststhat the high deviation of • from unity
observedduring these days could be related to excess
calculatedvaluesreacha high-Sunmaximumof about NO oxidation due to speciesnot consideredin our cal1.8-2. Since• wouldbe unity if ozonewerethe only culation. As indicatedby Haugtustaineet at. [1996b], oxidant that convertsNO to NO2, the valuesshown these measured high values of • are generally associhereclearlyindicatethe roleplayedby otheroxidizing ated with high H20/Oa ratiosand NO• mixing ratios species suchasperoxyradicals.Duringsummer,theob- lowerthan about 20-30 pptv. This couldsuggesta posservedvariability is higher, and measuredvaluesreach sible oxidation of NO by speciesoriginatingfrom the a midday maximum of 2-4. The NO to NO2 conver- marine boundarylayer, suchas halogens,which reach sionalsoseemsto be underestimated by the modelfor the MLO site during upslopeconditions.This issuehas
somespecific days(July20, 22, 24, 25, 27, 31, August beeninvestigatedby Crawfordet at. [1996]and Davis
30,282
HAUGLUSTAINE
ET AL.' PHOTOCHEMISTRY
AND BUDGET OF OZONE DURING MLOPEX 2
4/1,5
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2
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I
i
Model ©Measurements D ate
7/1 6
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Model ©Measurements I D ate
Figure 4. One-hour-averaged derivedfrom observations (dots)and model-cMculated (solidline) photostationarystate parameter as a function of time for MLOPEX 2c and 2d. Regions of shadingindicate periods of free troposphericconditions.
et al. [19968,]in the frameworkof the PEM-West A campaign which took place in the western Pacific during September-October1991. Figure ld indicatesthat
al. [1996],chlorine,bromineand iodinelevelsof about 2-3x10*, 3-5x107, and 1-2x108 molecules cm-a, re-
spectively,would be necessaryto bring the model close the calculatedNO/NO2 ratio overestimates the obser- to the observedNO/NO2 ratio for conditionsprevailvationsby a factor reachinga maximum of 1.•-2 for the ing at MLO. These iodine and bromine levels appear specificdaysreportedabove. Accordingto Crawfordet to be 5-10 and 100-1000times higher respectivelythan
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
AND BUDGET
OF OZONE DURING
MLOPEX
2 30,283
the underestimate is largerthan a factor of 2 for MLO the tropicalboundarylayer [e.g.,Li et al., 1994;Sin#h, upslopeconditions. The total peroxyradicalconcentration canalsobe es1995; Davis et al., 1996a]. Furthermore,transportof the levels of organic Br and I generally measured in
marine boundary layer air to the site during upslope conditions also occurred during the other intensives, where good agreementwas achievedfor the photostationary state parameter. Contribution of undetected peroxy radicals derived from heavier hydrocarbonsor organicsemitted from local vegetationis anotherpossible causeof excessNO to NO2 oxidation during strong upslopeconditions.The possiblerole of biogenicemissionson the photochemistryat MLO will be examined in more detail in section 2.6. We note that the measured
timated from the deviation of the photostationary state
ratio from unity. Fromrelation(3) in whichthe role of halogensis neglected,we derive
PO2 •- (•b- 1)[Os]k• / ]•4
(4)
Measured1-hour-averaged valuesof [NO], [NO2],[Oa],
T, andscaled JNO2areintroduced intorelation (4)to
determine PO2. Due to combined instrumental errors
for the different measured quantities, the uncertainty
in PO2 derivedfrom q5is high, exceeding100%. Furplain a significantdeviationof the NO/NO2 ratio from thermore,it shouldbe kept in mindthat relation(4) is the measuredvalues.Isoprenelevelsweregenerallyless only valid for high-Sunconditions(m8-16 HST at this statebetweenOathan 20 pptv and reacheda maximum of 100-150 pptv latitude)(to ensurephotostationary NO-NO2). Figure 5 shows the day-to-day variationof duringupslopeconditionsonly occasionally [Greenbet# the i-hour-averaged PO2 concentration measured duret al., 1996]. Possibleinterferences in the NO2 measurementsfrom, for example, HONO or HO2NO2 can- ing spring(2c) andsummer(2d), the modelcalculated isoprenelevels at MLO were generally too low to ex-
not be ruled out. However,consideringthe good agree- total peroxy radical concentration,and the radical con-
centrationderivedfrom relation (4). During all seament achieved between the calculated and the observed NO/NO2 ratio duringotherseasons and duringsummer sons(fall and winter seasonsare not illustrated but reach (with the exceptionof specificdaysalreadymentioned), showsimilarfeatures),measuredconcentrations interferencesdo not appear to be the reasonfor the dis- a high-Sunmaximumvalueof about 20-30 pptv on average.As discussed by Hau#lustaine et al. [1996b]and agreementencounteredoccasionallyduring summer. by Canttell et al. [1996a],modelresultsare generally largerthan the measurements and reacha maximumof 2.3. Peroxy Radicals about40 pptv. Evenif the reported30-35% uncertainty The total peroxyradicalconcentration (PO2 = HO2 on the measuredconcentrationsis considered,a signif+ CHaO2+ •-].iRO2,i)wasmeasured usingthe Chem- icant discrepancybetweenmodel and measurementsis
ical Amplifier(CA) technique[Canttellet al., 1996a]. still obtained. The deterioration in performance of radThis instrument has been used for several campaigns ical amplifiers associatedwith humidity effect pointed and under different atmosphericenvironmentsincluding out by Mihele and Hastie [1998](potentiallya factor
Rural Oxidantsin the SouthernEnvironment(ROSE), of 2 for MLO upslopeconditions)would help resolve and the TroposphericOH PhotochemistryExperiment this discrepancyduring upslopeconditions. No defini(TOHPE) [Canttellet al., 1993,1995,1997b].The rel- tive conclusionis available regardingthis effect for the ative uncertaintyof the measuredmidday radical con- instrument used during MLOPEX 2. The model incentrationis estimatedto be 30-35% [Canttell et al., dicatesthat about 60-70% of the peroxy radicalsare 1995].The CA instrumentalsoparticipatedin the Per- HO2, with the remainder CHaO2 radicals. Heavier peroxyRadicalIntercomparison Exercise(PRICE) heldat oxy radicalsgenerallycontributea negligibleamount at
the Schauinsland Tropospheric OzoneResearch(TOR) station, locatedin the Black Forest[Volz-Thomaset al., 1997]. Duringthis intercomparison, the National Center for AtmosphericResearch(NCAR) CA measuredhigherconcentrations than the other CAs, but
the site (_ 40
> 40
o 0
0
0 oQOO /
9/
oC•e&o oø•O / o •
5o
2o
10
-t--F'r: R2=0.14 •_ _ NF'r:R2=0.48
0
0
10
20
50
40
50
60
0
10
20
30
40
50
60
PO2 (me(•s), pptv
PO2 (mees), pptv
MLOPEX
2d
6O
60
5O
50[
/o• ß
o
•,, o ./
/
>ol
> 4O
....'":
__o5O
b:
a_ 20
..
.
0
o•
10
10
'
: •
""
0
0
0
10
20
30
40
50
60
0
PO2 (meos), pptv
10
20
30
40
50
60
PO2 (meos), pptv
Figure 6. Correlationbetweenmodel-calculated and measuredtotal peroxyradicalmixingratio
(pptv)forthedifferent MLOPEX2 intensives forfreetropospheric (FT) conditions (solidcircles) and non-free-tropospheric (NFT) conditions (opencircles).The unity line is represented by the solid line, and the dashedlines representlinear fits.
areasnear106cm2/cma).Since Figure6 andfilteredoverperiodswith prevailingfree loading(e.g.,surface tropospheric conditionsat the site, our analysisdoes the lifetime of peroxy radicals with respect to homonot confirm this conclusion. geneouschemistrywaslongduringMLOPEX 2 (HO2 The possiblerole playedby heterogeneous removal lifetimeof 100-150s for middayconditions),the possilossmust be considered.Figure of radicalson aerosols[Mozurkewich et al., 1987;Han- bility of heterogeneous 7 shows the correlation for high-Sun and clear-skyconson et al., 1992]has been considered by Canttell et ditions between measured PO2 concentrations and the al. [1996b]asa possible explanation of the discrepancy between modeled and observed concentrations during
aerosolscatteringextinction coefficientsmeasuredwith
the measured condensation nuclei(CN) MLOPEX 2. Crosley[1995]hasexaminedthe homoge- a nephelometer, neouslossrates of radicals under typical environments counts,theblackcarbon(BC) concentration [Bodhaine,
and hascalculatedthe magnitudeof uptake coefficients 1996],and the SO2mixingratio. In general,poorcorreneededto provide a significant.influenceon the PO2 lations are obtained,and only 5-10% of the variability negativecorrelevels. It appearsthat heterogeneous loss of peroxy (r 2) is captured.The only significant radicalsmight be importantfor highaerosolcontentor, lation (80%of the variability)is obtainedwith the exdepending on the magnitudeof the actualuptakecoeffi- tinction coefficientduringwinter (2b). However,only cientrelativeto gas-phaseprocesses, evenfor moderate four observations are included in this correlation, and
30,286 HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 P02 Correlation, 10-14 HST, Clear Sky 0.8
.....................................
i 1212a I[J2b•:ffi2c
0.4
Ei2d'
0.2 0
rained at night with the CA during MLOPEX 2 is unclear. Possible explanations include interference in the instrument from HO2NO2 or PAN-like compoundsor productionof radicals through unknownheterogeneous
processes [Canttell et al., 1996b]. Recently,Canttell et al. [1997a]indicatedthat reactionof 1-30 pptv of CHaSCHa(DMS) with NOa to produceperoxyradicals offersthe best current hypothesisto rationalize the nighttime generation of radicals. It should be noted
-0.2
-0.4
that our model calculations(without DMS) indicate
-0.6 -0.8
Neph.
CN
S02
BC
higher PO2 mixing ratios of about 15 pptv during the night of July 23-24, and more than 10 pptv for nighttime during April 20-23. Interestingly, these higher nighttime concentrationscoincide with high-isoprene
Figure 7. Observed correlation coefficient r between episodesduring the precedingday at MLO (isoprene total peroxy radical mixing ratio and aerosolextinc- mixingratiosreachedmorethan 100pptv), andsuggest tion coefficient (Neph.),condensation nuclei(CN), sul- the formation of peroxy radicalsfrom biogenichydrofur dioxide(SO2)and blackcarbon(BC) for the dif- carbonswhich persistduring the night or the possibility ferent MLOPEX 2 intensives(clear-skyand high-Sun for nighttime production, as suggestedby Platt et al.
conditionsonly, 10-14HST).
2.4. Hydroxyl
these results are not reproducedduring other seasons.
Radical
Hydroxyl radical concentrationswere measured dur-
Recently,Canttell et al. [1996b]indicateda reason- ing MLOPEX 2c and MLOPEX 2d by Eisele et al. able correlation between HO2 removal on aerosols and [1996] with the ion-assistedtechnique. No data are CN during MLOPEX 2. However,this conclusionwas available during the fall and winter measurementpemainly based on the analysis of data collected dur-
riods.
The
instrument
has been used for field
mea-
ing onespecificday (i.e., July 23, 1992)with particu- surementsand intercomparison campaigns[e.g.,Eisele larly largeCN concentration (reachingmorethan 2000 and Tanner, 1991; Eisele et al., 1994, 1997]. The toparticles/cma). Althoughsomeheterogeneous removal tal uncertainty of the measured OH concentration is of HO2 or PO2 on specificdayswith high aerosolloading estimatedto be about 50%. This uncertainty is pricannot be ruled out, no obvioussupport for systematic marily a result of uncertaintiesin the photodissociation heterogeneous removalstemsfrom our analysisof the crosssection of H20 for absolute calibration, absolute MLOPEX 2 data set. It should be noted that the re- optical flux measurements,flow velocity measurements, lationship between PO2 and aerosolscould be partially andvariabilityin calibrations[TannerandEisele,1995]. maskedby other processes and that CN or BC are only It should be added that recent measurements indicate qualitative surrogatesfor aerosolsurfaceareas. Thus that presently acceptedcrosssectionsof H20 should be more laboratory measurementsand atmosphericobser- increased by 30% [Canttellet al., 1997c].As a consevations are neededto investigatethe potential role of quence,the OH measurements madeduring MLOPEX 2 heterogeneous removalof peroxy radicalsin further de- and presentedin this study have been correctedaccordsail. ingly and increasedby 30%. Figure 8 showsthe dayAnother important disagreementbetweenmodelsim- to-day variation of 1-hour-averagedmeasuredconcenulations and measurements of PO2 arises from the trations superimposedon the model calculated values. nighttime peroxy radical mixing ratios, which reach 10- During spring(2c), considering the uncertaintyin the 20 pptv in the observationsbut are generallylessthan measured values(50 %) andwith the exceptionof some 5 pptv in the model. The persistenceof peroxy radicals specificdays, reasonableagreementbetweenmodel and during nighttime has been simulated by Ridley et al. measurementsis obtained. On April 19 the measured [1992]for MLOPEX 1 conditions.Smallnonzeronight- concentrations are underestimated by the model by as time concentrations of CHaO2 of only 2-3 pptv were much as a factor of 2 for high-Sun conditions. The calculated. More recently, the peroxy radical chemistry reasonfor this disagreementis unclear since no pecuin the remote marine boundary layer was investigated liar features appear in the measuredNOx, Oa, and CO by Monkset al. [1996]. PO2 mixingratiosof about 1 concentrationsduring that day. A possibleexplanation pptv were observedduring the night and were mainly is the low humidity recordedduring most of the day attributed to the long nocturnal lifetime of CHaO2. and consequenthigher uncertainty in the H20 mixing Platt et al. [1990]suggested productionof peroxyradi- ratio used to drive the model. On April 22, high OH
calsthroughreactionof nitrate radicals(NOd) with or- concentrations of 15x106 molecules cm -3 were meaganiccompounds(e.g., dimethyl sulfide,terpenes,and sured and calculated by the model. These high conisoprene). The causeof the higher mixing ratio ob- centrations are associatedwith high NOx mixing ra-
HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY
AND BUDGET OF OZONE DURING MLOPEX 2 30,287
15 10
.....
5 0
7/16
7/17
7/18
7/19
7/20
7/21
7/22
7/23
7/24
7/25
7/26
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7/28
7/29
7/30
7/31
8/01
8/02
8/03
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8/05
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8/07
8/08
8/09
8/10
8/'11
8/'12
8/13
lO
5 0
I
Model ßMeasurements I D ate
5 0
4/15
4/'16
4/'17
4/'18
4/19
4/20
4/21
4/22
4/25
4/24
4/25
4/26
4/27
4/28
4/29
4/30
,5/01
5/02
,5/03
5/04
5/o5
5/o6
5/07
5/08
5/09
s/•o
5/11
5/•2
5/15
5/14
15 10
5 0
Model
Figure 8. One-hour-averaged measured (dots)andmodel-calculated (solidline)hydroxylradical
concentration (106molecules cm-s) asa functionof timefor MLOPEX2c and2d. Regions of shadingindicate periodsof free troposphericconditions.
tio (reaching150pptv) resultingfromlocalsources on generallyoverestimatesthe measuredvaluesby a facthe island transported to the site in upslopeflow. On May 10-11, water vapor concentrations measuredwith the NCAR portable mesonetshowvery low values,below the instrumental detection limit. During summer
tor of 1.5-2.
The
calculated
concentrations
reach 5-
10 x 106 molecules cm- s while measured concentrations
weregenerally lowerthan5x 106molecules cm-s. Sim-
ilar resultswere obtainedby Eisele et al. [19961and (2d), a verydifferentpatternis obtained,andthe model Hauglustaineet al. [1996b].As seenin Figure8b, using
30,288 HAUGLUSTAINE ET AL.: PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 MLOPEX '
12
'
'
I
'
...•...
'
I
'
'
'
'
'
'
'
'
I
'
'
2d
'
•0 .- %8 o ///'"/' .
I'
8
oo
.
.
oøo /
/
,,'
,,'
r•O 0 0 0 ,'"0
'--•. 0(•5 •// b-
¸
'
_ (•. _ NF-T': R2=0.56
0
T
I
...e... FT: R•=0.80
IF'T:R2=0.75
_ • _ NFT:R2=0.69
10
?
'
,.,oo0•,o(•r2 ½
o
ø
4
.
2 ß
2
4
6
8
0
0
10
ß
2
4
6
12
10
10
oo
øøo o
? F
qDo
g o
00/0%/
oøo2 '--'ø -'/o o
0
2
4
_u
/
•' •'
t Oo½ /
o
0 '"
6
0
HST
_•o Lo o •"-' o
8
10
o
; if / 0
• , , I , , , I , , , I , • • I • • • I , • , 0
12
ooo
o ,.../øo.• _o ;--'/...•o
/ / '•/00
10
MLOPEX 2d, 10-14
HST
12
6
8
OH(meos),106 cm-3
OH(meos),106 cm-3 MLOPEX 2c, 10-14
'
12
OH (meas), 106 cm-3
0
2
4
6
8
10
12
OH(meos),106 cm-3
Figure 9. Correlation betweenmodel-calculated and measuredhydroxylradicalconcentration 6 3
(10 molecules cm- ) for the springandsummer MLOPEX2 intensives for freetropospheric (FT) conditions (solidcircles)andnonfreetropospheric (NFT) conditions (opencircles).The
correlations are alsopresented (lowerpanels)for high-Sunconditions (10-14HST). The unity line is representedby the solidline and the dashedlinesrepresentlinear fits.
the measuredhydrocarbons,peroxides,formaldehyde, and sulfur dioxide to constrain the model does not significantlyimprovethe comparisonbetweenmodeland measurements presentedin thosepreviousstudiesfor the summer intensive. Even if the disagreementbetweenmodel and measurementsduring summeris generally within the OH measurement uncertainty,no reason can be given as to why the measuredvaluesdecreasedby 30-50%from the springto the summerintensive.
The correlation between calculated and measured OH
As mentionedfor NO/NO2 and PO2, a largefraction of this correlation is driven by the diurnal cycle in OH
concentration.Figure 9 showsthat for high-Sun(10-14 HST) conditions,the correlationsare reducedto 38% and only 5% during spring and summer,respectively. A better agreementis achievedduringfree tropospheric periods, but only a few measurementswere obtained during high-Sun daytime free troposphericconditions, so this conclusion tween calculated
is not robust. and measured
A consistent OH
bias be-
has also been ob-
tained in several previous studies for continental rural
concentrationsis illustrated in Figure 9. For all day locations[Perner et al., 1987;Poppeet al., 1994;Eisele conditions,the modelreproduces 70% and 60% of the et al., 1994].MorerecentlyMcKeenet al. [1997b]found data variabilityduringspringand summerrespectively. a consistent50% overpredictionof in situ OH measure-
HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 30,289
ment by a photochemical box model under relatively
clean(NOz H202
and OH/HO2 is approximated by:
[OH] k•2 [NO]+ k•a [Os]+ k•4 [HO2] (5) whichis strictlyvalid only for OH/HO2. Indeed, (6) the calculatedOH/PO2 ratio showsa strongcorrela[CO] [HO2] k• [CO] tion with R during both seasons.More than 70% of the OH/PO2 variability is capturedby the variability The intersectof OH/HO2 with R = 0 at a value of in R. The calculated correlations show similar slopes about 0.004 (for typical MLO 10-14HST conditions) during springand summer. The calculatedOH/PO2 in Figure 12 gives the value of the secondterm of the reaches a maximum of about 0.02, with a mean value right-handsideof relation(6) and providesthe lower near 0.007 and 0.01 during spring and summer, re- limit of the OH/HO2 ratio. While the uncertaintyof measuredOH/PO2 is high spectively. The measuredmean OH/PO2 ratios are that some about 0.01 and 0.015 during spring and summer, re- (50-60%),resultsfromFigures10-12 suggest spectively,with maximum valuesreachingup 0.04-0.05. of the basicfeaturesgoverningthe radical chemistryunThe measurementsindicate that only 5% and 30% of
der the cleanMLO environmentmight not be accurately
the OH/PO2 variability is associatedwith •, during reproduced in the box model simulations. Reaction of springand summerseasons,respectively. OH with heavierhydrocarbons (not includedin the sim-
30,292
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
ulations)or lossof radicalsthroughheterogeneous reactionson aerosolsor by surfacedepositionhave been proposed to explain the overprediction of radicals by the models[Trainer et al., 1987; Canttell et al.• 1996b; Eisele et al., 1994; McKeen et al., 1997b]. Recently, Stevenset al. [1997]indicatedthat the disagreement be-
tweencalculatedandmeasured HO2/OH andRO2/HO2
AND BUDGET
OF OZONE DURING
MLOPEX
2
of OH with very largeorganicmoleculesnot producing RO2or producinglarge RO2not detected by the CA instrument, OH + X -• products, the calculated PO2 concentration
is decreased to values even lower than
measurements during the day. The ratio OH/PO2is brought to a value closeto the observations,but the NO/NO2ratio is significantlyincreasedand reaches0.9
for high-Sun conditions. As discussedin section 2.3, reaction on aqueous tionsfor higher hydrocarbons.However,this conclusion aerosolshas been proposedas a removal processfor at Idaho Hill, Colorado, may be due to inaccuraciesin the kinetics of the RO2+ NO and RO2+ HO2reacdoes not affect the results obtained for the low NO• and low hydrocarbon environment encounteredat the MLO
peroxyradicals[Canttellet al., 1996b]. Possiblesur-
2.5.
cm3/molecule/s), ues(200 pptv of [X] OHwith is only a reaction slighlydecreased rate kx -since 10-•2 re-
face removalprocesses of peroxy radicalshave alsobeen proposed to explain model overpredictionof radicals site. The possibleremovalof radicalsby heterogeneous processesand the potential role of biogenic emissions [Traineret al., 1987].If an RO2lossis introducedinto the model, RO2+ X -> products,in sufficientamount on OH are discussedin the following sections. to bring high-Sun PO2concentrationsto observedvalAdditional
Removal
Processes
for
Radicals
As discussedin the previous sections, the overestimate in the model of measured total peroxy radical
cyclingof HO2is not the primary OH productionterm, concentration (PO2) duringall seasons andof measured and the NO/NO2ratio is increasedand reachesa value OH during summersuggeststhat someradical removal of 0.8 for high-Sun conditions. If the same amount of IX] is introduced to RO, RO2+ in the model X -> RO, in the OH case is sharply of an RO2 inprocessesmay not be included in the model. In order recycling to illustrate the consequencesof OH or PO2 sinks not creased to a maximum value of 10 x 10e molecules cm -3 accountedfor in the photochemicalbox model, artificial peroxy radical and OH removals have been considered and PO2is evenincreaseddue to enhancedproduction. Canttell et al. [1996b]and Eisele et al. [1996] in sensitivity simulations. We have selectedJuly 17 as have indicated that an additional model OH loss with a specific day, since the calculated OH concentration overestimates the measurementsby about a factor of an unidentifiedcompoundwhichproduceslarge unde2 (Figure 8b). Figure 13 showsthe diurnal variation tected RO2radicals is generallyconsistentwith both of OH, PO2, OH/PO2, and NO/NO2 for the reference OH and peroxy radical measurements.However, our caseand severalsensitivitystudiestogetherwith mea- resultssuggestthat none of the hypothesesconsidered sured values. If a methane-like(or an isoprene-like) in the sensitivitysimulationsgive satisfactoryresults compound is introduced in the model at a concentra- in reconcilingcalculatedand measuredOH, PO2 and
NO/NO2ratio simultaneously. Severalprocesses could also be invoked simultaneously to bring calculated OH, with OH kx = 10-•ø cm3/molecule/s) to reducethe PO2and NO/NO2closer to observations at the same calculatedOH to the measuredvalue,OH + X (+02) compound reducingOH and -• RO2, additional peroxy radicals are produced and time (e.g.,a methane-like
tion sufficient(600 pptv of IX] with a reactionrate
the calculated total peroxy radical concentrationis increasedfrom about 40 pptv in the referencecase to 50 pptv for high-Sun conditions, a value further overestimating the PO2 measurements. Consequently,the
ratio OH/PO2 is significantlyreducedbelow the observed values. In that case, more NO is converted to
a RO2losswithoutalkoxyradicalformation).Evenif
sucha combinationof processes appearshypothetical,it may not be totally ruled out on the basisof our results.
2.6. Potential Role of Biogenic Emissions in Upslope Flow
NO2, and the NO/NO2 ratio is decreasedto a value The possibleremoval of OH by reaction with biocloserto the observedratio. If a CO-like compoundis genic volatile organiccompounds(VOCs) (e.g., isointroducedin the model(800 pptv of [X] with a reac- prene,pinenes,methylbutenol)has been considered to tionratekx = 10-•ø cm3/molecule/s), OH + X (+O2 explainthe overestimateof OH measurements by pho-> HO2 there is a competingeffectbetweenadditional tochemicalmodelsin continentalareas[Traineret al., HO2production by this reaction and reducedproduc- 1987; Perner et al., 1987; Poppe et al., 1994; Eisele tion of HO2 and CH302from lower OH. The total et al., 1994; Thompson, 1995;McKeenet al., 1997b]. PO2 concentrationand hencethe NO/NO2ratio are In this previouswork,isoprene-equivalent mixingratios only slightly affectedin this simulation in contrast to reaching2-4 ppbv wereneededto bring modelresults the caseof an isoprene-likecompound. However, the in agreementwith observations.At Hawaii, the rain ratio OH/PO• is decreased slightlybelowobservedval- forest, appears at an elevation of 450-1700 m on the ues as a consequenceof the lower OH concentration. If eastsideof the island. Measurements of isopreneemiswe consideran OH sink (500 pptv of IX] with a reac- sionsduring MLOPEX I in 1988indicatethat emission tion rate kx = 10-•ø cm3/molecule/s) that doesnot ratesfrom the Hawaiianflora are low comparedto conresult in any peroxy radical formation, or the reaction tinentalemissions in deciduous forests[Greenberg et al.,
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
Table 2. MeasuredIsopreneMixing Ratios
(pptv) onSelectedDaysDuringMLOPEX 2c and 2d
lower than 10-20 pptv. More important, the highest isoprenelevelsrecordedat the site occurredon April 20
and 21 (•_300pptv), and OH is underestimated in the
Day April April April April
AND BUDGET OF OZONE DURING MLOPEX 2 30,293
20 21 28 29
July 17 July 18 July 22 July 23 July 24 July 25 July 29 August 1 August 3 August 9 August 10
Time of Day (HST)
Isoprene
1500 1400 1600 1100
360. 317. 63. 21.
1500 1700 1600 1500 1500 1500 1300 1500 1500 1500
3. 6. 19. 47. 91. 53. 89. 14. 14. 75. 6o
model on these specificdays. If isopreneis considered as a tracer for biogenicemissions,it appearsfrom Table 2 that there is no clear correlation
between the overes-
timate of OH in the model and biogeniccompoundsat the site.
To test the possibleeffect of biogenicVOCs on OH at the site, we haverun an idealizedLagrangiansimulation. In this sensitivity simulation, we assumethat the air massoriginatesfrom the rain forest on the island below MLO, leavesat 0900 LT and is transportedduring about 3 hours to the experimentalsite, reachingMLO at noon, when upslope conditions usually occur. We
assumemixing ratios of longer-livedspecies(i.e., 03, H20, NO•, CO, VOCs) fixed to the valuesobserved
at noon at MLO on July 17, and photolysis rates are calculated for MLO conditions. No mixing of the air mass with the surroundingenvironment is considered here, nor possibleremoval of solublespeciesby wet deposition or cloud scavenging.Figure 14 showsthe cor1992]. Isoprenemeasurements from severalislandloca- responding time evolution of isoprene, methylacrolein tions werereportedby Greenberget el. [1992]and are (MACR), OH, and PO2, alongthe air masstrajectory generallylower than 100 pptv, except in the rain for- for different isopreneinitial concentrations. With zero est where 155 pptv were measured. The transit time initial isoprene concentration, the maximum OH concm-3 at noon from the forestedregionto the site is quite variableand centration,reaching8.3x106 molecules takes at least several hours. Due to the high reactiv- when the air mass reachesMLO, is consistent with the ity of isoprenewith OH, concentrationsat the site were calculations already reported in Figure 8b for July 17. low on average(5 and 6 pptv duringspringand sum- The total peroxy radical mixing ratio reachesabout 40 mer, respectively, for upslopeconditions)[Greenberg et pptv at noon, and the NO/NO• ratio reachesa maxi1500
el., 1996]. The measuredisopreneconcentrations are mum valueof about 0.7 (not shown).In a secondsimconsideredin our simulationsand, as shown in Figure 8b, the observedlevelsappear too low to affect OH significantly and explain the model overestimateobtained during summer.
ulation, an initial isopreneconcentrationof 300 pptv is considered.This value overestimatesby a factor of 2 the
measurements reportedby Greenberg et el. [1992]for the rain forest. However, the measurementsof Green-
As stressed by Eiseleet el. [1996],the low isoprene berget el. [1992]weremadeat the surfacewithin the concentrationsmeasured at the site do not imply the absenceof other long-livedbiogenicemissionsand their
rain forest with low light intensity and low emission capacity of emitters, and limited exchangewith the re-
productat MLO. Eiseleet al. [1996]haveshownthat
gionof maximumemissions (top of canopy). It is not unreasonable to expectthat the mixingratio washigher abovethe canopy[J. Greenberg,personalcommunication, 1997].Becauseof the rapid oxidationby OH, the
the overestimate of OH in their photostationary state model seemsto occur preferentially when daytime local wind blew from the northeast. They indicated that
northeasterlywinds are thought to passover regionsof the island coveredby the rain forest and contain more emissionsfrom vegetation or their oxidation product. However, the local wind measurement is a poor indicator of the complex airflow pattern over the island of Hawaii [Chen end Nash, 1994]. Table 2 showstypical mixing ratios reported on selecteddays during spring and summer. In summer, OH is significantlyoverestimatedby the modelon July 23, 24, 25, and 29 (Figure 8b), and isoprenelevelsof 50-90 pptv weremeasured.
isoprenemixing ratio decreasesfrom its init'ial value in the forest to less than 1 pptv at noon, as the air mass reachesMLO. As isopreneis oxidized, the MACR concentration increases, reaches a maximum of about 60 pptv between 1000 and 1200 LT and then decreasesdue to MACR oxidation by OH. When the air mass reaches the site, the MACR mixing ratio is about 30 pptv. Be-
causeof the reactionwith isopreneand its degradation products, the calculated OH concentration at noon is decreasedby about 10% in comparisonto the baseline Lowisoprene mixingratios(_• 20 pptv) weremeasured simulation.This effectis smallcomparedto the signifion August 1 and 3, and a reasonableagreementis ob- cant (factorof 2 ) overestimate of measuredOH by the tained for OH in the model. However, on July 17, 18, model. In a third simulation, isopreneis introduced in 22, and August 10, a strong disagreementis obtained the modelin a sufficientamount(i.e., 1 ppbv) to rein simulatingOH and the isoprenelevelswere generally duce the calculated OH to its observed value at the site
30,294 HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 Methylocrolein
Isoprene 200 ß
7
> 1•0
\
,
150
/
\
•_ 0.8
,
lOO
c•0'6 ••' •-0.4
- 0.2 0.0
\,
" .....
oø""~,
5O
I'." ot
' ..... 12
i
15
,
'"' ,
i
12
18
'" "'".""-
1.5
18
Time of Day, hours
Time of Day, hours
Peroxy Rodicols
Hydroxyl Radica•
.
lOO
lO
.
,
,
12
15
-
.
80
E o
>
6
L•
4
o
2
cL
o/'
o
•/
6o 4o 20 0
0 9
12
15
Time of Doy, hours
9
Time of Day, hours
Figure14. Calculated timeevolution of OH concentration (10s molecules cm-3) withoutisoprene, isoprene (ppbv)andMACR(pptv)mixing ratios,andOH withisoprene (106molecules cm-3) alongthe transport of an air massoriginating downthe island(at 0600LT HST) and reachingMauna Loa Observatoryat noon HST.
(3-4 km). Forthesesim(about5X106 molecules cm-3). Isoprene rapidlyde- of the lowerfreetroposphere creasesfrom its initial mixingratio to a valueof 20-30
ulationsthe free troposphericmeasurements were aver-
overtheduration ofeachintensive. Table3 gives pptvwhentheair massreaches thesiteat noon.MACR aged freetropospheric concentrations andphysical reaches180 pptv at 1100 LT and 150 pptv at MLO. averaged used toconstrain themode] forMLOPEX1 In this case,the calculatedPO2 mixing ratio at noon parameters is about 60 pptv, comparedto the observed25 pptv. andfor eachintensiveof MLOPEX 2. As alreadydone andLiu et al. [1992] in the However,the calculatedNO/NO2 ratio is 0.55,a value by Ridleyet al. [1992]
caseof MLOPEX 1, in the modelthe concentrations parameters are tial mixingratio of 1 ppbvassumed for isoprene in the of key tracegasesand meteorological duringfreetropospheric abovesimulationis a typical valuefor continentalfor- fixedto meanvaluesobserved The photochemical boxmodelis the same est[Martinet al., 1991;Montzkaet al., 1995]butprob- conditions. ably overestimates the concentrations encountered on as describedin section2.1. However,for free tropo-
closeto the observedratio. As mentionedabove,the ini-
the island. These results indicate that neither isoprene
spheric simulation, the isoprene chemistry is notcon-
andconsequently, the numberof reactions is nor its oxidationproductsappearto be presentin suffi- sidered, cient amountsat the site to affectOH significantly.The reducedto 650. The simulationsare performeduntil a statediurnalcycleis reached, andthe budget role playedby other longer-lived biogeniccompounds steady may not be ruledout, and isoprene-equivalent concen- terms are then averagedover 24 hoursSincethe retrationsof about 1 ppbv wouldexplaina largepart of sultsreportedhereare intendedto simulatethe North around3.4 km, the photolysis the discrepancy betweencalculatedand observedOH. Pacificfree troposphere However,no clearcorrelationbetweenthe OH overesti- rateswerecalculatedassumingthe surfaceat sealevel mate in the modeland isoprene(as a indicatorof some and a surfacealbedo of 10%. The O3 quantum yields by Shetteret al. [1996]are used.Because OH consumerof biogenicorigin) at the site has been reevaluated obtained.
of upwelling radiation,the photolysis ratescalculated in this caseare higherthan thosepreviouslyusedfor
3. Budget of Ozone and Other Species in the Free Troposphere
MLO conditions in section 2.
3.1.
Method
The hydrolysisof N205 and NO3 producing HNO3 on wet atmosphericaerosolsurfacesis introduced on
the basisof a pseudo-first-order lossrate [Hauglustaine
The MLO site has the advantagethat characteristic et al., 1996a]. We assumea temperature-independent
nighttimeto early morningdownslope flow can be fil- reactionprobability7 of 0.1 [DeMote et al., 1997]. tered to provide measurementsthat are representative The total aerosolsurfacedensityA (m2/m3) is esti-
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
AND BUDGET OF OZONE DURING MLOPEX 2 30,295
Table 3. Averageand Standard Deviation of MLOPEX 1 and 2 Free TroposphericMeasurementsUsed in the Simulations
H20, ppthv NO•, pptv 03, ppbv CH4, ppbv C2H6, pptv C•H4, pptv C•H•, pptv C3Hs, pptv C3H6, pptv C•H•, pptv CO, ppbv CH20, pptv H202, pptv CH3OOH, pptv HNO3, pptv PAN, pptv
Fall (2a)
Winter(2b)
Spring(2c)
Spring(1)
Summer(2d)
3.7 4- 2.3 26.3 4- 10.25 36.0 4- 8.7 1729.8 4- 14.3 511.2 4- 111.1 13.5 4- 8.2 80.0 4- 32.6 33.1 4- 19.5 6.8 4- 4.5 2.1 4- 16.6 88.3 4- 9.8 130.6 4- 24.2 847.0 4- 260.0 379.0 4- 100.0 76.1 4- 40.5 11.5 4- 10.5
2.0 4- 2.1 32.5 4- 11.6 42.8 4- 6.0 1730.2 4- 12.5 1028.7 4- 258.4.0 10.3 4- 9.7 274.8 4- 147.8 171.6 4- 122.3 1.9 4- 1.1 43.8 4- 30.4 114.7 4- 24.5 119.4 4- 25.8 830.6 4- 423.2 212.4 4- 96.4 77.9 4- 72.8 29.1 4- 24.7
1.7 4- 1.4 40.5 4- 15.8 63.0 4- 9.8 1731.4 4- 12.0 1027.0 4- 200.0 4.4 4- 1.7 230.6 4- 68.6 76.1 4- 44.4 2.8 4- 1.1 33.4 4- 17.7 128.6 4- 21.5 122.3 4- 31.5 1271.1 4- 566.4 427.6 4- 183.2 139.5 4- 75.7 36.4 4- 30.6
2.6 4- 1.3 32.4 4- 13.1 43.4 4- 12.4 1655.8 4- 40.5 753.5 4- 151.3 24.4 4- 8.4 74.6 4- 22.1 33.1 4- 14.7 8.7 4- 4.1 11.6 4- 5.9 136.0 4- 11.1 104.9 4- 42.5 1051.6 4- 265.0 136.8 4- 51.3 116.2 4- 57.0 17.0 4- 14.3
5.0 4- 2.9 29.7 4- 12.5 38.8 4- 16.1 1683.6 4- 15.2 466.2 4- 121.0 3.5 4- 3.0 33.0 4- 22.1 18.5 4- 10.7 1.9 4- 1.8 7.1 4- 3.2 68.5 4- 15.2 161.2 4- 53.8 1518.4 4- 486.3 569.9 4- 144.5 83.5 4- 63.3 8.9 4- 11.7
mated from the measuredaerosolscatteringcoefficient destruction rate is dueto the reactionof O(•D) with
(crop) [Bodhain½, 1996],assuming a scatteringefficiency H20. The remainder of the destruction is associated with the reactions of ozonewith HO2 (25-40%),and, face/scattering surface ratio(A = 2or,p).Theestimated to a lesserextent,with OH (10-15%). Othercontribureactioncoefficient is 8.?x10-6, 6.3x10-6, 2.7x10-5, tions, suchas OH + NO2, play a minor role in the O• and?.3x10-6 s-• for fall, winter,springandsummer, budget. The total O• destructionrate rangesfrom 1.7 respectively. ppbv/d in winterto 4 ppbv/d in summer.The calculatedO• destruction ratesleadto an O• lifetimeagainst
factor of 2 and taking into account the aerosol sur-
3.2.
Ozone
chemical loss (=[O•]/Lo•)ofabout 26and10days dur-
Plate 1 showsthe majoroddoxygen(O•) production ing winter and summerseasons,respectively.As calcuand destructionprocesses for conditionsrepresentative latedby Ridleyet al. [1992]and Liu et al. [1992]for of eachMLOPEX 1 and 2 intensive(i.e., fall 1991,win- MLOPEX 1 andby Daviset al. [1996b]for PEM-West Ox budgetis thedifter 1991-1992,spring 1992, late spring 1988, and sum- A (fall 1991),thefreetropospheric ference between two large and similar terms, with the mer 1992) and averagedover 24 hours. The budget net O• photochemical production being slightlynegaanalysis is basedon oddoxygen(O• = Oa + O(aP) + tive. The net productionrate rangesfrom a near balNO2 + 2xNOa + 3xN205 + 2xHNOa + 2xHNO4 + PAN) to accountfor cyclingwithin the family. How- ancein winter to a destructionof-1.4 ppbv/d during ever, sinceOa is the most abundant componentof O• summer. Resultsobtainedduring the MLOPEX camin most of the troposphere,the O• and Oa budgets paignsare consistentwith the idea that the tropical can be viewed as equivalent. About two thirds to three lower troposphereprovides a chemical sink for ozone
et al., 1987;Ridleyet al., fourths(60-75%) of the O• productionrate is associ- [Liu et al., 1983;Chameides ated with the reaction of HO2 with NO, and the re- 1987, 1992; Thompsonet al., 1993;Davis et al., 1996b; mainder with reaction of CHaO2 with NO. Reactions of Jacobet al., 1996]. otherperoxyradicals(RO2) with NO representonly a smallcontribution(about 3%) to the total production 3.3. Hydroxyl and Peroxy Radicals rate. The total productionrate rangesfrom 1.6 ppbv/d Plates2, 3, and 4, showthe major pathwaysfor proin winter(2b) to 2.6 ppbv/dduringsummer(2d), due ductionand destructionof OH, HO2, and CHaO2, reto higher photolysis rates and radical concentrations spectivelycalculated for each MLOPEX intensive and calculated during the latter period. The MLOPEX 2 averagedover 24 hours. As expectedfor these short-
springintensive(April 15- May 15, 1992)tookplace2 lived species,a nearly exact balancebetweenproducweeksearlierthanMLOPEX 1 (May 1 - June4, 1988). tion and destructiontermsis calculated.Comparison The O• productionis larger during MLOPEX 2c than of winter (2b) and summer(2d)conditionsshowsa MLOPEX 1 and reachesabout2.5 ppbv/d, in compar- strong seasonalvariation in both production and deisonto 2 ppbv/d for MLOPEX 1, due to larger NO• structionprocesses, stressing the fact that chemistryis levelsmeasured duringspring1992(40 pptv) in com- muchmore activeduring the summerperiod. About parisonto 1988 (32 pptv). About 40-60% of the O• 50-70%of the OH production (Plate2) is dueto pho-
30,296
HAUGLUSTAINE
ET AL.' PHOTOCHEMISTRY
AND BUDGET
OF OZONE DURING MLOPEX 2
tolysisprocesses (mainlyOs photolysisand to a lesser et al., 1996; Jacob et al., 1996; Heikes et al., 1996a,b; extent that of peroxidesH202 and CHaOOH). The rest Brasseuret al., 1996]. For MLOPEX conditions, previis associatedwith recycling from HO2. Even in this low-NOs environment,about 20-30% of the production is attributed to the HO2 + NO reaction. About 40-60%
ouswork indicated an overestimateofH202, CH3OOH,
and CH20 by modelsby up to a factorof 3 [Liu et al., 1992;Zhou et al., 1996].
Figure 15 comparesobservedand simulated concenof the OH destructionis due to reactionwith CO (4060%) and, to a lesserextent, ca4 (20-30%). During trations of H202 and CH3OOH filtered over free troposummer, while the reactions with CO and CH4 repre- sphericepisodesand averagedover the duration of the sent the major loss for OH, the summed contribution different measurement periods. Calculated and meaof seven other minor reactions, each contributing less sured concentrations are also reported in Table 4. As than about 10%, is comparableto the contributionof shownin Figure 15a and Table 4, during the springand the dominant processes. summer periods of MLOPEX 2, the measured H202 As expected, the free tropospheric budget of HO2 mixing ratios are 1.3 and 1.5 ppbv respectively.Calcu(Plate 3) showsthat morethan 50-60%of the produc- lated concentrationsappear to be on the high side of tion stems from the reaction of OH with CO and, to a measuredvaluesduring these two seasons.During fall lesserextent, with O3. However,eight other minor reac- (2a) and winter (2b), three setsof measurements are
tions, eachcontributinglessthan 10%, have a summed available[Staffelbach et al., 1996]. The measuredmixcontribution to the HO2 production comparableto that ing ratios are lower than during springand summerand of the CO + OH reaction. Destruction of HO2 occurs reach only 570-850 ppbv. The calculated H202 mixing mostly by formation of hydroperoxidesand by reaction ratios overestimate the measured values by a factor of with NO and Oa. As reportedby Ridleyet al. [1992]for 1.5-2.5 during these seasons. The average concentraMLOPEX 1, it is interestingto note that even in this tion of CHaOOH (Figure 15b) is overestimated by a low-NOs environment, the rate of the reaction of HO2 factor of 1.2-2 by the model during all seasonsduring with NO is comparableto and often larger than the rate MLOPEX 2. The uncertainty in the rate constant for of its reactionwith Os. About 2/3 of the CH302 pro- the CH302 + HO2 reaction(whichis believedto be the
ductionis due to initial methaneoxidation(Plate 4), and the remaining to CH3OOH oxidation. The CHaO2 destruction is about equally divided among reactions with HO2 and NO.
only sourceof CH3OOH), is a factorof 2.2 for Mauna Loa conditions[DeMote et al., 1997]. When this reaction rate is decreasedby a factor of 2.2 in the model, the concentrationof CH3OOH is reducedby about 30%, bringing the model closerto observations.However,as a consequenceof the lower destructionrates of HO2 and CH302, the total concentration of peroxy radicals is in-
Plate 5 showsthe budget of RO• = OH + HO2 + CHaO2 under free tropospheric conditions as derived from the individual budgets from Plates 2-4. The main sources of radicals as calculated by the model
creasedby about 20%. Consequently, the mixing ratio
are Os photolysis and subsequent reactionof O(•D)
of H202, already overestimated by the model, is further
with watervaporandhydrogenperoxidephotolysis(80- increasedby about 25%. 90%). Formaldehyde andmethylhydroperoxide photolIn orderto investigatethe possiblerole playedby wet ysiscontributefor only 15-20% to the total production removal of H202 and to a lesserextent CHaOOH, an of radicals at this altitude. The major lossesof radi- artificial constantremovalrate of thesespecieshas been calsarethroughperoxide(H202, CHaOOH)production consideredin the model. Table 4 givesthe calculated (60-70%) and water vaporformation(20-30%). The mixing ratios of peroxidescalculated with a first-order calculatednet productionof radicalsrangesfrom about removal rateof 1.2x10-5 s-• (corresponding to a life1.5xl0 -5 molecule cm -3 s-•
in winter to 2.5x10 -5
time againstwashoutof i day) and of 2.3x10-6 s-• moleculecm-3 s-• in summer. The budgetresults (corresponding to a 5 day lifetime). A washoutrate for the altitude of MLO contrast those of the upper for H202 of lx 10-5 s-• hasbeenderivedby Martin et troposphere, where recent comparisonsof models and al. [1997]from measurements obtainedin the boundmeasurementshave stressedthe importance of acetone ary layer over the Atlantic Ocean and is used here as and peroxides as important sourcesof odd hydrogen an upper limit for MLOPEX conditions. The value of
[ Wennberget al., 1997].
2.5x10-6 s-1 waspreviously usedby Liu et al. [1992]
3.4. Peroxides and Formaldehyde
and is consideredin this study for comparisonwith this previouswork. This additionalsink decreases H202 by
As illustrated in Plates 2-5, peroxidesand formaldehyde provide a sourceof radicalsin the mid troposphere and, consequently,play a significantrole in the budget and distribution of species. Furthermore, H202 is a major oxidant of sulfur dioxide to sulfate in the liquid phase. Several studies have shown a major shortcomingin our current understandingof peroxidesand
about30-40%and CH3OOH by 20-30%,dependingon the seasonin the caseof the 5 day lifetime removalrate. Includingthis removalobviouslyhelpsto bringthe calculatedperoxidemixing ratios closerto measurements, and even lower than the observations in the case of a 1
day lifetimeremovalrate. However,as pointedout by
Liu et al. [1992]and Hauglustaine et al. [1996a],be-
CH20 in the remoteatmosphere[Liu et al., 1992;Zhou causeof the episodicnature of precipitation,it is over-
HAUGLUSTAINE
ET AL.- PHOTOCHEMISTRY
AND BUDGET OF OZONE DURING MLOPEX 2 30,297
FreeTropospheric OxBudget Net Production' -0.56 0.00
-0.78
-0.44
-1.59
Production
RO2 + NO
• CH302 + NO
FI H02 + NO ._o
0
ii i1•
i
Destruction
O H20 + O(•D)
/ I
-3
• 03 + HO2 [ 03+
-4
OH
0 OH + NO2
-5
I
I
I
I
2b
2c
1
2d
Intensives
Plate 1. Photochemical budgetof O• for the differentMLOPEX intensives (fall 2a; winter2b; spring2c;spring1988;andsummer2d) duringfreetropospheric conditions (ppbv/d).
Free TroposphericOH Budget 2Ol
I
Production
• CH3OOH+hv 13 H202+hv HO2+O3 • HO2+NO
15
10
El H20+0(1 D) Destruction
OH+CO
OH+CH4 OH+CH3OOH OH+H202 OH+ CH20
-5
-10
OH+H2 OH+HO2 OH+03 OH+NO2
-15
-20
I
I
I
I
2a
2b
2c
I
2d
Intensives
Plate 2. Photochemical budgetof OH for the differentMLOPEX intensives (fall 2a; winter2b;
spring 2c;spring 1988;andsummer 2d)during freetropospheric conditions (105molecules cm-3
30,298 HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2
FreeTropospheric HO,_Budget 2Ol
I
15
10
5
II
Production
Ii CH3OOH+hv I OH+O3 El CH20H02 I H2+OH I CH20+OH I HNO4+ M 1:3H202+OH II CH20+hv I CH302+NO [3 CO+OH Destruction
-5
[] HO2+HO2 I HO2+NO I HO2+O3 El HO2+CH302 I HO2+NO2 ß HO2+CH20 HO2+OH
-10
-15 -20
2a
2b
2c
I
2d
Intensives
Plate 3. Photochemical budgetof HO2 for the differentMLOPEX intensiveperiods(fall 2a; winter 2b; spring2c; spring1988;and summer2d) duringfree tropospheric conditions(105 molecules cm-3 s- •).
FreeTropospheric CH30=Budget Production
I CH3OOH+OH • CH,•+OH
I
Destruction
-2
• CH302+HO2 -4
I CH302 + NO
-6 -8
CH302+CH302 I
I
I
I
I
2a
2b
2c
1
2d
Intensives
Plate 4. Photochemicalbudget of CH302 for the differentMLOPEX intensiveperiods (fall
2a; winter2b; spring2c;spring1988;andsummer2d) duringfreetropospheric conditions (105 molecules cm-3 s- •).
HAUGLUSTAINE
ET AL.' PHOTOCHEMISTRY
AND BUDGET
OF OZONE DURING
MLOPEX
2 30,299
FreeTropospheric ROxBudget 20 Production
15
HN04 +u
CH20+hv 10
CH300H+hv H202+ hv
H20+O(1D) Destruction
I-I OH+ NO2
-5
I OH+H02 HO2+HO2
-10
D HO2+ CH302 I HO2+ NO2
-15
I CH302+CH302 -20
I
I
I
I
I
2a
2b
2c
1
2d
Intensives
Plate 5. Photochemical budgetof RO• (=OH'+ HO2 + CH302) for the differentMLOPEX
intensive periods (fall2a;winter2b;spring2c;spring1988;andsummer 2d) duringfreetropospheric conditions (105molecules cm-a s-1).
FreeTropospheric NOxBudget Net Production -20.5 -18.0
ß -55.9
-24.1
-47.6
Production
30
20
• HNO3+hv
10
HNO:•+OH I PAN+hv
-10
i
• PAN+M
.2o / -30
Destruction
-40
• NO2+0H
-50
-60
I NO2+CH3C03
-70
N205+Aer.
-80
2a
2b
2c
1
2d
Intensives
Plate 6. Photochemical budgetof NO• for the differentMLOPEX intensives(fall 2a; winter2b; spring2c;spring1988;and summer2d) duringfreetropospheric conditions(pptv/d)
30,300 HAUGLUSTAINE ET AL.: PHOTOCHEMISTRY
AND BUDGET OF OZONE DURING MLOPEX 2
FreeTropospheric NOxBudget 40
Net Production -28.5 -25.9
: -49.4
-56.9
-56.1
Production
3O
E] HNO3+hv HNO•+OH I PAN+hv -lO
E] PAN+M
-20 /
I I I I
C• -40 Z
-$0
Destruction
• NO2+OH I NO2+CHsCO• N205+Aer.
-70
-80
I
2a
2b
2c
1
2d
Intensives
Plate 7. Photochemicalbudget of NOs for the different MLOPEX intensiveswhen acetoneis
considered in the simulations(pptv/d).
simplifiedto simulaterainout processes with a constant removalrate. If rainout werethe only processreponsible for the disagreementbetween observedand calculated CH3OOH, one would expect frequent air masseswith larger CH3OOH mixing ratios. However,evenwhen the day-to-day variability is considereda systematic overestimateof CHaOOH by the modelis obtainedduring
measurementsare overestimatedby the model. Significant overpredictionsof measuredformaldehydeby the modelshave also been reported by severalauthors
for remoteor marineenvironments.Liu et el. [1992] showedpredictedformaldehydelevelsthat were about a factorof 3 higherthan measuredduringMLOPEX 1.
LoweendSchmidt[1983]reportedmeasurements in the mostof the year(particularlyduringfall andsummer). Atlantic to be about two thirds of those predicted. Figure 16 showsthe observedand simulated concen- Heikeset el. [1996a]and Zhouet el. [1996]calculated trations of CH20. Two types of simulationshave been CH20 concentrations 2 to 3 times higherthan obserperformedwith the model.The first set of results(Fig- vations in their analysis of the MLOPEX 2 data. Reure 16a) corresponds to simulationswhere both per- cently,Jecobet el. [1996]reportedmodelcalculations oxidesand formaldehydewere calculated. Since previousresultsindicatean overestimateof peroxidesby the
that overestimateby a factor of 3 to 4 the measurements
collectedduringTransportand AtmosphericChemistry model,we haverun a secondsetof simulations(Figure Near the Equator-Atlantic(TRACE-A) in the bound16b) in which solelyformaldehydewas predicted,with ary layer over the tropical Atlantic Ocean. InterestH202 and CH3OOH constrained to their observed val- ingly, simulationswith peroxidesconstrainedto their ues. When formaldehydeand peroxidesare simultane- measuredvalueyield a muchbetter agreementbetween ouslycalculatedby the model,simulatedCH20 mixing observedand predictedCH20 mixing ratios. Except ratios overesti•natemeasurementsby a factor of 1.3-2. during summer when the calculatedvaluesstill overesIt shouldbe noted that in total, five differenttechniques timateobservations by a factorof 1.7, a fair agreement wereusedto measureCH20 duringMLOPEX 2 [Heikes is obtainedwith the aqueous-scrubber enzymefluoreset el., 1996a]. As is clear from Table 4 and Figure cencemethod[Heikeset el., 1996a]duringMLOPEX 2 16, becauseof variations in analytical-proceduralfield and even MLOPEX 1. A similarconclusion regardblanksandcalibrations[Heikeset el., 1996a],thereis a ing the improvement of calculatedCH20 with respect significantspread among the different techniqueswas to observations when CHaOOH is constrained has also obtained. However, taken together, all the different beenreachedby Zhou et el. [1996]usinga steady
HAUGLUSTAINE ET AL.' PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2 30,301
model and averaged over 24 hours for all intensives.
2000
(a) H202 1500
As pointedout by Jacobet al. [1996],in assessing the
,.-'d ,,'
ß
importance of NOs production from PAN thermal decomposition and photolysis,
(R15)
PAN
> NO2 + CH3CO3
1000
(R16) /
bo
PAN + hu ---+ NO2 + CH3CO3
and NOs lossthrough PAN formation,
5OO
(R17)
NO2 q- CHaCOa + M •
PAN + M
0
1500
1 ooo
5OO
0
2000
in a model constrained with observed NOs and PAN
and with CHaCOa assumed to be at steady state,
Model, pptv
the reactionrates of (R15), (R16), and (R17) need '
8OO
ß
ß
i
i
i
ß
/
(b) CH300H
ß
to be corrected for cycling within the family PANs
(=PAN+CHaCOa). FollowingJacobet al. [1996],the
,.-'
reaction rates are correctedaccordingto
600
,
k•s- k•s
LPANx
(7)
400
LpAN• PJ•A•V•+ kx• + kx6
(8)
PPAN• PJ•A•V•+ kxs+ k•e
(9)
200
/
kx,- kx7 i
0
,
,
200
,
i
,
,
400
,
i
600
,
,
800
Model, pptv
Figure 15. Relationshipbetweenobservedand model-
calculated(a) H202 and (b) CHaOOHfor the different MLOPEX intensives(pptv).
state box model. Our resultsindicatethat a significant part of the formaldehydeproblemmight be related to a shortcomingin our understandingof the peroxidesand, in particular, methylhydroperoxidechemistry. However, this conclusionneedsto be tempered by the fact that a significantoverpredictionof formaldehydeby the modelis still obtainedduring summer,pointingtoward a possibleadditional discrepancyin the representation of CH20 chemistry in the model. Part of the remaining discrepancycould be attributed to dry deposition of formaldehydeat the surfacearound the site [Zhou
where PJ•A•V•and LpAN• are the production and loss rates of PANs, respectively. For MLOPEX conditions
the correctionfactorsk•5/kts and k•7/kt7 are in the range 0.24-0.3 and 0.2-0.4, respectively. As shown in Plate 6, model results indicate a systematic imbalancein the NOs budget, with production
balancingonly 20% and 45% of the lossduring summer (2d) and winter (2b) respectively.A largepart of this deficit is associatedwith HNOa formation not compen-
satedby sufficientrecyclingof HNOa to NOs [Hauglustaine et al., 1996a]. The calculatedlifetime of NOs againstchemicallossrangesfrom about I day in winter to 0.5 day in summer. From our results, it appears that
an additionalsourceof NOs rangingfrom 18 pptv/d in winter to 48 pptv/d in summerwould be required
in the model to maintain the nitrogen oxides at their observedvalues. To balance the NOs budget, a recycling of HNOa to NOs occurringwith a i-day lifetime would be required. However, the nitric acid lifetime et al., 1996]. Simulationshavebeenrepeatedadopting against photolysisis estimated as about 30 days near a removalrate for formaldehyde of 4x 10-6 s-• corre- 3.4 kmo This problem has been encounteredpreviously
spondingto a depositionvelocityof about 0.4 cm s-• [Jacobet al., 1996]. As shownin Table 4, CH20 is de-
by Liu et al. [1992]duringMLOPEX 1, Chatfield[1994]
creasedin that caseby about 10-15% in comparisonto the reference case. However, the measured formaldehyde concentrationis still overestimatedby about 50% during summer.
(CITE-2), Fan et al. [1994]duringthe ArcticBoundary LayerExpedition(ABLE-3B), Singhet al. [1996]during PEM-West A, Jacobet alo [1996]duringTRACEA, and morerecentlyby Jaegldet al. [1998]duringthe
3.5. Nitrogen Species
during Chemical Instrumentation Test and Evaluation
Subsonic Aircraft Contrail and Cloud Effects Special
Study(SUCCESS)campaign.Chatfield[1994]proposed
Plate 6 showsthe photochemicalproduction and loss that rapid reaction of HNOa with CH20 in acid aerosols ratesof nitrogenoxides(NOs) calculatedwith the box might take place and yield NOs and formic acid. Fan
30,302
HAUGLUSTAINE
ET AL.: PHOTOCHEMISTRY
AND BUDGET
OF OZONE DURING
MLOPEX
2
Table 4. ComparisonBetween Measured and Calculated Mixing Ratios of H202, CHaOOH,
and CH20 (pptv) AveragedOver IntensiveDuration for MLOPEX 1 and 2 Free Tropospheric Conditions
Fall(23)
Winter(2b)
Spring(2c)
Spring(1)
Summer (2d)
H2 02 Model
Model-removal URI Coil TDL HPLC
1610.0
508.3/1062.0 ... 847.0+ 260.0 635.0 q- 230.0 794.0 q- 278.0
1490.8
1706.8
361.1/862.7 830.6 + 423.2 589.0 q- 216.0 574.0 4- 260.0 ............
534.1/1125.2 1271.1
q- 566.4 ......... .........
1966.9
582.2/1260.4 1051.6 q- 265.0
1859.1
728.5/1351.7 1518.4
q- 486.3
CH3 00H Model
722.2
Model-removal URI Coil HPLC
324.6/554.1 ... 379.0 q- 100.0 354.0 4- 131.0
569.5
646.9
196.4/390.5 212.4 4- 96.4 282.0 4- 107.0 ............
278.6/489.5 427.6 q- 183.2 .........
633.0
257.2/466.7 136.8 q- 51.3
759.2
417.3/629.7 569.9 4- 144.5
CH•. O
Model a
242.8
194.3
216.2
201.4
280.3
Model b Modelb-removal
180.4 162.0
132.1 116.9
179.5 159.4
116.0 104.7
242.3 220.7
URI Coil TDL
159.8 4- 65.2 130.6 q- 24.2 87.8 4- 54.3
145.1 q- 48.9 119.4 4- 25.8 158.9 4- 84.7
198.8 q- 59.0 122.3 q- 31.5 .........
104.9 q- 42.5 ...
147.2 q- 67.2 161.2 q- 53.8
URI, University of Rhode Island coil collection;TDL, tunable diodelaser;Coil, aqueouscoil collection; HPLC, high performanceliquid chromatography[seeStaffelbachet al., 1996;Heikeset al., 1996a;Zhou et al., 1996; Mackay et al., 1996]. a Peroxides calculated.
b Peroxides constrained.
et al. [1994]proposedthat the reactionHNOa with used the correlation derived from PEM-West B to esCH20 couldyield hydroxymethylnitratethat could be timate acetone levels from CO measurements obtained photolyzedto produceNOs. The heterogeneous reac- during SUCCESS. Assuming the same correlation for tion of HNOa on black carbon aerosolsyielding NOs conditionsprevailing at Mauna Loa during free tropohasalsobeenproposed[Lary et al., 1997;Hauglustaine sphericepisodes,we derive from measuredCO during MLOPEX, acetonemixing ratios of 414, 515, 660,705 et al., 1996a]as a possiblemechanism. and 292 pptv for MLOPEX 2a, 2b, 2c, 1, and 2d, respectively. For comparison, Greenberg et al. [1996]reported 3.6. Role of Acetone a value of about 700 pptv for MLOPEX 2d conditions. $inghet al. [1995]havepointedout the importanceof For MLOPEX conditions,the budget of OH and HO2 acetonephotolysis asa sourceof HO• (=OH+HO2) and as illustrated in section 3.3 is dominated by water vaPAN under dry conditions in the upper troposphere.
por and, to a lesserextent, peroxides. When acetone
Jacobet al. [1996]calculatedfor TRACE-A conditions is consideredin our simulations, HO• increasesby less an increaseof HO• of 15% at 8-12 km when 650 ppbv than 1%. The impact on peroxidesand CH20 is also of acetonewas included. McKeen et al. [1997a]es- lessthan about 1%. However,acetonehas a significant timated that, in the upper troposphere, acetone can
accountfor a significantand even dominant sourceof HO• duringthe SUCCESScampaign.Measurements of acetoneduring the PEM-West B missionshowa strong positive correlationbetweenacetoneand CO over the
impact on PAN formation and, consequently,on the nitrogen budget. As illustrated in Plate 7, including acetone and the subsequentadditional PAN formation from peroxyacetylradicals producedby acetonefurther increasesthe NOs deficit. In that case, an NOs source
westernPacific[Singhet al., 1995].Similarresultswere rangingfrom 24 pptv/d in winter to 56 pptv/d in sumobtainedby Fan et al. [1994]for ABLE 3B andMauzer- mer is required to maintain NOs at the observedlevel all et al. [1998]for TRACE-A. McKeenet al. [1997a] in the model.
HAUGLUSTAINE
ET AL.' PHOTOCHEMISTRY
CH20 300
....
i ....
i ....
i ....
i , . ß .,• ß
o
25O
o
,.
o
200
ß
o
150 ß
•
100
'/
b 1
AND BUDGET OF OZONE DURING MLOPEX 2 30,303
over a mean diurnal cycle. The present results indicate that, evenwhen a detailed simulationof the photochemistry includingthe role of varioushydrocarbonsis considered,the comparisonwith observationsis not significantly improved. These results suggestan excessNO oxidation to NO2 by some unidentified speciesnot included in our simulations as undetected peroxy radicals formed from the oxidation of organicsemitted by the vegetation or by halogensoriginating from the marine boundary layer. However, estimated concentrationsof these speciesat the MLO site do not support this hypothesis. The correlationsbetweencalculated and mea-
,.
5O
ß
,
,
,
i
....
1 O0
5O
i
....
150
i
....
200
i
....
250
300
Model, pptv
and measured hydroxyl radical concentrations. However, during summer, the model tends to overestimate the OH measurementsby a factor of 1.5-2. Furthermore, a systematicfactor of 2 overestimateof the daytime total peroxy radical concentration by the model is obtained during all seasons.As suggestedby Eisele et
300
(b)
25O
suredNO/NO2 ratiosfiltered overhigh-Sunconditions are generallysmaller(40-50%duringMLOPEX 2a, 2c, and 2d and only 25% during winter). During spring, fairly good agreement is obtained between calculated
..-'
200
al. [1996]and Canttell et al. [1996b],severalhypothe150
seshave been consideredto explain these discrepancies,
..f-d' 8
100
50
including additional removalof radicalsby reaction on aerosolsor by unidentified biogenic organics. In con-
'
50
1 O0
1,50
200
2,50
300
Model, pptv
trast to previouswork [Canttellet al., 1996b;Eiseleet al., 1996],no evidencefor thesehypothesesis found in the overallMLOPEX database.Additionally, no single hypothesis simulaneouslyprovides satisfactory results
Figure 16. Relationship betweenobserved and model- for NO/NO2, OH, and peroxyradicals.The deterioracalculatedCH20 when peroxides(a) are not con- tion in performanceof radical amplifiersassociatedwith strainedand (b) areconstrained in the model(pptv)o humidityeffectpointedout by Mihele andHastie [1998] (potentiallya factor of 2 for MLO upslopeconditions) 4.
Conclusions
In this paper a photochemical box model has been used in conjunction with the MLOPEX measurements
to evaluate the processesand speciescontrollingthe photochemistry at the Mauna Loa observatory(MLO) and provide some insight into the ozone and nitrogen budgetsin the free troposphereover the remote Pacific. The main objective of the present study was to assess the ability of the model to simulatetime-varyingconditions and the day-to-day variability of the measurementscollectedduringMLOPEX 2 in variousair masses and during the four seasons.We have emphasizedthe simultaneous simulationof abundances andpartitioning of the odd nitrogenand odd hydrogenspecieswhenconstrained by time-varying measurementsof key chemical tracers and physical quantires.
Analysisof variabilityof the NO/NO2 ratio at the site indicatesthat the model generallyreproducesthis photostationarystate. However,during summertime,the model occasionallyfails in reproducingthe NO/NO2 ratio. A similar discrepancyhas been obtained by Hauglustaineet alo[1996b]for clear-skyconditionsand
would help resolvethis discrepancyduring upslopeconditions. No definitive conclusionis availableregarding this effect for the instrument used during MLOPEX 2. The issueclearly requiresfurther investigationwith improved intrumentation. Analysisof the budget of speciesduring free troposphericconditionsindicatesnearly balanced production and destruction of ozone during all seasons. The net productionrate of ozoneis slightly negativeand ranges
from nearly zero in winter to about-1.4 ppbv/d during summer, indicating that this region of the tropical Pacific troposphere acts as a net photochemical sink
for ozone.The budgetsof radicals(OH, HO2, CHaO2) illustrate the seasonal variation of production and destruction processesof these compounds, and the fact that the photochemistry is much more active during summer.
A strongimbalanceis found in the NO• budget during all seasons.Our results indicate that an additional
sourceof NO• rangingfrom 18 to 48 pptv/d is required to sustain the observed nitrogen oxide concentrations in the model. This imbalance is more pronouncedwhen acetone is included in the model, due to additional formation of PAN and consequentsink of NO2. The tea-
30,304 HAUGLUSTAINE ET AL.: PHOTOCHEMISTRY AND BUDGET OF OZONE DURING MLOPEX 2
Table 5. Agreement Reached in Simulating the MajorChemicalSpecies DuringMLOPEX 2 Intensive Periods Fall (2a)
Winter(2b)
Spring(2c)
Summer (2d)
NO/NO2
good
good
good
OH PO2
...... poor
poor
fair poor
poor poor
OH/PO2 HNOa/NOx H202
poor poor poor
poor poor poor
poor poor fair
poor poor good
fair
CHa OOH
poor
poor
poor
poor
CH20
good
good
good
poor
Good,theresultsarewithintheobserved -t-la;fair, theresultsaremostof the timewithintheobserved -t-la;poor, the resultsarenot (or onlyoccasionally) within the observed -i-la.
son for this imbalancein the NOx budget is not clear but may havesevereimplicationsfor ozonebudgetestimates from photochemicalmodels. With the exception of summertime,a reasonableagreementis achieved between measurements and model for formaldehyde if methylhydroperoxide is constrainedto its observed value. On the other hand, the model tends to overestimate the various sets of measurements of H202 and
Acknowledgments. Helpful comments on the manuscriptby F. Flocke and J. Greenbergare gratefully acknowledged.Thanks to B. Bodhaine, J. Greenberg,B. Heikes,G. Htibler, G. Kok, K. Lantz, Y.-N. Lee, T. Staffelback,J. Walega,X. Zhou,and other MLOPEX participants for allowing accessto their results for this study. The National Center for Atmospheric Researchis operated by the University Corporation for Atmospheric Researchunder the sponsorshipof the National Science Foundation.
CHaOOH during all seasonsby a factor of 1.5-2.5. Inclusion of a constant removal rate such as washout for
thesespeciescertainly helpsreconcilemodel and measurementsin terms of averagedconcentrations. However, becauseof the episodicnature of precipitation, it is unlikelythat washoutwouldbe solelyresponsible for the peroxidedisagreement,and the origin of the inconsistencythereforepoints toward a shortcomingin our understandingof peroxidechemistry. As a summary,Table 5 givesthe level of agreement reached in simulating the photochemicalspeciesfor MLOPEX conditions. The good agreementobtained for the NO/NO•. photostationarystatelendssomeconfidencein our understandingof the local photochemistry. The understandingof the photochemistryin this regionof the Pacific clearly benefitsfrom the use of
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[email protected])
(Received February27, 1998;revisedApril 28, 1999;
ercise: A joint TOR/OCTA experiment at Schauins- acceptedJune 17, 1999.)
