Jun 2, 1992 - NAâ¢gley Research Center, Hampton, Va. 2SAIC ... Pump Corp., Camden, New Jersey. .... on the order of 2 to 3 years that is related to the QB0.
GEOPHYSICAL RESEARCH LETTERS,VOL. 19,NO. 11,PAGES1109-1112, JUNE2, 1992
OBSERVATIONS OF REDUCED OZONE CONCENTRATIONS IN THE TROPICAL STRATOSPHERE AFTER THE ERUPTION OF MT. PINATUBO
W. B. Grant • J. Fishman • E. V. BrowelP V. G Brackett 2 D Nganga • ,
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A. Minga 3, B. Cros 3, R. E. Veiga 2, C. F. Butler 2, M. A. Fenn 2, G. D. Nowicld 2 Abstract.The eruptionof Mt. Pinatubo(15øN,122øE)on June15 and 16, 1991, placeda largeamountof SO2and
(20-28km) with a largechangeoccurring betweenMay and August1991, reachinga peak in October/November, and crustal materialin the stratosphere.Basedon measurements continuing into 1992with reducedmagnitude.The altitude ofdecreases of stratospheric ozoneafter previousvolcanic of the mostpronounced reductionagreesvery well with the eruptions, it wasexpected thattheaerosols deposited intothe altitudeat whichthe highestconcentrations of aerosolswere stratosphere (bothdirectlyandas a resultof SO2conversion measured independently with an airhomelidar system. intoparticulate sulfate)by thiseruptionwouldgiveriseto significant ozonedepletions.To checkfor suchan effect, ExperimentalProcedures andResults ozone profilesobtainedfrom ECC sondesbeforeandafter Ozonesonde stations at Brazzaville and Ascension Island theeruptionat Brazzaville,Congo(4øS,15øE),andAscensionIsland(8øS, 14øW),are examined. Aerosolprofiles were establishedprimarily to measuretroposphericozone determined from a lidar systemin the westernPacific(4ødistributions in a regionwhere anomalouslyhigh integrated 6ø1,,1, 125øE)showthat mostof the materialinjectedinto the troposphericozone amountshad been indirectlyinferred stratosphere is locatedbetween18 and 28 km with highest usinga satellitemethoddescribedin Fishmanet al. [!990]. mountsat 24-25 km. For the period 3-6 monthsafter the
The stationsusethe electrochemicalconcentrationcell (ECC)
eruption, decreases in ozoneare foundat 16 to 29 km, with developed by Komhyr[1969] and manufactured by Science peakdecreases as largeas 20% foundat 24 km. Integrated PumpCorp., Camden,New Jersey. Their operationis based between16 and 28 km, a decreaseof 13-20 Dobsonunitsis uponthe reactionof ozonewith iodideto producetri-iodide observed when the ozonesonde data after the Pinatubo (iodine). The reactionis inducedby pumpingair throughthe eruption are comparedwith thoseprior to theeruption.The cell with the resultingelectriccurrentbeingdirectlyproporaltitudeat which the most pronouncedozone decreaseis foundstronglycorrelateswith peak aerosolloadingdeterminedby the lidar. In addition,a smallincreasein ozone density is foundaboveabout28 kin. Mechanisms thatmight explain theresultssuchasheterogeneous chemistry, radiative
effects, anddynamicsare discussed. Introduction
Theassociation betweenstratospheric aerosolsandreductionsin stratospheric ozonehas beennotedsincethe mid1960s. Decreasesof mid-latitudeozone resultingfrom transport polewardof tropicalair masses werefoundafterthe eruptions of Mt. Agungon March 17, 1963 [Gramsand Fiocco, 1965;Pittock,1966],Fuegoin October1974[J/iger andWege,1990], andE1Chichonon April 4, 1982[Angell,
tional to the ozone concentration
in the air stream.
It is
possiblethatthepresence of sulfateaerosols in theair stream could affect the measurementof ozone, but this does not
agreewith the verticalprofilesof ozone. The accuracyof ECC sondesis 10% or betterin the stratosphere, and their precisionis in the 5 % to 8 % rangebetween10 and 31 km [Barneset al., 1985]. The Brazzavillelaunchescommenced on June6, 1990, andare expectedto continuethrough1992. Interruptions haveoccurredfrom August6 to September19, 1990,andfrom May 23 to August15, 1991. The Ascension Islandlaunchescommenced on July 28, 1990, andlikewise are expectedto continuethrough1992. No launcheswere madefrom September12 to November21, 1990, and from May 23 to October10, 1991. Exceptfor the prolonged interruptions,ozonesondes were launchedapproximatelyon a weeklybasis. Figure1 depictstheintegratedamountof ozonebetween16
1988;NewellandSelkirk,1988;JagerandWege,1990]. and 28 km between June 1990 and December 1991. This Verylittleeffecthasbeennoticedneartheequator[Angell, figurehasbeenderivedfrom an analysisof 71 sondesthat 1988;Chandra andStolarski,1991]. reachedan altitudeof 30 km or higher(of the 79 launched Inthisstudy,we present a setof ozonesonde measurements during that period). Unfortunately,becauseboth stations obtained in thetropicsin 1990and1991. Thedataindicate were not operational throughoutall of JuneandJuly 1991, a reduction in the amountof ozonein the lower stratosphere we havea two-monthdata gap centerednear the time of the eruption. Between May and August, a decreaseof 12
DobsonUnits (D.U.; 1 D.U. = 2.69 x 10•6 tool. 03 cm4)
NA•gley Research Center, Hampton, Va.
2SAIC, Hampton, Va. 3Universit6 Maien Ngouabi, Brazzaville, TheCongo Copyright 1992by theAmerican Geophysical Union. Papernumber92GL01153
0094-8534/92/92GL-01153503.00
occurred with an additional decline of 7 D.U.
between
Augustand November. The numberaboveeach monthin Figure 1 showsthe numberof sondesfor eachmonththat have gone into the analysisdepictedin this figure. The
dottedline on eithersideof the curvein thisfigurerepresentsthe standarderror of the averagesfor each month. Sincethis shorttime seriesis not long enoughto showthe effect of the quasi-biennialoscillation(QBO), it must be augmented with additionaldata. 1109
1110
Grantet al.' Observations of Reduced OzoneConcentrations
INTERGRATED OZONEDIFFERENCE (28km - !6kin) ! 8o
Before Pinotubo
.
ZD
15o •
density of ozone at25.5kmdecreased subsequent totheMt.
After Pinotubo
.
-,_,,-
14.0
•
13o
0
12o
•
11o
.
Pinatuboeruption. The Octoberand November1991 concentrations are 3.7 x 1012real. cm'3. Prior to those measurements, the lowestSAGE II monthlyaverages had been4.1 x 1012mol.cm'3. TheSAGEII ozonedensity at 25.5 km exhibitsthe greatestvariationthroughout a period on the order of 2 to 3 years that is relatedto the QB0
[Hilsenrath andChandra,1989;McCormick et al., 1989;
CD 100 •
measurement in thetropicsup to an altitudeof 29 krn. The ozonesonde measurements, on the otherhand,showthatthe
.....
Stondord
Schuster et al., 1989;ZawodnyandMcCormick,1991].
error of the rneon
From the informationin the two figures, we can estimate the ozone decreasebelow 28 km that is related to volcanic
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OND
1991
Month
Fisure t.
Column ozone amountsbetween•6 •d 28 km
aerosols.If we extrapolatethe pre-Pinatubo meancolumn amountbetween16 and 28 km (124 D.U) and compare it with the October/November low (104 D.U.), the difference is --.20 D.U. If on the otherhand, we assumethattheQB0
minimumof January1991(117D.U.) wouldhavecontinued
determined from ozonesandes]aunch• from Bruz•viHc •d
into Octoberand November, the integralis still ---13 D.U.
Ascension Is]•d from June•990 throughD•ember •99•, averaS• by month.
lowerthananyprevious valuemeasured.Usinga clirnatologicalcolumn valueof 250D.U. [e.g.,NewellandSelkirk, 1988],themeasured columnozonedecrease below28 kmis 5-8%. However,increases in ozoneabove28 km would maskpartof thisdecrease for anyinstrument thatmeasured
A longerdata recordat low latitudesis availablefrom measurements obtainedby theStrataspheric AerosolandGas Experiment !I (SAGEII) whichhasbeenmakingcontinuous 03 measurements since November1984. The average monthlyconcentrations between10øNand 10øSat 25.5 km from November1984 throughMay 1991are shownas the solid line in Figure 2. The ozonesonde measurements at 25.5 km are shownby thedashed line. Between January and May 1990,theozonesonde measurements fromNatal,Brazil (6øN,35øW),are usedasthemonthlyaverageconcentrations [Kirchhoffet al., 1991, plus Kirchhoff,privatecommunication]. As canbe seenfrom thisfigure,theozonesonde data andthe SAGE II datatrack eachothervery well andgeneral-
ly agreeto betterthan 5%. After the eruptionof Mt. Pinatubo, however, SAGE II data are not availablebecause
theopticallythickvolcanicaerosols interferewiththeozone
only columncontentozone.
The averageprofilesshownin Figure3 compareozone measured duringthesecond withthethirdquarters of 1991 for Brazzaville. The differencebetweenthe two profileson
thisfigureis shownby the dash-dot line. Although not shownhere,thethirdandfourthquarterdatafor Brazzaville are similar and the loss shown at AscensionIsland between
thesecond andfourthquartersof 1991is similarto whatis observed at Brazzaville.The uncertainty in thedifference is estimatedto be 0.2-0.4 x l0 n mol. cm'3. The Brazzaville data indicatelower ozoneconcentrations between20 and 29 km after the eruptionof Mt. Pinatubo.
Themostpronounced difference at bothstations isat 24-25 km,reaching 18%fortheAscension Islandsiteand20%for theBrazzavillesite. Anotherinteresting aspectthatshows up
SAGE II and OZONESONDE
in these comparisons is a slightincrease in ozone above 28
MEAN AT 25.5km
km. The enhancement is 5%-7% for Brazzavilleand - 10%
for Ascension Island. Valuesto higheraltitudes couldnotbe Before Pinaruba
A e,'
Pinaruba
MEAN PROFILE COMPARISONS at BRAZZAVILLE,CONGO
(BeforeondAfter Pinotubo)
3O ; z w
--SAGE mean at. 25.5km J'."
Ld 3 1...... Ozonesandes (117 sandes) mean a•: 25.5km / 0 SAGE I1calc isfor10s- 10nlat. band /
n Z
N
O
0
Note
2
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84.
0
Ozone•onde calc isforNatal, Ascension, and Brazzaville / . , , , , , . . , , , , , • , , , ! .... , . l.
25 L :
2o:
85
88
87
88
89
90
91
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:
15:
'•'. -- Before Pinaruba, 2ndOtr 91(7sandes)
Time
After Pinaruba, 3rdOtr 91(7sandes) nd rd
Figure2. Monthlyaverage concentrations of ozoneat 25.5 km between 10øN and 10øSmeasuredby the SAGE II satelliteinstrumentbetweenNovember1984andMay 1991
(solid)areshown withthemeasurements atthesamealtitude byozonesandes atBrazzaville, Congo, Ascension Island, and
5
........... D.??. r.?.n.c.e..!2,,,, .-,, ,5,,, .O.!r.) .............. •
'•'",,'-1
0
1
2
3
4
5
Ozone Density (Mol/cm 3 x 1012)
Natal,Brazil(dashed line). Notethatno measurements from
Figure 3. Comparison ofthesecond andthirdquarter 1991
SAGE II at 25.5 km are available after Pinatubobecauseof
ozone sondedata for Brazzaville. The difference between
interference from volcanic debris.
the two curvesis givenby the dot-dashline.
Grant et al.' Observationsof ReducedOzone Concentrations
obtained usingozonesondes sincetheballoons generally burst near31 kin.
1111
thirdandfourthquarters of 1991. Bothsetsof profilesshow peakvaluesnear24-25 km, and bothfall off at aboutthe samerate below 24 km. Between 25 and 28 km, there is
UV DIAL
Aerosol Profiles
evidencethat the ozonedecreasefalls off moreslowlythan doestheaerosolloading. Despitethe separation in time and TheUV DIAL (differential absorption lidar)system is used location,the agreementis very good. for measuringozone and aerosolprofilesfrom an airborne A possibleexplanation for the observed resultsis heterogeplatform [Browell, 1989].It generates wavelengths at 1.064 neouschemistryinvolvingthesulfuricacidvolcanicaerosols. in /•mand600nmthatareused primarily foraerosol measure- Normally,chlorineis boundby nitrogenoxidecompounds mentsand287 and 300 nm for tropospheric ozone,or 301 the form of CIONO2. However,in thepresence of sulfuric and311nm for stratospheric ozone. It wasdeployed in the acid aerosols,the chemistrycan be perturbed,with, for NASADC-8 overthe PacificOceanfromtheSeptember 16 example, the odd nitrogen speciesbeing more readily to October21, 1991, to participatein the NASA Global convertedfrom NOx (NO + NO2)into nitricacid(HNO3)on Tropospheric Experiment's PacificExploratory Mission-- the surfaceof the aerosols. SinceNOx helpsbind chlorine, West(PEM-West). Sincethis was a tropospheric mission, its reduction can shift the chlorine balance from HC1 in favor theUV wavelengthswere tuned to 287 and 300 nm, which of C10, allowingC10 to destroymoreozone[Hofmannand limited the ozone measurements to below -18 km. When Solomon,1989; Michelangeliet al., 1989; Brasseuret al., theaircraftaltitudewas greaterthan 10 km, the 300-, 600-, 1990; Rodriguezet al., 1991; Prather, 1992]. Indeed, and1064-nmlidar signalswere ableto provideusefuldataon Mt. Pinatuboaerosolsat altitudesup to -30 km. The data
forthe600-nmchannelwere usedfor the presentanalysis. Conversions from aerosolbackscatter to surfacearealoading weremadeusingthe findingsof J•igerand Hofmann[ 1991] for E1 Chichon volcanic aerosols at about the same time after
theeruption. A typicalaerosolprofile measured at 4ø-6øN, 12øEon October!3, 1991, duringthe PEM-Westmissionis shown in Figure 4. The peakaerosolsurfacearealoadingis foundnear24-25 kin, with significantaerosolloadingfrom 18-28kin. The aerosoldistributiongenerallymatches the profileof the ozonedecreasefor bothsites. Discussion
To establishthat the aerosolprofilesmeasuredover the northern PacificOceanin Septemberand Octoberrelateto theozoneprofilesmeasured overthesouthern AtlanticOcean
fromSeptember to December,we notethecontinuity of the aerosol layer at 22-28 km that hasbeenobserved by the SAGE II satelliteobservations[McCormickand Veiga, 1992]. A strongcorrelationexistsbetweenthe aerosol profilesin Octoberand the ozonedecrease profilesin the AEROSOL SURFACE AREA LOADING
(40 - 6ON,October 13, 1991) 30
.
.
.
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.
,
.
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increasedlevelsof HNO3 were foundnear volcanicaerosols from E1 Chichon [Arnold et al., 1990]. While no such similar measurements have yet been publishedfor Mt. Pinatubovolcanic aerosolsin the tropics, Johnstonet al. [1992] did showa decreaseof the integratedcolumnamount of NO2 over Lauder, New Zealand (45øS, 170øE). These findingswould be consistentwith the premisethat enhanced
catalyticdestruction of ozoneis takingplaceas a resultof more stratosphericaerosols. However, none of the model
calculations citedsuggests that therehasbeenor wouldbe largelayer ozonedecreases in the tropics. In additionto the heterogeneous chemistryreactions,the aerosolsare responsiblefor changingthe solar radiation fieldsin waysthatalsocontributeto reduceozone[Adriani et al., 1987; Michelangeliet al., 1989]. The aerosolsreduce the flux at wavelengths below 300 nm insidethe layer, thus reducingthe photolysisrate of 02. They alsoincreasethe flux at wavelengthsgreater than 300 nm inside the layer, which increasesboth the photolysisrate and temperatureof 03. For an aerosollayer with an opticaldensity(O.D.) of 0.25, Michelangeliet al. calculateup to a 7% reductionof O3 near the peak of an aerosollayer, and a 3 % increase above the layer. Above the aerosollayer, there is an increasein flux causedby reflectionof solarradiation,which increasesthe photolysisrate of 02. For Mt. Pinatubo,the maximum O.D. measuredwas 0.5 [Stowe et al. 1992]. Thus, thesemechanisms couldaccountfor a largefractionof the observedtropicalchangesin ozone. Finally, there is the possibility that dynamical effects causedby heatingin the aerosollayer [LabitzkeandMcCor-
mick, 1992] couldcausesomeof the observedchangesin 22
ozone densities.
Summaryand Conclusions 1½
Two independent datasets,oneof ozonefrom ozonesonde measurements,and one of aerosolsfrom an airborne lidar
systemsuggestthat significantozonedecreases may have occurredas a resultof the injectionof debrisby the Mt. Pinatubo volcano in June 1991.
The amount of this reduc-
Totol Surfoce Arco (microns2/rn 3 x 107) tion maximizesat 24-25 km, near the peak of the aerosol Figure 4. A profileof theaerosol surface areameasured distribution,althougha deficit is observedthroughoutthe between19 and 28 km. The highest using theUV-D!ALsystem at600nmtaken at4ø-6øN, 125øE lower stratosphere onOctober13, 1991. Above28 kin, the dataareprobably dueto noise,ratherthanaerosols.
differences observed before and after Mt. Pinatubo at these
altitudesis 18% to 20%. There are not enoughdata on
1112
Grant et al.: Observations of ReducedOzoneConcentrations
P. V., R. L. McKenzie,J. G. KeysandW. A. profilesof otherchemicalspecies in thetropicalstratosphere Johnston, Matthews. Observations of depleted stratospheric NO2 to determinehow muchof the observedchanges to attribute followingthe Pinatuboeruption,Geophys. Res.Lett.,•, to theheterogeneous photochemistry, radiationredistribution, 211-213, 1992. and dynamicalmodels. Acknowledgments. The authors thank N.M. Mayo, LockheedResearchCorporation,andW.J. McCabeandB.L.
Kirchhoff, V. W. J. H., R. A. Barnes,and A. L. Torres. Ozone climatologyat Natal, Brazil, from in situ ozone-
sondedata,J. Geophys.Res., 96, 10,899-10,909,1991. Komhyr,W. D. Electrochemical concentration cellsforgas Meadowsof NASA Langley,for helpingto operatetheUVanalysis,Ann. Geophys.,25, 203-210, 1969. DIAL systemduringthe PEM-West mission,andS. Mayor, temperaSAIC, for helpingwith the computercalculations.This Labitzke,K., andM.P. McCormick,Stratospheric research wasfundedin partbyNASA'sGlobalTropospheric ture increasesdue to Pinatuboaerosols,Geophys.Res. Lett., 19, 207-210, 1992. ChemistryProgram. McCormick, M.P.,
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(ReceivedMarch 31, 1991; acceptedMay 12, 1991)
