Sep 20, 1997 - to play a critical role in activating chlorine there [Solomon et al., 1986; Molina et ... linkages between intrusions of stratospheric air over Kansas.
JOURNALOF GEOPHYSICAL RESEARCH,VOL. 102,NO. D17,PAGES21,411-21,429, SEPTEMBER 20,1997
Heterogeneouschlorine chemistry in the tropopauseregion S. Solomon,1,2S. Borrmann,3 R. R. Garcia,2 R. Portmann,1 L.Thomason,4 L. R. Poole,4 D. Winker,4 and M.P. McCormicks Abstract. Satelliteobservations of cloudopticaldepthsandoccurrence frequencies areusedas inputto a two-dimensional numerical modelof thechemistry anddynamics of theatmosphere to studytheeffectsof heterogeneous reactions on cloudsurfacesuponchemicalcomposition and ozonedepletionin thetropopause region.Efficientreactions of C1ONO2with HC1andH20, and of HOC1with HC1,arelikely to takeplaceon the surfacesof cirrusclouds[Borrmannet al., 1996]andperturbchlorinechemistry, muchastheydo onpolarstratospheric cloudspresentat higheraltitudesandcoldertemperatures. Becauseof theverylow predictedbackground abundances of C10 nearthetropopause, suchreactions couldenhance thelocalC10 mixing ratiosby up to 30-foldat midlatitudes.Substantial perturbations arealsopredictedfor related chemicalspecies(e.g.,HC1,HOC1,C1ONO2,NO2, HO2) in themidlatitudeandtropical tropopause regionsdueto theseheterogeneous reactions.If cirruscloudsoccurwith sufficient frequencyandspatialextent,theycouldinfluencenot onlythe chemicalcomposition but alsothe ozonedepletionin the regionnearthe tropopause.Becauseof variationsin observedcloud occurrence frequencyandin photochemical anddynamicaltimescales, thepresenceof cirrus cloudslikely hasits largesteffecton ozonenearthemidlatitudetropopause of thenorthern hemispherein summer. 1.
Introduction
The discovery and explanation of the Antarctic ozone "hole" led to fundamental changes in understanding of stratospheric chemistry. In particular, heterogeneous reactionsof hydrochloricacid and chlorine nitrate on ice and nitric acid/ice polar stratosphericcloud surfaces were shown to play a critical role in activatingchlorine there [Solomon et al., 1986; Molina et al., 1987; Tolbert et al., 1987], markedly altering the composition fi•omthat of a purely gas-phase system and greatly enhancing chlorine-catalyzed ozone depletion. Cloud formation depends upon temperature and the availability of condensablevapors. In spite of the extreme drynessof stratosphericair, cloudsoccur fi•omabout 15 to 26 km above polar regions (particularly Antarctica) due to very low temperaturesthere [e.g., Poole and Pitts, 1994; World Meteorological Organization (WMO) /United Nations Environment Programme (UNEP), 1991]. While the tropopause region at subpolar latitudes is considerably warmerthan the Antarctic stratosphere,it can be quite wet, allowing formation of cirrus clouds. Murphy et al. [1990] noted the frequent observation of ice saturation at or just above the tropopause over Norway (59øN) and emphasized that the ice saturation in this region was associatednot with
exceptionally cold tropopauses but with increased water vaporcontent. Borrmannet al. [1996, 1997] recentlypointed out that cirrus clouds near the tropopausemight lead to
chemistrysimilar to that of type II (water ice) polar stratosphericclouds. They suggestedthat these clouds could affectthe abundancesof key radicals such as C10 near the midlatitude tropopause and thereby perturb the local chemistry,perhaps leading to ozone depletion [see also Reichardtet al., 1996]. In the presentpaperwe examinethese
chemical perturbations in more detail, using global observationsof cirrus cloudsfi•omsatellite studiestogether with a numericalmodelof stratospheric ozonechemistryand transport.
Depletionof ozonecloseto the tropopauseplays only a limited role in influencingthe total ozone columntrends,due to the low abundancesof ozone found there comparedto higher altitudes, but is important to the role of ozone depletion in radiative forcingof the Earth's climate system [e.g,Lacis et al., 1990; Ramaswamyet al., 1992]. Further,a key test of scientific understandingof ozone depletion processesis the accurate simulation of both the total column
trend and the shapeof the observeddepletion profile. The latterhasprovenparticularlyproblematic for presentmodelsat midlatitudes (see WMO/UNEP [1994]andreferences therein). Previous interest in cirrus clouds in the vicinity of the tropopause has been largely prompted by connections to IAeronomy Laboratory, NOAA, Boulder, Colorado. 2National Center forAtmospheric Research, Boulder, Colorado. radiative forcing of the climate system, meteorological dynamics,and atmospheric optics [e.g.,Whitneyet al., 1966; 3Institut ftirPhysik derAtmosph•ire, Johannes Gutenberg Universit•it,Mainz, Germany. Sassenet al., 1989;Sassen,1991;•lensenet al., 1994]. This 4Aerosol Research Branch, NASALangley Research Center,literaturehasshownthat cirruscloudsarecertainlyobserved Hampton,Virginia. tropopause.Tropical 5Department ofPhysics, Hampton University, Hampton, Virginia. at andslightlyabovethe meteorological cirrusareparticularlywidespreadand often optically thick, Copyright 1997bytheAmerican Geophysical Union.
but there is abundant evidence for cirrus cloud occurrencenear
the midlatitudetropopauseas well, particularlysubvisible Papernumber97JD01525• 0148-0227/97/97JD-01525509.00
cirrus. Sassen [1991]used polarization lidar data taken in
Utah andWisconsinto demonstrate that multiple-ringed solar 21,411
21,412
SOLOMON ET AL.: HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION
haloe are relatedto optically thin cirrusice cloudsobservedat and up to a kilometer above the local tropopause. Study of suchphenomenaappearsto have a long history, as Sassen [1994] suggestedthat concentric circle symbols found in Native American art may have depicted this phenomenonas much as several thousand years ago. Lidar observationsby Ansmann et al. [1993] quantified the developmentof cirrus decks near the tropopause over northern Europe and demonstratedtheir persistenceover periods of hours. The lidar studies of Sassen [1991],Ansmann et al. [1993], and Winker and Vaughan [1994] show that such clouds are generallyobservedin layersof thicknessesof the orderof 1 km or
more.
Sassen et al. [1995] provided observational evidence for linkages between intrusions of stratosphericair over Kansas and formation and maintenanceof cirrus in the tropopause region. They reportedconcurrentmeasurements not only of particles but also of ozone and concluded that the cirrus cloudswere in direct contactboth with stratospheric air at the tropopauseand with layers of troposphericair of very recent stratospheric origin. They also found evidence for a connection of Pinatubo aerosol and vapors of stratospheric origin with enhancedcirrusformationnear the tropopause[see also Jensen and Toon, 1992; Wylie et al., 1994]. Hence there is evidencethat cirrus clouds can occur not only in purely tropospheric air but also in air with chemical composition reflecting a stratospheric contribution, particularly at the interfacesbetween stratosphericand troposphericair. Satellite measurements provide a meansof quantifying the global distribution of cirrus clouds near the tropopause,and are a cornerstoneof the analysis presented in this paper. Stowe et al. [1989] and Wylie et al. [1994] showedevidencefor widespreadcirrus cloud occurrencein the vicinity of the tropopauseusing both infraredand ultraviolet nadir-viewing measurements. Thosedataprovidesomeinformationregarding cloud altitudes and horizontal variability. Because of its limb-scanning geometry, high vertical resolution, and multiple wavelength measurements,observations from the StratosphericAerosol and Gas Experiment(SAGE) II permit detailed study of cirrus cloud distributions at visible wavelengths. All of these satellite studies have revealed frequentcirrus clouds near the tropopausein the tropics. High-altitude cirrus clouds were also observed, albeit less often,in the extratropics. We discussthe observedvariability of cloudsand chemistryin detailbelow(seesections2 and3). The goal of this paper is to further examinethe chemical perturbationspointed out by Borrmann et al. [1996, 1997], which could be causedby cirrus clouds in the vicinity of the tropopause,and to provide a first estimateof their possible impact on ozonetrendsin this region. We make use of a twodimensionalchemical/dynamical/radiative model [Garcia and
cirrus cloud surfaces,the background composition (e.g., abundancesof HC1, ozone, and other gases) present where cirrus clouds occur,and the dependenceof chemicalchanges uponcloudsurfaceareaandfrequencyof cloudoccurrence. It will be shown that cirrus clouds very likely induce transient perturbations in compositionnear the tropopause in many regions of the globe, and they appearto contribute to ozone depletionthroughchlorinechemistryparticularly in northern midlatitudes in the summer season. Section 4 presents calculationsof the expectedinfluence of cirrus clouds upon ozonein the vicinity of the tropopausebasedon the SAGE II climatology and model calculations, and summarizes sensitivity studies that elucidate the magnitude of uncertainties in the estimated ozone loss. Finally, the key conclusions are summarized
2. Global
in section 5.
Observations
Distributions
of Cirrus
Cloud
and Occurrence Frequencies
Wang et al. [1996] presenteda detailed analysis of the climatology of cloud inferred from SAGE II measurements during the period from 1985 to 1990. Wang et al. [1996] comparetheir results with those of several other cloud climatologies,including those of Warren et al. [1986, 1988], Stoweet al. [1989],and Wylie et al. [1994]. In addition, Liao et al. [1995] compareSAGE II high-level cloud results with thoseof the InternationalSatellite Cloud Climatology Project (ISCCP). There is broad agreementbetween these cloud
databases, all of whichrevealthefollowingfeatures: frequent highcloudsnearthe tropicaltropopause, maxima in optically thick cloud frequenciesover Indonesia, Africa, and South
Americanlandmasses,and lessfrequentbut measurable high cloudsat midlatitudes in the regionof the tropopause. Kent et al. [1993] showed that the ratio of aerosol
extinctionat 0.525 gm to that at 1.02 gm measuredby SAGE (hereinafter referredto as0.5 to 1.0 [anratio) could be usedto discriminatebetweenstratospheric aerosolsand cirrusclouds and supportedtheir cloud identificationusing airbornelidar data. Thelargesizesand optical depthsof the particles[see Kentet al., 1993]confirmthattheymustbe composed primarily of water, although trace amountsof HNO3 are likely to be taken up on the surfaceas well [e.g., Diehl et al., 1995; Hanson, 1992] perhaps affecting surface chemistry as discussedbelow. Depolarizationdata [e.g., Sassen,1991; Ansmann et al., 1993] suggest that the clouds are predominantlywater ice, althoughthere is evidencethat some clouds are composedof liquid supercooleddroplets [e.g., Sassen, 1992]. Note that aircraft contrails would also be
detected by SAGEII if theydisplaysignificantopticaldepths
and extinctionover the field of view (seebelow). To study the impact of cirrus on the averagestate of the atmosphere for this work, a climatologyof cirrus properties Solomon, 1983, 1994; Garcia et al., 1992; Solomon et al., has been derived using SAGE II measurements from a more 1996] to evaluate chemicalprocessesrelating to cirrus cloud limitedtime periodthanthat usedby Wanget al. [1996]. Both occurrence for typical air parcels. The limitations and optically thin and optically thick clouds are included in the assumptionsinherentin using sucha model for the problemof present analysis; the former are more prevalent near the cirrus cloud chemistry will be discussed below. Section 2 tropopause and are likely to be more important for describesSAGE II observationsof cirrus cloud distributions, photochemistrythere as discussedbelow. The detection of frequency,and optical depth. Section 3 describesthe cirrus presenceof cloud in SAGE II observationsis predicatedon cloud and other related chemistry near the tropopause. particle size differencesbetweencloud and aerosolas reflected Section3 also probesthe sensitivity of chemicalcomposition in the measuredratioof 0.5 to 1 gm aerosolextinction [Kent et in the tropopause region to uncertainties in chemical and al., 1993]. Only data from 1988 and 1989 were used here, cloud parameters. Among the key questions that will be sincethis period hasthe lowest volcanicaerosolloading in consideredare the chemicaltransformations that take place on the SAGE II record and observations of cloud can be most
SOLOMON ET AL.: HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION
21,413
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,
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-
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90
Latitude (deg) % cloud
o
20
occurrence
40
60
80
Plate 1. Observations of the zonally averagedoccurrence frequencyof (a) subvisualand (b) opaquecirrus clouds basedupon the StratosphericAerosol and Gas Experiment (SAGE II) extinction data, from Wang et al. [1996]. The data represent6-year averagesof the periodfrom 1985to 1990. The dottedline denotesthe averagedlocationof the tropopause(see text).
clearly distinguishedfi'omthose of the backgroundaerosol latitude and 1-km altitude bins between 80øN and 80øS and 10.5 and 20.5 km. Note that the 10th and 90th percentile surfaceareasare typically about3 timessmallerand largerthan the medianvalues,respectively.As will be shown below, the effects of clouds near the tropopause upon chemical compositionare generally less sensitive to the derived cloud surfacearea than to their frequencyof occurrence. Cloud occurrencefrequencyis a key factordeterminingthe very sensitive to composition or to the details of the size chemicalperturbationsinducedby cirrusclouds. It is defined distribution. On the other hand, the conversion factor can be here as the ratio of SAGE II occultations displaying clouds modifiedby the shapeof the particles,but no clear meansof comparedwith all occultations(as in the work of Wang et al., considering this effect is available. In addition to cloud [1996]),seasonallyand spatiallyaveragedas above for surface eventsidentified by the methodof Kent et al. [1993], clouds area. Extensive spatial coverageof the globe is obtained in are assumed to occur at and below altitudes at which the SAGE II data through such averagingon a seasonalbasis, as measurementsare terminated by the presence of dense cloud. shown by Wang et al. [1996]. Wang et al. estimatethat the The nominal (cloud free) altitudes for the termination of uncertaintyin the observedseasonalfrequenciesis less than aerosolextinctionprofilesare 5.5 and 1.5 km for the 0.525 and 10% within the lower stratosphere.However, SAGE II makes 1.02 gm extinction measurements, respectively. A seasonal measurementsonly at sunrise and sunset, so these climatology of cirrus surfacearea density is obtained by observations cover restricted local times. computingthe zonally averagedmedianvalue observedin 10ø Wanget al. [1996]notethe occurrence of subvisibleclouds layer then.
The cloud surfacearea(total particle surfacearea per unit volume of air) is deduced from 1.02 pm extinction measurements, using the empirical relationship derived for sulfate-aerosol by Thomason et al. [1997]. For largeparticles (such as cirrus ice crystals) the factor for converting from extinction to surfacearea density is nearly constant and not
21,414
SOLOMON ET AL.: HETEROGENEOUSCHEMISTRY IN THE TROPOPAUSEREGION
5
39.3
38.8
38.4
37.9
37.4
36.9
36.4
35.9
35.4
34.9
34.4
Latitude (dcg)
Figure 1. Observationsof a midlatitude cirrus anvil in the tropopauseregion. A featureresultingfrom convectivepenetrationof the tropopause is seennear the middleof the cloud mass. The tropopausewas locatedat 12.7 km in this region accordingto the National Center for EnvironmentalPrediction(NCEP) analysis,whichmay not capturelocal variability. Data from the the Lidar-In-space TechnologyExperiment (LITE) 532 nm channel,acquiredduringSeptember1994 [seeWinkeret al., 1996].
near and above the midlatitude climatological tropopause,as platform,the lidar gives a highly detailed view of cloud shown,for example,in Plate 1. Becausethe local tropopause structure. At the tropopause,LITE is sensitiveto clouds with can vary by severalkilometersabout the meanzonal value, it extinction as low as about 0.002 /km. Figure 1 shows cannot be concluded that all of these clouds indeed occurred observationsof cirrus clouds readily detectedby LITE fi'om within the stratosphere.While comparisonto meteorological stronglidar returns near the tropopauseat midlatitudes. The analysescan provide some insight into the location of clouds primary cloud shown in this figure extendsfi'omat least 8 km observedby SAGE comparedto that of the tropopause,such up to roughlythe tropopause(locatedat 12.7 km basedupon analysesdo not generally resolve small-scaledisturbances. the National Center for EnvironmentalPrediction (NCEP) analysis),andthereappearsto be a penetrating Many of the midlatitude clouds that affect the tropopause meteorological region are probably associated with tropospheric turret up to 15 km. Note that the main cloud mass extends disturbances, for example through stratosphere/troposphere over several degrees of latitude, from about 35øN to 37øN. folding events [see Sassen et al., 1995] and through SAGE II sampling did not cover this particular cloud, so a convection (especially in summerand in the tropics). We direct comparisonbetweenthis LITE data and SAGE II is not discussthe distribution of the clouds comparedwith that of possible. While this cloud event is only a single example,it the tropopausein moredetail below and present sensitivity does illustrate that large cloud structuresthat should be well studies to elucidate the dependence of our results upon resolvedby the SAGE II samplingcan be encounterednear the midlatitudetropopause. As shownbelow, suchclouds can be frequencyand surfacearea. The vertical resolution of SAGE II observations is I km, importantfor chemistryevenif they occuronly a few percentof and
the
horizontal
resolution
is
about
200
km.
the time.
Figure 2 presents the latitudinal distribution of the frequencyof occurrenceof clouds at 13.5 km as measuredby SAGE II for four seasons. In the tropics this level is in the troposphere,but it is above the average tropopause much of vertical resolution, the horizontal scale of the clouds is not the time at middle and high latitudes. Few clouds are well known. SAGE II measurements will overestimate the observed above 60ø latitude at this altitude. Figure 2 spatialextent of cloudswhile simultaneouslyunderestimating illustrates that cirrus clouds are present near the midlatitude tropopausemost often in summer,and they are present more their surfaceareadensityif they are smallerthan200 km. Insight into the spatial extent of cirrus clouds near the often in the north than in conjugate seasonsin the southern Figure 3 further illustrates these points by tropopauseis provided by observationsfi'omthe recent space hemisphere. shuttle flight of the Lidar-In-space Technology Experiment showing the vertical profiles of measuredcloud occurrence (LITE) for 10 days in September1994 [Winker et al., 1996]. frequency for 45øS and 45øN in the summer and winter LITE is a backscatterlidar operating at the wavelengths of seasons.Note that clouds are occasionallyobservedas high 1064, 532, and 355 nm. The 10 Hz laser pulse rate gives a as 14-16 km in summer at 45øN, consistent with Plate 1. horizontal resolution of 740 m, and the return signals are Wang et al. [1996] also report higher frequenciesof highsampledat a verticalresolutionof 15 m. While the data are of altitude cloud in the afternoonthan in the morning [see also, limited duration due to the use of the space shuttle as a Minnis and Harrison, 1984]. The observed variations in
Inhomogeneitiesof cloud structurewithin the field of view are one sourceof uncertainty. While the vertical depth of cirrus clouds typically observed by lidar in the vicinity of the tropopause is comparableto or greater than the SAGE II
SOLOMON ET AL.' HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION Estimated Surface Areas at 45øN and 45øS
SAGE II Observations of Cirrus Clouds 1.0
21,415
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16
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-40
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Figure 2. Observations of the zonally averagedcirruscloud SurfoceAre(] (cm2-'/crn 5) occurrencefrequency(opaqueand subvisible)from SAGE II observationsduring 1988-1989,for four seasonsat 13.5 km Figure 4. Estimated zonalIy and seasonallyaveragedmedian geometricaltitude. Note the highfrequencies obtainedin the cirrus cloud surface areas at 45øN and 45øS for the winter and tropics and the seasonalmigration of the region of peak summer seasons (scc text), based on SAGE II cloud frequency.
observations.
diurnal, latitudinal, and seasonal cloud frequency suggest a
connection to convective processes, although in situ nucleation through slower uplift is also a possible cloud formationmechanism(see the detailed theoretical analysis by Jensen et al. [1996]). In the region near 11-12 km, cirrus cloudsare observedat 45øN by SAGE II from 10 to 50% of the
altitude clouds reflect lifted local tropopauses,or how often they occur above the local tropopause. Figure 4 shows the vertical profiles of cloud surface areas inferred fromthe measuredcloud extinction for 45øN during winter and summer. Note that the median inferred averaged
time, dependingupon seasonand altitude. At altitudes as high as 13-15 km they are observedbetweenabout1 and 10%
surface areas rangefromabout5xl 0-8 cm2/cm 3 to about5xl 0-7 cm2/cm 3 nearthetropopause. Section 3 examines the chemical
of the time at 45øN.
I(•
[ -
It is not clear how often these high-
1' [ I I[[][
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15-
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[ I [ at 45ø
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It is now well known that ice and liquid surfacesfacilitate reactionsof relatively long-lived chlorine compoundssuch as HC1, HOC1, and C1ONO2 that are not possible in the gas phase. These compounds serve as temporary lower
stratasphericreservoirs for chlorine liberated from chlorofluorocarbons, and the chemical balance between such
-
speciesand the C10 radical plays a major role in determining ozone depletionrates. Laboratorystudieshave demonstrated that the following reactions take place on water ice surfaces with an efficiency of the order of 0.2 or more [Tolbert et al.,
-
1987; Molina et al., 1987; Hanson and Ravishankara, 1991, 1992; ,,tbbatt and Molina, 1992; ,,tbbatt et aL, 1992; see Jet Propulsion Laboratory (JPL), 1994, for a review]:
HC1 + C1ONO2 --> C12+ HNO 3 .01
Occurrence
(1)
.I
Frequenc•
Figure 3. Observations of zonally and seasonally averaged cirrus cloud occurrencefrequency from SAGE II observations at 45øN and 45øS for the respective winter and summer seasons.
Considerations
_
i,.001
Chemical
-
-
_
[
]
•
- • -.
[ I I [ Ill[
processesthat may take place near the tropopause for such surfaceareasand cloud frequencies.
HC1 + HOC1 --> C12+ H20
(2)
H20 + C1ONO2 -->HOC1+ HNO3
(3)
HC1 + HOBr -->H20 + BrC1
(4)
21,416
SOLOMONET AL.' HETEROGENEOUS CHEMISTRYIN THE TROPOPAUSE REGION
All of thesereactionsconvert chlorine fromrelatively longlived speciesto shorter-livedones,and reactions(1) and (3) also concurrentlyconvert nitrogen oxidesinto the longerlived formof HNO3; both factorsalter chemicalcomposition
5O
40
and can impact ozone [Solomon et al., 1986]. In addition, changesin NO x and HNO3 abundancescan affect reactive
hydrogenradical concentrations(OH, HO2), and reaction(3) alsorepresents a directsourceof reactivehydrogen. We next consider how the availability of ozone and reactive chlorine near the tropopause may affectthe chemical ratesand impactsof reactions(1) through(4). It is known that the transition fromstratosphericto troposphericabundances
of ozonelies belowthe thermaltropopause at middleand high latitudes,mainly due to downwardtransportof stratospheric air. For example,a recent review by Bethan et al. [1996] documents the spatial distribution of ozone and its relationship to temperature over Europe, based upon ozonesonde observations. They show that the thermal
tropopausegenerally occurs about 1 km above the region where ozoneabundances reflect a predominantlytropospheric characterat midlatitudes. Tuck et al. [1997] reach similar conclusionsusing observationsof long-lived tracerssuch as chlorofluorocarbons.Thus it may be mostappropriateto think of the extratropical tropopause as an interface region containingair displayingboth stratosphericand tropospheric
,•30
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Figure 5. Model-calculated ozone and total inorganic chlorine(Cly)abundances in summer at 49øN. Theobserved summeraveragedozonemixing ratio profile from Edmonton,
Canada(52øN),ozonesondes in 1990is shownfor comparison.
character.
When formationof high-altitude cirrus is directly coupled to local deep convection,then the inorganic chlorine content of the air within the cloud updraftswould be greatly reduced through condensation of HC1, implying heterogeneous chemistryprimarily at the interfacebetween stratosphericair and the cloud rather than throughout the cloud. However, subvisible
cirrus
cloud
formation
near
the
midlatitude
tropopauseis often relatednot only to local deep updraftsbut also to other meteorologicalconditionssuchas outflow at the cloud top that mixesthe updraftwith ambient air. Further, somecloudsare likely to be relatedto slow, larger-scaleuplift [seeJensenet al., 1996], or to aircraftcontrails [Jensenand Toon, 1997]. Under such conditions the chlorine content within the cloud would reflect the compositionat altitude rather than that in updrafts. Theseconsiderationssuggestthat cirrus clouds occurring above, at, or slightly below the midlatitude thermal tropopauseare likely to encounterair containingsubstantial amountsof ozone (and other chemicalswith stratospheric
stratosphereand uppertroposphere.As in the work of Bethan et al. [1996], the thermaltropopausein the model occursin a
regioncontainingelevatedozoneindicativeof a stratospheric contribution. Hence the similarity between observed and
modeled ozone provides partialsupport forthemodeled Cly profile, although other processes(most notably removal
through precipitation) couldalsoaffect theClyabundance of the lowermoststratosphere. Murphy et al. [1990] presentevidencefor heterogeneous removalof nitric acid near the tropopause,but nitric acid couldbe moreeffectivelyremovedthan hydrochloricacid due to formationof nitric acid hydrates(seebelow). Further,the
observationsof Keim et al. [1996] imply high HNO3 abundances nearthe midlatitudetropopause,perhapsbecause of reevaporation,which would similarly redistribute and perhaps even increase local HC1 abundances in some
locations. There areafewmeasurements of HC1andClyin the lowermost midlatitude stratosphere and
uppermost
tropospherereported in the literature, including those of sources suchasCly). If thisisthecase, thentheexactlocation Lazruset al. [1977],Gandrudand Lazrus [1981],and Farmer of cirrus clouds relative to the tropopause, while et al. [1976].These authors suggest abundances of Clyand quantitativelyimportant, is not qualitatively critical to their HC1nearthe midlatitudetropopauseof the orderof 100 pptv chemicalrole. The above reactions will be significant for based upon filter and infrared absorption measurements compositionif even rathersmall amountsof inorganicchlorine (respectively),comparableto thoseshownin Figure5. (orderof 100 parts per trillion by volume (pptv)) are present, Reactions (1), (2), and (4) are complex processesthat as shown below. involve uptake of HC1 onto particles and subsequent Figure5 shows theverticaldistributions ofozoneandCly interactionswith gas-phasespecies. Hence the abundanceof calculated by our two-dimensional model for the summer HC1 present not only in the gas phase but also in and on seasonat 49øN without including cirrus cloud chemistry. cirruscloud particlescan affectthe rates of these processes. Calculatedozonewith cirruscloudchemistrydiffersfrom these Here we assumethat HC1 uptake and availability is not the valuesby up to 15% at somealtitudes. The calculatedozone limitingfactorin controlling the ratesof reactions(1), (2), and profile is comparedto the averageof summermeasurements at (4) on cirrus ice particles (at least for temperaturesbelow Edmonton (52øN) in 1990 in Figure 5; similar values are about220 K andHC1 mixing ratios above 50 pptv, seeDie hi observed at other ozonesonde stations near this latitude in et al., [1995]). This assumptionis based upon laboratory that season. The figure suggests that the ozone profile studiesthat suggestsignificantsurface coverageand rapid calculatedby the modelfor this latitude is in good agreement reaction for HCI concentrations smallerthanthoseimpliedby (within about 20%) with observationsthroughoutthe lower Figure 5 near the tropopause(D. Hanson, personal
SOLOMON ET AL.' HETEROGENEOUS
CHEMISTRY
communication,1997). Laboratory measurements suggest that HC1 forms a surfacelayer on ice but that diffusioninto the bulk is limited [Hanson and Ravishankara, 1992; Abbatt et al., 1992]. We assumethat these reactions proceedwhenever the
HC1mixingratiois greaterthan 50 pptv (about2 x 108 molecules/cm 3 at 100mbar,allowingfor at leasta monalayer of surface coverage of HC1 for surface areaslessthan 1 x 10'7 cm2/cm 3, seeFigure4) andturn themofffor smallervalues. This cutoff is likely to be conservative, since partial surface coveragewill be associatedwith large surfaceareas,and the two factorswill compensatefor one another to some extent. Further, the efficiency may not decrease linearly as surface coverage decreases (D. Hanson, personal communication, 1997). The rate coefficientof a heterogeneousreaction on particles whose radius is smallerthan the meanfree path of impinging gas moleculesis given by
khet= 7S v/4
(5)
where 7' is the efficiencyor gammavalue, S is the particle
I000000
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I0 -6
Surface Area (cm2-/cm3) Figure 6. Timescalefor reactionof C1ONO2 on HCl-cooted ice surfaces(reaction (1)) as a function of surface area. The
timescalefor photolysisof C1ONO2is shownfor comparison. surfacearea, and v is the molecular speed [e.g., Turco et al., The range of averagedcirrus cloud surfaceareasinferred from 1989]. This equation holds for particles with radii smaller than about 1 grnnear the tropopause,but for largerparticles, SAGE II is also indicated, correspondingto timescalesfor chemical reaction of a few hours or less. the efficiencyis reducedby a factorrelatedto the ratio of the radiusto the molecularmeanfree path [see Turco et al., 1989]. For 10 grn particles near the tropopause the efficiency of reaction is reducedby about a factorof 10, while for 20 grn
particles,it is reducedby about a factorof 20. While a key focus of many optically thick cirrus cloud studies is the observationof somevery large particles with radii as large as 1000 grn [e.g., Heymsfield and Platt, 1984; Dowling and Radke, 1990], it is clear that smaller particles are also present, particularly in subvisible cirrus. For example,Bartmann et al. [1996] and Stroem et al. [1996] present observationsof cirrus clouds near the tropopause in which substantial numbersof small particles are present (radii less than a few
microns),togetherwith somelargerparticles. A key issuefor our study is the fraction of total surface area obtained in particlessmallerthan about 1 grn in subvisible cirrus. We presentbelow a sensitivitystudyin which the estimatedtotal surfaceareafrom SAGE II is reducedby a factor of 3, providing an indication of how the results would be affected if, for
example,two thirdsof the total surfaceareawere containedin very largeparticles. It will be shown that the results are not very sensitive to surface area, implying that the size distribution is not critical so long as all of the particles are not large. Apart from this sensitivity test, all the results presentedin this paperare calculatedfor particleswith radius smallerthan about 1 gm. Figure 6 showsthe calculatedrate of heterogeneousuptake of C1ONO2 in reaction(1) versuswater ice surfacearea. For
surface areasof the orderof 1 x 10'7 cm2/cm 3 (as shownfor midlatitudesin Figure 4) the timescalefor uptake of C1ONO2 throughreactions(1) or (3) is only a few hours, implying that C1ONO2 will be rapidly convertedto the reactive gasesC12 and
HOC1
on
cirrus
ice
cloud
surfaces.
In
the
sunlit
atmospherethe C12 formed will photolyze on a timescale of seconds. Rapid recycling of the active chlorine thus released will occur as long as NO 2 or Ha 2 is available to form C1ONO2 or HOC1, amplifying the active chlorine production in cirrus cloud eventsthat can last for many hours. The low nitric acid abundancesnear the tropopauseand the
relatively large mean sizes and extinction of the particles deduced from 0.5 to 1.0 grn ratios implies that the SAGE II observationsreflectprimarily water (but not necessarilyice) particles. There is not enoughHNO3 present at the relatively warm temperatures of the midlatitude tropopause to allow conversion of cirrus ice cloud particles to nitric acid trihydrate or other crystalline acid hydrates [Hanson and Mauersberger, 1988]. The particles are hence likely to be mainly but perhaps not exclusively composedof water ice. Somenitric acid hydrate may also be present, and uptake of trace amountsof HNO 3 on water ice surfacesis likely [Tolbert et al., 1990]. Ice particles can nucleate on supercooled sulfuric acid aerosolsat temperaturesa few degreesbelow the frost point [Tabazadeh et al., 1997], suggesting that the presenceof liquid aerosols may be tied to cloud formation. Some observationsof depolarizationshow that cloud particles may be liquid sulfuric acid/water droplets at times [e.g., Sassen, 1992]. Locally high water content or cold temperatures could be responsible for observations of enhancedliquid aerosol extinction near the tropopause,just as occursat times in polar regions. Reactions (1) and (2) can take place as rapidly on liquid sulfuric acid/water droplets as they do on water ice, depending upon the size and water contentof the droplets [e.g., Hanson et al., 1994; Keim et al., 1996; Michelson et al., Strataspheric ozone loss enhancedby aerosol reactions at temperaturesabove 200K, submitted to Science, 1997]. Similarly, reactions (1) and (2) proceed rapidly on both water ice and water ice surfacescontaining traceamountsof HNO3 accordingto laboratory studies [dPL, 1994,and referencestherein],so the type of surface(pure water ice, dilute liquid solution, or water ice containing trace HNO3) is not critical to those processes. On the other hand, reaction (3)slows down significantly when trace amountsof HNO3 are present on water ice surfaces [Hanson and Ravishankara, 1992], which can be thoughtof as "spoiling" or
21,418
SOLOMON ET AL.' HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION -9
'"poisoning"of the surface. Reaction (3) also becomesslow relative to reaction (1) in dilute liquid sulfuric acid/water droplets. The chemicalroles of these competingreactions are further examined
SAGE
II
data
49 ø N,
,
135 mbar
•
q
-I0
We now presenta seriesof model calculationsto probe the possibleimpact of suchcloud eventson chemicalcomposition in the lowermost stratosphere. SAGE II observations reveal the frequency of occurrence of clouds and their optical extinction(which can be convertedto an estimatedsurfacearea as describedin section 2) on a seasonally averagedtimescale as a functionof geometricaltitude. We usethosedata as input to the two-dimensionalmodel, with the goal of characterizing the chemistry experienced by a typical air parcel on a seasonallyaveragedbasis. SAGE II makesmeasurementsas a function of geometricaltitude. The data are interpolated to each model grid point in latitude and geometric altitude (although the model is formulated in pressure coordinates, geometricaltitudes of eachgrid point are computedprior to interpolation). The tropopauseof the model is defined by climatology,so in an averagedsense,the vertical coordinates of the
,
below.
and those
of the
model
should
ith
•
cirrus
Nocirrus • ................ CIO
•
-13
IO
•
0
I
I
60
I
i
120
I
I
180
I
•
2_40
I
300
360
---'---' 10-9= I -'I [ I ' I ' I _
-IO
be
comparable. Chemical processes(1) through (4) above are considered using assumed efficiencies of 0.3 for all four processes for simplicity and recognizing the substantial uncertaintiesin these factors [JPL, 1994]. Someof the surface reactions (1)through (4)proceed even more efficiently on water-rich solid surfaces[Hanson and Ravishankara, 1991; Abbatt et al., 1992], but they could occur somewhat more slowly for large surfaceareas if there is insufficient HC1 for completesurfacecoverage,or if the cirrus particles are liquid. The completemodel chemistryfor gas-phaseand stratospheric aerosol backgroundconditionsis describedby Solomonet al. [1996] and Portmann et al. [1996] and referencestherein. Individual air parcels in particular dynamical situations will not be representedby such an approach, which at best simulatesthe zonally and seasonally representativestate of the atmosphere. Insofar as there may be regions where high cirrus clouds preferentially occur (e.g., regions of strong convection), air parcels traveling around the globe would experiencemore frequent events as they pass through those locationsas compared to others. The two-dimensional model presentedhere does not simulate such variability but rather representswhat would be obtained if the averaged cloud frequency measuredby SAGE II were representative of all longitudes and local times. The SAGE II data suggest that optically thick cirrus clouds tend to occur preferentially in certain regions, but the subvisible cirrus clouds of primary interest here tend to be much more evenly distributed over longitude [Wang et al., 1996]. The chemistry certainly depends upon how often the clouds are assumedto occur, whetherthey occurin the sunlit or dark atmosphere,and upon how long they are assumedto last during each such event. The available data do not fully constrain these parameters,so we illustrate the dependence of our results upon those uncertainties with sensitivity studies. We assumethat each cloudevent persistsfor 12 hours (0.5 days), beginning at local noon, and occurs at appropriate intervals to matchthe observed frequency from SAGE II for each season,latitude, and altitude (e.g., for a location with a cloud frequencyof 10%, a cloud is assumedto be presentfor 0.5 days every 5 days, while for a 2% frequency,clouds are assumedto be presentfor 0.5 days every 25 days).
i
-14
I0
0
,
I 60
•
I
•
12_0
I 180
•
HOCl I
240
•
-
I
300
36o
Day of Year Figure 7. Calculatedresponsesof HC1, C1ONO2,HOC1, and C10 for the model level just abovethe tropopauseat 49øN to cloud events based upon the SAGE II data throughoutthe year. Solid lines show results when cirrus clouds are included
in the calculations, while dashed lines depict results when cirruscloudsare not considered. Daily noon valuesare shown for each species. Note that the prescribedfrequencyof cloud occurrenceand mean cloud surfaceareasvary seasonallybased upon the SAGE II measurements.
Figure 7 showsmodel resultsfor key chlorinatedspeciesat 49øN for local noon for a pressureof 135 mbar (which is the level just above the model tropopausein summer)under 1990 conditions of chlorine and bromine loading (as in WMO/UNEP [1994]). Note that the figure depicts only the noonvaluesand not the diurnalcycle of eachgas. Small dayto-day variations in this and later figures reflect planetarywave-driven excursions in solar insolation, which are explicitly calculatedby the model. Figure 7 illustrates that cirrus cloud events,even if quite
rare,cancauseremarkabletransientchangesin C10 and other chlorine species near the midlatitude tropopause. Noon valuesof C10 are enhancedby factorsof 5-30 as comparedto the control case in which no cirrus cloud chemistry is included,reflectingthe efficiencywith which reactions(1)
SOLOMON ET AL.' HETEROGENEOUSCHEMISTRY IN THE TROPOPAUSEREGION
21,419
through (4) activate the chlorine reservoirs, which are
At lower altitudes in the model (e.g., at the tropopause), considerablymore abundantthan the backgroundmixing HC1represents a largerfraction oftheavailable Clycompared
ratiosof C10 in the lower stratosphere. It is useful to consider the contrast between polar stratosphericand cirrus cloud chemistry. In the heart of the polar stratosphericcloud region near 15-20 km in polar regions,abundancesof HC1 and C1ONO2 are unlikely to be exactlyequalbut are comparable, sothat reactions(1) through (4) can readily convert nearly all of the available chlorine to activeforms [see Websteret al., 1993a;Rocheet al., 1994]. In the tropopause region, HC1 is expected to be much more abundantthan C1ONO2 or HOC1, but these reservoirs are nonetheless present in greater concentrations than the unperturbed C10 amounts. Hence the C10 enhancements caused by cirrus cloud chemistry at these altitudes are controlled both by the abundancesand by the rates of recyclingof HOC1and C1ONO2,which attackthe morelonglived reservoir, HC1. The resultsare therefore sensitive to the chemistry controlling the relative ratios of all of these chlorinated gases. In the region fi'om 10 to 20 km the
abundances of C1ONO2andHOC1relativeto thoseof NOy andClyarethoughtto dependmainlyuponthe amountof ozone, temperature,and photolysis rates of several species, particularly those of ozone,C1ONO2,HOC1, and NO2. The abundancesof OH and HO 2 are also important, and higher values in this region (e.g., from acetonephotolysis,seeArnold et al. [1997]) would imply reducedHC1 and larger amountsof C1ONO2 and HOC1. If there were to be larger abundancesof C1ONO2 comparedto HC1 as suggestedby someobservations in the lower stratosphere[e.g., Websteret al., 1993b], larger perturbationsto C10 and related gaseswould occur during cirrus cloud events. However, recent observations of C10 and
NO2 suggestabundancesof C1ONO2near 20 km that are close to thosepredictedby theoreticalmodels [Stirnpfieet al., 1994] such as the one presentedhere. Transient reductions in the
calculated HC1 amounts andhencein the HC1/Clyratio do resultfrom the cloud events, as indicated in Figure 7, but the C10/C1ONO2 ratio follows simple photochemistry,as in the work of StimA/le et al. [1994], exceptfor brief periods during
to HOC1 and C1ONO2, and the peak C10 mixing ratio calculated at the meteorological tropopause at this latitude following cirrus events is somewhatsmallerthan at the level shownin Figure 7 (by abouta factorof 2 to 4 dependingupon seasonand latitude, accordingto our calculations). However, the cloud frequencyat the tropopauseis greater than that at higherlevels(Figure 3). The net effectof these two factorson composition and ozone as a function of altitude is discussed further below.
There are very few observations of reactive gases in the tropopauseregion. Borrmann et al. [1997]found evidencefor C10 maximaof severalparts per trillion by volume and ozone mixing ratios above 100 ppbv in air fi'omabout 9 to 12 km containing cirrus clouds. Borrmann et al. [1997] also presentedevidencefor remarkablespikesin C10 measurements fi'omabout 9-15 km of morethan 40 pptv during ascent and descent
of the
ER-2
aircraft
at midlatitudes.
These
C10
abundances greatly exceed those predicted fi'om gas-phase chemistrybut are comparableto the model results shown here including cirrus and in Borrmann et al. [1997]; see also Keim et al. [1996]. Note that the photochemical lifetime of C10 enhancedthrough such chemicalprocessesis several days or even weeks, so perturbedvaluesof C10 will be observedlong after the dissipationof the cirrus clouds. If the cirrus clouds occuronly a few percentof the time, it is hence more probable that high values of C10 will be observed after cloud events rather than during them. Figure 7 showsthe changesin HOC1 and C1ONO2 that can also be expectedto persist after the clouds have dissipated. Figure 8 shows the calculated C10 abundanceswith and without consideration of cirrus cloud chemistryat 100 mbar and 5øS in June,essentially at the tropopause in the tropics. At this location, subvisible clouds appearto be present on a daily basisin the SAGE observations(Plate 1), making their calculated impact more of an overall change in chemical
the cloud events themselves.
Maximum
calculated
values of C10
after the cirrus
events
reach 100 pptv in winter and about 8 pptv in summerat this model grid point, where cirrus clouds are present only about 2% and 9% of the time, respectively,basedupon SAGE data. The modeled recovery of chlorine species fi'om each cirrus event is characterized by a cascade fi'om the shortest- to longest-livedspecies,beginningwith C10 liberated initially, which is then convertedto HOC1, then to C1ONO2, and then to HC1. Note the calculatedenhancements of C1ONO2 and HOC1 following the events. The net timescalefor reformation of HC1 in this region is rather long as shown in Figure 7, severalweeks in winter and severaldays in summer. Further, it is noteworthy that the timescale for reformation of HC1 actually becomeslonger due to the cirrus events. The cirrus cloud chemistryconverts NOx to HNO3 (reactions (1)and (3)), reducing the amount of NO. The reformation of HC1 proceedsmainly through the reaction sequenceinvolving conversionof C10 to C1 via NO followed by reaction of C1 with CH4. Reductionof gas-phase NO thereforeincreasesthe HC1 formationtimescale. In winterthe cirruscloudprocessing is sufficiently frequent relative to the long timescale for reformingHC1 that recoveryis incompletebetween successive eventsat this particular location.
2.0e-12
-- ClO I00 mbor
Summer Tropics
1.8e-12
1.6 e-12
1.4e-12
1.2e-12
ß - .........
- ....
No cirrus With
cirrus
I.Oe -12 80
190
200
Day of Year
Figure 8. Calculated response of C10 at the tropical tropopause to cloud events based upon the SAGE II data during northernsummer. Clouds are persistentin this region (see Plate 1), making the effect of the cirrus clouds continuous rather than transient.
21,420
SOLOMON ET AL.: HETEROGENEOUS CHEMISTRY IN THE TROPePAUSE REGION
IC•91 ' ,.. , . , . , . , ,..... 49øN
135
i(•o
mbar
tie
o
I015
No cirrus
Frequency / 2
........
Surfoce
,
0
id 9
.........
.....
•
•0
,
Aree/3
•
,
60
•
90
,
•
12 0
, ,,•............. 150
180
I ßI ' I ' ß
R(•)-R(4) i(•i2
.......
No cirrus
No CIONO2+H20 No HOCI+
........ o
HCI
ClON02+H20 only $0
60
90
120
150
180
Day of Year Plate 2. (Top) Calculatedresponseof C10 just abovethe tropepauseat 49øN to cloud eventsbasedupon the observedfrequencyof occurrencein the SAGE II data and assumingcloudsoccur half as often, or that they contain one third as much surface area for reaction. (Bottom) Calculated responseof C10 just above the tropepauseat 49øN to cloud eventsassumingdifferent heterogeneous reactionsor setsof reactions.
species such as CIO rather than a highly transient one. Clearly, the actual effecton particular days or at particular locationscould be different from these averagedvalues. If for
example,abundancesof ClONe2 are temporarilyenhanced becauseof lightning-produced NOx, the impactof subsequent cirrus events could be greater than the average.Regions far from convection probably would display less frequent
as a representativelocation and one of particular relevancein the context of midlatitude
ozone trends.
Effectsof uncertaintiesin cloudfrequencyand chemistryare illustratedin Plate 2. The top panel showsthe calculatedC10 for 49øN, 135 mbar when it is assumedthat cirrus clouds occur
at half of the measuredSAGE II frequency,or when the surface areais reducedby two thirds. The main effectof the decreased perturbations. However, thelowamounts of Clyatthetropical frequency is directly reflected in the time series. The tropepause along with limited abundancesof HOC1 and decreasedfrequencyis alsosufficientto influencethe degreeof ClONe2 there(lessthan 1 pptv at 100 mbarin the model) recoverybetweensuccessiveeventsat this particularlocation, suggestthat the perturbationsto C10 generally expectedfor illustrating the importantrole that can be played by event the tropics are likely to be smaller in absoluteterms(albeit frequencywhen the HNO3 and HC1 chemicalrecovery times are long. more persistent)than thoseat middle and high latitudes. On the otherhand,the effectof reducingthe surfacearea(or, In the following, we continueto focuson 49øN at 135 mbar
SOLOMON ET AL.' HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION
21,421
equivalently,the ¾value)is generally less critical to the C10 enhancements. This reflects the very rapid activation of
caseas shown,due to more C1ONO2reactingwith HC1 via (1) rather than via reaction(3). As noted earlier,the abundancesof HC1 and other gases chlorinethroughreactions (1) to (4). In the darkany C1ONO2 and HOC1 presentwill be quickly convertedcompletelyto presentlocally during cirruscloud eventscouldbe affectedby activechlorine. Smalleror larger surfaceareas(or y values) factorssuchas precipitationof the particles,upward lofting of alter the time required for such conversion, but when these tropospheric air containingreducedCly, and other poorly times are rapid comparedto the duration of the event, the understood and spatially variable processesthat may vary surfacearea only affectshow long the conversion takes and from one cirrus cloud event to another. However, Figures6 - 8 not whether or not it occurs(see Figure 6). In the sunlit and Plate 2 demonstrate that substantial local enhancements atmosphere,surfacearea variationsin cirrus cloudsalso have a in C10 will occur after cirrus cloud events at any altitude and limited effect on the chemistryof interestherebut for a different latitudewhere even a few partsper trillion by volume of HOC1 reason. C1ONO2 will be approximatelyin equilibrium with and/or C1ONO2 are present along with HCI in sufficient C10 driven by the following equation: quantityto drive reactions(1), (2), (3), or (4), or combinations thereof. The reactions are sufficiently effective, and the C1ONO2-- k (C10)(NO2)/(khet +J) (6) background abundances of C10 sufficiently small, that the cloudsneed not be very frequentnor need they persistlong for so that when khet>>J large local enhancementsin C10 to be produced.
khet (C1ONO2)-- k (C10)(NO2)
(7)
4. Possible Impacts on Ozone
Hence for effective processing as C10>>C1ONO2, the
Figure 9 presentsthe calculated annually averageddecadal
abundance of C1ONO2will be reduced forfasterkhet,but the trendsin ozone versusaltitude at 49øN from the presentmodel productkhet (C1ONO2) remainsessentially unchanged. Put differently, when the transientreservoirC1ONO2(or HOC1) is driven to small values under sunlit conditions by rapid heterogeneousreactions,the rate of further reaction is limited by reformation of the reservoir (by reaction of the liberated C10 with NO2 or HO2) and not by the rate of khet. Plate 2 showsthat C10 is not very sensitiveto reducedsurfacearea at this particular location in the winter and summer,when the cloud surfaceareasinferred fromthe SAGE II data are larger than they are in spring, illustrating the importanceof these saturation processes. It is useful
to consider
the chemical
roles
with and without considerationof cirrus clouds, comparedto the analysis of midlatitude northern hemisphereozone trends using ozonesondeand Umkehr data by Miller et al. [1995]. Corresponding changes in the total ozone column computed with cirrus clouds
undergoingsubsequentconversionthrough reaction(2), as
-1 and -1.5%
for midlatitudes
comparedto calculations neglecting cirrus cloud chemistry. The error bars for the ozonesonde observations represent statistical uncertainties and not the total error. Many of the
Ozone Trend at Northern
of each of the
heterogeneousreactions (1) through (4) for the midlatitude lowermost stratosphere. Analysis of the rates shows that reaction (4) plays a much lesserrole than reaction (2) in the present model near the tropopause at midlatitudes, due to much smaller calculated abundancesof HOBr as comparedto HOC1 there. Plate 2 (bottom) illustrates the relative roles of the other three reactionsby presentingresultsfrom model runs in which particular reactionsare turned off. Note that these processesare not completely independent. Nonlinearities arise, for example,through the HOC1 formedin reaction (3)
are between
Mid-latiludes
5O
I
ß
]
I
Ozonesonde
(1968-1991)
45
0
Umkehr
(1968-1991) 4O
---
Model, no cirrus clouds 1980-1990
35
E
•
Model, with
•30
v
cirrus clouds 1980-1990 ,
25
well as from chemical nonlinearities due to factors such as the
impactof the combined set of reactions on NOx. However, Plate 2 shows that if C1ONO2 + H20 were to be the only heterogeneous reactiontaking place, its effectson C10 would be of greatestimportancein winter at midlatitudes with small effectsin summer.Considerably larger and longer-lasting effectson C10 are expectedif reactions (1) and (2) occur on cirrus clouds. In summer, conversionof HC1 to active chlorine
2O
15 I
I0
ß I
-12
•
I
-I0
i
I
-8
-6
-4
-2
0
2
4
Ozone Trend (%/decade)
is dominated by reaction (1)due to the relatively large Figure 9. Annually averagedmeasuredand model ozone C1ONO2 abundances presentthenrelativeto HOC1 (which in trends (%/decade) at northern midlatitudes. The ozonesonde turn reflecthigh NO2 fromdestructionof HNO3) as compared and Umkehr measurements (1968-1991) consideredby Miller et to the winter season at this location. In winter, both reactions al. [1995] are shown as the open and solid symbols with (1) and (2) play a role in activating chlorine from its statisticalerror bars. Model calculations(49øN) assuming reservoirs,with reaction(2) being the most importantprocess constantcirruscloudsas definedby the SAGE II measurements at this particular latitude and altitude. If reaction(3) is shut and including the known trends in total chlorine and bromine off entirely(through,for example,poisoningof ice surfacesdue [seeWMO/UNEP, 1994]are shownas the heavysolidline. The dashed line shows the model calculation when cirrus cloud to uptake of trace amountsof HNO3), there is little effecton chemistryis not considered. C10 in winter. SummerC10 actually could increase in this
21,422
SOLOMON ET AL.: HETEROGENEOUS
CHEMISTRY
ozonesondestationsconsideredin Miller et al.'s analysis are closeto 49øN (+5ø). Volcanic perturbations influenceozone loss profiles for several years after major volcanic eruptions [e.g., Hofmann et al., 1994], but analysis of long recordsover several
decades
is
intended
to
remove
such
transient
perturbationsand provide a long-termtrend representativeof the background state. The model was run for the appropriate conditionsof chlorine and bromine input for the period 19801990, for background stratospheric aerosol conditions appropriate to late 1978, with and without the addition of cirrus
clouds.
The ozone depletion near 40 km occursat slightly higher altitudesin the model than suggestedby the Umkehr data, but boththe modelandthe data reveal maximum depletionin this region comparedwith lower altitudes near 30 km. The modeledozone depletion increasesbetween30 and about 20 km but then decreasesin the lowermost stratosphereat northern midlatitudes when cirrus cloud chemistry is not considered. The observations,on the other hand, suggest increasingozone depletion in this region, with a maximumin the lowermost stratospherethat broadly follows the model when cirrus cloud chemistry is considered. Bojkov and Fioletov [1997]presentan ozonesondeanalysisusing height above the tropopauseas the vertical coordinate to reduce variance in the tropopauseregion; they also concludethat maximumozone lossesare obtained close to the tropopause. Logan [1994] also presentsevidence for large ozone lossesin the lowermoststratospherein her comprehensiveanalysis of ozonesondeobservations,and McCormick et al. [1992] and Harris et al. [1997] provide additional evidence for ozone depletionclose to the tropopauseat midlatitudes. Logan [1994], Miller et al. [1995],and Bojkovand Fioletov [1997] note that there is considerablevariability in ozone trends among different stations. In particular, European stationsgenerallyreveal substantialincreasesin tropospheric ozone, while Canadian stations [see also, Tarasick et al., 1995]tend to display negative trends in troposphericas well as stratosphericozone. While these differencescould reflect local pollution (such as enhanced ozone in the upper troposphere/lowermoststratosphere near European flight corridors,or vertical transport of lower-altitude pollution), the differencesamongstationsare alsolikely to be due in part to measurement uncertainty,limitations of sampling,changes in calibration,and other experimentalaspects. Nevertheless, the trends observed in
the
lowermost
midlatitude stratosphere suggest that significant ozone depletionis likely to be taking place in this region, since any increasesrelatingto local pollution (suchas is clearly seenin the lower tropospherein Europe) will only serve to offset such depletion. Increasingozone depletion near 10-15 km comparedwith higher altitudes need not be caused solely by local cirrus cloud chemistry. Such a profile of ozone depletion at midlatitudescould,for example,resultfrom transportof ozonedepletedair from the polarvortex [e.g., Tuck,1989], fromlower altitudes through diffusion, or from temporally and spatially varying depletions relating to volcanic aerosol fluctuations [Solomon et aL, 1996]. In addition, there are many uncertainties in photochemistry in the tropopauseregion. However, the key result of Figure 9 is that cirrus cloud chemistryin the tropopauseregion at northern midlatitudes probably makes a significant contribution to the observed trendsthere, as suggestedby Borrmann et al. [1996, 1997].
IN THE TROPOPAUSE REGION
We next considerthe impact of cirrus cloud chemistryfor ozonedepletionas a functionof seasonand latitude. Section 3 showsthat cirrus cloudsshouldbe expectedto enhanceC10 nearthe tropopauseeven in the tropics. Whether or not such enhancements influenceozonein the tropopauseregion(and, in particular, its trends over the past decades of human additionof chlorine to the atmosphere)dependsprimarily on the competitionbetween transport and photochemistryand betweenchlorineand otherchemical processesat the relevant altitudes.
Figure 10 shows the calculated diurnally averaged timescalefor photochemicaldestructionand vertical advection of odd oxygenat 135 mbarfromthe Garcia-Solomonmodel in northern hemispheresummerand winter. Note that cirrus cloud chemistryhas not been included in the photochemical lossratesshownbut will be examinedlater in Figures 13 and 14. It is important to emphasizethat such timescalesare strongly dependentupon altitude, and the results described here for the lowermoststratosphere are distinctlydifferent from higher levels where differentphotochemistryand dynamics must be considered.
109•- ,
,
,
,
•
,
NH Summer -4
• I :
P=135mbar 1
p Oh:mica, 10ss
0
,
..•
c>108 "-'-.
(.t307 th
i06
-60
-40
-20
0
2_0
40
60
109
I [ I [ I ] I i I [ I • •
IO8
_
NH in. t. 11___ -
107 Vertical
-40
-20
0
Latitude
advection
20
40
60
(ø}
Figure 10. Model-calculateddiurnally averagedtimescales for chemical destruction and vertical advection at 135 mbar
versus latitude in northern hemispheresummer and winter. Note that chemical processesare substantiallyslower than advection at midlatitudes in winter and in the tropics but becomemostcomparablein summer.
SOLOMON ET AL.: HETEROGENEOUS
CHEMISTRY
IN THE TROPOPAUSE REGION
21,423
differences obtained for the level near 22 km between the cases with and without cirrus clouds reflect small calculated
dynamicalchangesrelated to the larger depletions at lower altitudes. It is clear that the sonde data are subject to very
largeuncertainties,as notedabove. While the averageof all stations considered by Miller et al. [1995] displays the indicated seasonalvariation, the changesfrom one season to another are certainly within statistical uncertainty and are different for the Canadian stations as comparedto European locations. Nevertheless,it is interesting that both the model
and the analysis of Miller et al. [1995] suggest largest depletionsnearthe tropopausein summer. The analysisof Bojkov and Fioletov [1997] displays a similar seasonal variation for the Canadian station at Edmonton, although a
Northern Hemisphere Mid-Latitude Latitude
Ozone Trends
(ø)
Figure11. Calculated change in ozone at 135mbarfor 1990 ascompared to 1980levelsof totalchlorine andbromine, with
4:3 mbar
and without considerationof cirruscloud chemistry.
ß
-2
Understanding the competition between photochemical destruction and transport as revealed by Figure 10 is key to evaluating where and when ozone densities near the tropopauseare mostsensitiveto chemicalperturbations. In the winter hemisphere,vertical advection transports ozone relatively rapidly through the lowermost stratosphere. Vertical
and horizontal
diffusion
add to advection
-4
-6-
so that the
Model, with cirrus Model, no cirrus Ozonesonde (,1977-1991)
o
total transporttimescaleis even shorter, implying that it is N relatively difficult for photochemicalperturbationsto affect o ozone in the transport-dominated winter lowermost -8 stratosphere.In the tropics,transportis again believedto be 0 morerapid than photochemistry,particularlyin the region of 0 maximumupwelling, which shiftswith seasonand limits the possibleimpactof photochemistry on ozonetrends. In summer • at middle and high latitudes, two factors make ozone vulnerable to photochemistry near the tropopause: _• -photochemical timescalesare generallyshorterthan in winter, • andthe timescalesfor transport (both horizontal and vertical) are considerablylonger than in winter. Figure 11 shows the calculated change in ozone in -4 -northernhemisphere summerof 1990comparedwith 1980at the --
60 i
120
I
i
model's 135 mbar level, for runs with and without cirrus clouds. Consistent with the argumentsgiven above, there is
I
i
I
:300 I
I
360 I
-
•
/
••
////
ß --
--
little changein the tropics or in the winter hemisphere,but significantozonechangesof severalpercent are calculatedfor the northernhemispherein summerat this pressurelevel. The SAGE II data suggest more frequent cirrus clouds in the
-6
northern than in the southern summer(see Figures 2 and 3). The calculateddepletions obtained in southern hemisphere
--
-8
' o
60
120
summer(not shown) are substantiallysmallerthan those in
180
240
300
360
Day of Year
northernhemispheresummer.
Figure 12 showsthe seasonalvariation of the calculated Figure 12, ozone trend from 1980 to 1990 at 49øN latitude with and without consideration of cirrus clouds at two different
i
240
135 mbar
--
•
I
180
from
1980
Seasonalvariation of the calculated ozone trend to
1990
at
49øN
latitude
with
and
without
considerationof cirrus clouds at two different pressurelevels, pressurelevels, one at 43 mbar(near22 km)and the other at one at 42 mbar (near 22 km) and the other at 135 mbar (near 135 mbar (near 13.5 km). Figure 12 also displays the 13.5 km), along with observations from the analysis of observations from the analysis of northern hemisphere northern hemisphere midlatitude ozonesonde data by Miller midlatitudeozonesondedata by Miller et aL [1995]. The small et al. [1995] for the period 1977-1991.
21,424
SOLOMON ET AL.' HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION
2.5ppbv • •25ppbv
per month
49øN
per month
/
,
' [-I"-- W"h I
T
/
/
I0
I0
|
NOx
ClO,,/BrO,,
HO,,
!
•
/
5I ..... ,111 ........ i ........ I ........ i ,,•..... 1..... ,,], ,,/,,,,1 ........ i .....J5 I
i0-19 i0-18 i½17 i0-16 i0-15 i0-14
I0
Ox Loss Rate (I/sec)
I00
I000
I0000
Ozone (ppbv)
Figure 13. (Left) Calculateddiurnally averagedrates of odd oxygendestructiondue to Ha x, NOx, and ClOx/BrOx chemistry(see text for definitions)from 5 to 20 km at 49øN in summer. The solid lines show results including cirrus clouds, while dashedlines do not considercirrus cloud chemistry. (Right) The calculatedozone mixing ratio profile at the samealtitudes.
springmaximum in ozonedepletionis observedover Europe [seealsoMiller et al., 1995]. Many factors influence the seasonalvariation of ozone depletion. Transportof ozone-depletedair from the polar vortex could, for example, lead to large spring ozone depletions.Changesin dynamicscanalsoaffectozonebut are
againmostlikely to maximizein winter and spring. While somefractionof ozone depletion producedearlierin the year could persist into the summerseason,the occurrenceof a summerdepletion maximum,if real, argues for a local photochemicalloss. In the present model, the summer depletion maximumoccurswhen cirrus cloud chemistryis consideredmainly becauseof the vulnerability of ozone in that season due to relatively slow dynamical and fast
49øN in summer and the monthly averaged composition
perturbationscausedby the cirrus clouds as a function of altitude. Although this region is dominatedby HOx-driven ozone lossesin the summerseason [Wennberg et al., 1994], chlorine/bromine chemistry is significant, and coupling betweenthe two is very important,as will now be shown. The
reactionsof Ha2 with C10 and BrO are consideredhereas ClOx/BrOx-driven odd oxygenloss mechanisms alongwith the reactionof C10 with BrO, while the dominantpure Ha x-
driven odd oxygenloss is due to the reactionof Ha2 with ozone. The calculatedenhancementsin C10 at and slightly
abovethe tropopauseare substantialand allow ClOx/BrOx chemistryto competeeffectivelywith purely HOx-driven Ox lossprocesses.Indeed,a usefulway to understand the strong photochemical processes as shownin Figure 10; the summer potentialfor impactof the cirruscloud chemistryon ozoneis
to notethatit enhances theC10/Clyratioby factors ofmany,
maximumin cirrus cloud occurrencefrequency at northern midlatitudes in the SAGE II climatology (Figures 2 and 3) also contributes but is less important.
implying that anthropogenicchlorine will be much more
Figures 13 and 14 show the calculatedrates of various catalyticdestructioncyclesin the lowermoststratosphere for
2O
cloud chemistryalso enhancesHa x in this region (mainly
I I IIIII1[I I IIIIII I 71'IIIIIlyI I IIIIII Summer,
HO,,
- 49ON
ClO
/ ' ,('
24 - hour
15
effective for ozone destruction when these processes are considered than it would otherwise be. Further, the cirrus
/]
/
-
\
[ t ][[]]]l I/] ill[ 20 /
IO requiredto add i0% to
/
C 10x-induced
_ average "7_-'"/
15
II •l
-10
I I rl I IllIll
0 -14
I
IO-13
I I I IIHI
I •
I I IIIIJ
IO-12 IO-II Mixing Ratio
I
I I II
IO- 4
IO-13
IO- 2
Mixing Rotio
Figure 14. (Left) Calculateddiurnallyaveraged C10 andHa2 profilesfrom5 to 20 km at 49øNin summer. Solid and dashedlines showcalculationswith and withoutcirrusclouds,respectively.(Right) IO abundance requiredto increaseC1Ox-induced odd oxygenlossrate by 10%.
SOLOMON ET AL.: HETEROGENEOUS CHEMISTRY IN THE TROPOPAUSE REGION indirectlythroughthe effect of reaction(1) on NOx and HNO3 abundances),augmentingthe total chemicalloss due to HOxdriven chemistry as an indirect result of human chlorine buildup. While the amplitude of the calculated changes in HOx are smallerthan the calculated changesin C10 (Figure 14), they make substantial contributions to the absolute ozone loss rates as shown in Figure 13. The abundancesof radicalsnear the tropopauseas shown in Figure 14 imply ozone loss rates of the order of 10 ppbv/month. Since the ozone abundance in this region is only of the orderof a few hundredparts per billion by volume and transportis slow (Figure 11), the calculatedenhancements to ClOx/BrOx and HOx-driven ozonedestructioncan result in ozone
trends
over
the decade
of the
1980s
of the
order
of
severalpercent, as shown in Figures 9 and 12. Note that such ozone loss rates are sufficientto depleteozone slowly over the courseof many days (as in Figure 12) but are not rapid enough to produce locally large anticorrelations between ozone and cirrus clouds and hence fall short of explaining the measurementsof Reichardt et al. [1996]. Figure 14 showsthe calculatedradical abundancesnear the midlatitudetropopause. The presentmodel predicts that BrO abundancesbelow about 14 km are lessthan 1 pptv (based on current understanding of the source gases, including halons and methyl bromide), but there are virtually no direct measurementsavailable in this region to test this key parameter.If therewere to be substantial convective transport of short-lived bromine sourcesto the lowermost stratosphere or more rapid downward transport from above, these could substantially enhancethe BrO in this region and hence the ClOx/BrOx-driven ozone loss through the catalytic cycle involving the reaction of C10 and BrO [see Solomon et al., 1994]. Indeed, convection may be closely related not only to
21,425
lowermost stratosphere,or by additional chemical sources such as acetone reactions [see McKeen et a!., The photochemistryof acetonein the upper troposphere:A source of odd-hydrogen radicals, submitted to Geophys. Res. Lett., 1997].
As shown in Plate 2 above, frequency of cirrus cloud occurrenceplays a critical role in determininghow often high C10 values are attained, and hence the rate of ozone loss.
Sensitivity tests were run in which the cirrus clouds were assumedto be presenton twice as many days for half as long (0.25 days)as the basemodelcaseshown here (0.5 days), and on half the number of days for twice the duration (1.0 days). Substantialenhancementsin C10 can occur rapidly (as shown by Figure 6), making the number of cloud events important, but recyclingof the C10 so activatedis also significant under sunlit conditions as noted above, so that event duration also
plays a significant role, particularly in summer. The shorter assumed duration yielded smaller but more frequent C10 enhancements
and increased the fraction of the time that events
occurred in sunlight as compared to dark conditions at midlatitude. The annually averagedozone depletionincreased in this caseby a local maximumof about 1% at, for example, 49øN and 135 mbar. The longer assumedduration produced larger but less frequent calculated C10 enhancementsand somewhat smaller ozone depletion (by about 2% near 135 mbar). While cirrus clouds may be present in a particular locationfor periodsas long as severaldays at sometimes, flow through such systems implies that air parcel exposures to cloud chemistry are likely to be of shorter duration. The observation
of
a
diurnal
variation
in
subvisible
cloud
occurrence[see Wang et al., 1996, and referencestherein] also supports a mean duration of the order of hours rather than days. These sensitivity tests show that ozone depletion is the occurrence of cirrus clouds at midlatitudes but also to the transport of short-lived trace gases that could influence the generally more sensitive to uncertainties in the timeabundanceof halogen speciesin the lowermost stratosphere. integrated cloud occurrencefrequency than to the assumed Someevidencefor suchprocessesat midlatitudeswas recently duration for the same integrated frequency. If clouds were presentedin observations of dibromomethane[Kourtidis et present less frequently than deduced from the SAGE II climatology or if their horizontal or vertical extent were much al., 1996]. Given the very low abundancesof riO 2 and BrO smaller than the SAGE II field of view, the calculated impact predictedby the model in the key ozone lossregion from 10 to on ozone would be reduced. 15 kin, only about 0.1 pptv of lO would be required to add 10% to the C1Ox-driven ozone loss, using the measuredrate constantsfor iodine chemistryreportedby Gilles et al. [1997] 5. Discussion and Conclusions and Turnipseedet al. [1997], as shownin the right hand panel of the Figure 14. Such values are below the upper limit Observations of cirrus clouds by aircraft, ground-, and recentlysuggestedby Wennberget al. [1997]. EnhancedC10 space-basedlidar and a variety of satellite methods using from cirrus cloudsand considerationof seasonalvariability in differentwavelength ranges show that cirrus clouds occur in the timescale for transport in the lowermost stratosphereas the vicinity of the tropopause [Sassen,1991;Arismann et al., discussedabove are important differencesbetween this study 1993; Borrmann et al., 1996; Stowe et al., 1989; Wylie et al., and Wennberg et al. [1997] insofar as the impact of trace 1994; Wang et al., 1996], and there is evidencefor occasional amounts of iodine on ozone loss is concerned. Similarly, cirrus cloud formation slightly above the meteorological convective sources of chlorine radicals from short-lived trace tropopause [Sassen,1991;Murphy et al., 1990; Wang et al., chlorocarbons(even if present in far smaller amounts than 1996]. Cirrus cloud surfacesmay be composedof water ice or longer-lived gasessuch as CFCs) could also contribute to the liquid sulfuric acid/water supercooled droplets, but both availabilityand trendsin Cly (andhenceozone)in this kinds of particles facilitate rapid heterogenous chlorine region. Finally, sincethe reactionsof C10 and BrO with HO 2 reactions. Borrmann et al. [1996, 1997] pointed out that such represent key catalytic cycles driving ozone loss in this clouds might activate chlorine in the tropopause region, region, uncertainties not only in C10 and BrO but also in perhaps leading to ozone depletion. In this study, a more HO2 represent important uncertainties determining the detailedexaminationof the chemistryof the tropopauseregion absoluterate and trendsof ozone loss in this region. Reactive and of cirrus clouds has been presented using a cirrus cloud hydrogen (and chlorine) gasescould be affectedby transport climatology from SAGE II measurementscoupled with a of nonmethanehydrocarbons to the upper troposphere and numericalmodel of atmosphericchemistryand transport.
21,426
SOLOMON ET AL.: HETEROGENEOUSCHEMISTRY IN THE TROPOPAUSEREGION
This study supportsthe view that it is highly likely that cirrus clouds do perturb chemical composition in the tropopause region. Uncertainties in cloud locations and frequency are substantial (particularly insofar as their occurrenceabove the local tropopause is concerned), but
to our model, which predicts HC1 abundances near the tropopauseof the orderof 100 pptv. If HC1 were presentin muchsmalleramountsthan predictedby the model(e.g.,as a result of precipitation), then surfacepoisoning would be important and the impact of cirrus cloud chemistryon C10
availableobservationsstrongly suggestthat clouds occur
would be reduced but still not zero.
more than 10% of the time, at least at the tropopause.
In spite of uncertainties in chemistry,inorganic chlorine content, and cloud parameters, the analysis presentedin this suggest thatCly abundances nearthetropopause areof the paper shows that it is highly likely that cirrus clouds do orderof 100 pptv, with larger amountsbeingpresentat higher indeedenhancethe abundanceof C10 and otherwiseperturb latitudes (as is true for ozone and for related reasons). Any the compositionof the tropopauseregion. This representsa C1ONO2 and/or HOC1 present in the tropopauseregion is new aspect of heterogeneous chemistry of relevance to highly likely to rapidly react with HC1 on cirrus cloud tropospheric and lower stratospheric chemistry whose surfaces,suggestingthat C10 abundancescould increasefrom elucidation will require direct observation of the clouds and pptv or sub-pptv levels near the tropopauseto considerably relatedspecies. EnhancedC10 could impactmanyaspectsof larger values due to cirrus cloud chemistry. The magnitude chemistry in the upper troposphere and lowermost and duration of such perturbations depend upon season, stratosphere, suchas the ratio of NO to NO2 and the lifetimes altitude, and latitude. Activation of chlorine from HC1 should and vertical gradients of organic compounds(including also be expectedto lead to transient enhancementsin HOC1 propane,ethane,and methane). and C1ONO2 after cloud events, depending upon the Giventhe large ozonedepletionsobservedin the vicinity abundancesof HO2 and NO2 and the timescalefor reformation of the midlatitude tropopause over the past decade, of HC1. Hydrogen radical concentrationsare also affectedby particularly in clean environments such as over western cirrus cloud chemistryinvolving chlorine species,providing Canada [Logan, 1994; Miller et al., 1995; Bojkov and an additional perturbation and an indirect mechanismthat Fioletov, 1997], any perturbationto C10 in this region is of likely influencesozone loss. Nitrogen radical concentrations considerablescientificinterest. While the presenceof cirrus are expectedto be reducedby cirrus cloud chemistry. clouds near the tropopause plays only a small role in In the tropics, cirrus clouds are observedmuchof the time determining the total ozone column trends (less than 1.5% near the tropopause, suggesting that their impact on change in computed column at midlatitudes in these composition may be nearly continuous at least in some calculations), they are of particularimportancein determining locations. We estimate an average enhancementof C10 by the changesin ozone at the tropopauseand hencethe radiative about 50% at the tropical tropopause. Clouds are also forcing. observednear the tropopausein the extratropics,with greater Photochemical reactionscompetewith transportprocesses frequenciesin the northernthan in the southernhemisphere near the tropopausemost effectivelyin summerat middle and and greaterfrequenciesin summerthan in winter [Wang et al., high latitudes,when transportis relatively slow. Further,the 1996]. The relaxationof active chlorine to HC1 is expectedto SAGE II climatology indicates maximum cirrus cloud occur rather slowly in the lowermost stratosphereat middle occurrencenearthe tropopausein summermidlatitudes in the and high latitudes, over timescales of days or weeks northern hemisphere. Model calculations indicate that dependingupon seasonand latitude. Hence C10 enhancedby perturbations in C10x and HOx chemistry related to the cirrus clouds may persist long after a cloud has dissipated. presenceof cirrus cloudscan contributeto ozonetrends in the The peak calculatedenhancements in C10 in the extratropics tropopauseregion, particularly in summer at midlatitudes, but nearthe tropopauseare of the orderof factorsof 30. also in other seasons.Someozonesondeobservationspoint There are many factorsthat couldaffect these cirrus-induced towardmaximumozonedepletions during summerat northern chemicalperturbations. The impact on C10 dependsmainly midlatitudes,suggestinga local ozone loss mechanism there, upon the absolute and relative abundancesof HC1, C1ONO2, although the observations are highly variable and seasonal and HOC1, which in turn vary with season and latitude. trends are uncertain. Larger enhancementsin C10 could occur on a transient basis Here we have studiedthe likely impactof cirruscloudsfor a if, for example,lightning enhancesthe abundanceof NOx and specific time period when they are believed to be most henceC1ONO2. The frequencyof occurrenceof the clouds is accurately observed because of the absence of volcanic also important,and can, for example,affectthe abundanceof aerosol. However, there is both observational and theoretical NOx and hence C10 and C1ONO2, due to the long evidence suggestingthat cirrus cloud propertiesnear the photochemicallifetime of HNO3 in the tropopause region. tropopausemay be affectedby volcanic activity, probably Heterogeneousreactionsinvolving C1ONO2,HOC1, HC1, and through cloud microphysics[densenand Toon, 1992; $assen H20 all play a role in determining the responseof C10 to et al., 1995; Wylieet al., 1994]. The cloud frequencycould cirrusevents,and their impactsare nonlinear. While many of alsobeaffected by the E1Nino Southern Oscillation(ENSO) the heterogeneous reactionsare likely to occurratherrapidly [Wanget al., 1996]. Further,the occurrence of cirrus clouds is on either solid or liquid surfaces for the wet conditions likely to be affectedby any trendsin temperature in the prevalent near the tropopause under conditions of cloud tropopause regioncausedby the ozonedepletionitselfor by formation,the possibilityof poisoningof water ice surfacesfor otherprocesses of relevanceto the thermalbudgetof this the reaction C1ONO2+H20 through uptake or formationof region(e.g.,trendsin CO2). Observations [e.g.,Oort andLiu, trace amountsof HNO3 has also been considered. However, 1993] suggest that the lowermost stratosphere/upper surfacepoisoning may not affectthe chemistrysignificantly, tropospherehas indeed cooled by at least severaltenths of a sincereactions(1) and (2) involving HC1 are likely to be most degreeover roughly the past decade. Perhaps most effective in enhancingC10 in the tropopauseregion according importantly,emissions from aircraft(contrailsand/or chemical Observations
of ozone
and
limited
measurements
of HC1
SOLOMON ET AL ß HETEROGENEOUS CHEMISTRY IN THE TROPePAUSE REGION
25
25
2o
2o•
21,427
15
I0 -I0
I0
-8
-6
-4
-2
0
-I00
% 03 Decreese (perdecode)
-80
-60
-40
-20
0
% 03 Decreose
Figure15. Cartoonrepresentation of thekeypointsof thispaper.Downward circulation brings03 andCly to the lowerstratosphere in middleandhighlatitudesandis strongest in winter. Cirruscloudsnearthe midlatitude andtropicaltropepause perturbchemical composition, muchaspolarstratospheric cloudsdo in the Antarcticstratosphere, throughreactionssuchas HC1 + ClONe2--> C12+ HNO3. Relativelyweak transportin summercompetes lesseffectivelywith this chemistrythanin winter. Observations suggest midlatitudeozonetrendsthat are largestin the tropepause region[fromMiller et al., 1995],perhapsin part becauseof the cirruscloudchemistrydiscussed here. Antarcticozonedepletionmaximizesnear 12-24 km,
wherepolarstratospheric cloudsarefrequently found.TheAntarctic ozonedepletion profileis basedupon the seasonalchangefrom Augustto October1990at the SouthPole Station(D. Hofmann,personal communication, 1997). Note the changeof scalefor the Antarcticozonedepletion(about10 timesgreater than the midlatitude depletion).
their competition with other processescontrolling ozone, such as transport. Our results suggestthat consideration of cloud distributions and frequency[Jensenand Teen, 1997]. cirrus cloud chemistry at midlatitudes may make important Clearly,if there were to be variability and/or trendsin the contributions to the shape of the ozone depletion profile in frequency of occurrenceof cirrus clouds or in their the lowermost stratosphere, which could play a role in distributions,this could add substantially to their impacton reconciling discrepancies between observed and modeled the ozone layer based upon the chemical considerations ozone depletionprofiles at midlatitudes. presentedhere. In short, the modeling results presentedin this paper Acknowledgments.We thankRoyMiller, Maria Neary, andTerry in preparationof the figuresandthe manuscript. suggestthat the surfaces of cirruscloudsnearthe tropepause Cookrefor assistance very likely provide sites for activation of chlorine (and We appreciatehelpful commentson the paper by the anonymous
effluents)havecertainlyincreased in recentdecades,are likely
to continue to increase in the future, and could influence cirrus
perturbations to relatedspeciessuchasNOx and HOx),much reviewers as well as M. L. Chanin, T. Deshler, D. Fahey, A. D. Hofmann,D. Hanson,A. Langford,H. Jaeger,A. R. as polarstratospheric cloudsdo at higheraltitudesover polar Heymsfield, Ravishankara,and A. Tuck. regionsand for similarreasons. Figure 15 summarizes this key resultof this paperby depictingthe vertical distribution of polar stratosphericand cirrus clouds along with the References observed vertical distributions of ozone depletion in midlatitudes
and in Antarctica.
Whether
or
not
cloud
chemistryinfluencesozone depends upon the occurrence frequency,spatialextent,anddistribution of the cloudsandon
Abbatt, J. P. D., and M. J. Molina, The heterogeneous reaction
HOCI+HC1-->C12 + H20 oniceandnitricacidtrihydrate: Reaction probabilities andstratospheric implications, Geephys. Res.Lett.,19, 461-464, 1992.
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CHEMISTRY
IN THE TROPOPAUSE
REGION
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