Feb 27, 2000 - low, 1974] doses were calculated together with the UV index. (UVI). The latter is defined as the daily maximum effective ery- themally weighted ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105,NO. D4, PAGES 5001-5007,FEBRUARY 27, 2000
Sensitivity of surface UV radiation and ozone column retrieval to ozone and temperature profiles B.Lapeta,• O.Engelsen,2,3 Z. Litynska,4 B.Kois,4 andA. Kylling5
Abstract. Thispaperdiscusses the influenceof ozoneandtemperature profileson surfaceUV radiation,and on total ozonecolumnderivedfrom globalirradiancemeasurements. Measured
ozoneandtemperature profilesfromLegionowo, Poland,areusedtogether withtypicalsurface andcloudless atmosphere conditions. Theeffectsof assuming a U.S. standard profilewithscaled ozonecolumninsteadof actualprofilesareanalyzed.Variabletemperature/ozone verticaldistribu tionsanddifferentsetsof ozoneabsorption crosssectiondatamaychangeerythemally weighted radiationby asmuchas 14% with respectto referenceconditions.The meanand standarddeviationsof errorsweregenerallybelow2% butincreased another2-3% for largesolarzenithangles. Uncertainties of up to 10% maybe causedby usingan inappropriate profile in total ozonecolumn retrieval.We analyzedthe underlyingprocesses causingthe uncertainties by selectingthreeozone andtemperatureprofile pairscharacterized by the sameunscaledtotal ozoneamountbut with different vertical distributions. Results obtained for cases with ozone redistribution from the strato-
sphere to thetroposphere areconsistent withearlierwork.However,if thetemperature profiles differsignificantly in thestratosphere, anozoneredistribution mayleadto a strongdecrease in UV dosesfor highsolarzenithangles.It is alsoshownthatdifferences in ozonemaximumheightas well asin ozoneconcentration in theuppertroposphere havea significantinfluenceon surfaceUV radiation.
1. Introduction
uncertaintiesin modeled UV irradiancescausedby the limited
accuracyof modelinput data.They foundthat replacinga standardozoneprofileby seasonal profilesmay leadto a significant,
SolarUV-B radiationis stronglyattenuatedby ozoneabsorption in the Earth's atmosphere.The effectivenessof ozone absorptionis a functionof the temperature andthe pathof propagation through atmosphericlayers that contain ozone. Hence the absorptionof solar UV radiationby ozonedoesnot only depend on the total ozonecontentbut alsoon the solarzenith angle,atmosphericscattering,and the vertical distributionof ozone and temperature.In radiative transfer models, standard ozone and temperatureprofiles are usuallyusedsince in situ measurements of ozoneandtemperatureprofilesare generallynot available. Br•ihl and Crutzen [1989] showed that a redistribution of stratosphericozone to the tropospheregives a decreasein the biologicalUV dose.In a similarstudy,Tsayand Stamnes[1992] analyzedthe effectsof shiftingozonefrom the stratosphere to the troposphereon erythemalUV radiation.They found that such a redistributiontendsto decreasethe erythemalUV dosefor a solar zenithangleof 55ø and to increasethe dose,by 1-3%, for larger solarzenith angles.In the latter study,syntheticozone profiles were used and ozone artificially redistributedfrom the stratosphereto the troposphere.Schwanderet al. [1997] estimatedthe
around +11%, increase in UV irradiance at 290 nm and a solar
zenithangleof 60ø for winterconditions. For otherwavelengths and solarzenith anglesthe differenceswere smaller.For summer profilesthe oppositeresultswere obtained. This paper discussesthe influenceof variable ozone and temperatureprofiles on surfaceUV radiation. Furthermore,the im-
plicationsfor ozonecolumnretrievalfrom globalUV irradiance measurements are discussed.Measuredozone and temperature profilesfromLegionowo,Poland,areusedtogetherwith an accurateradiativetransfermodelto quantifythe errormadeby using standardozone and temperatureprofiles comparedwith actual profiles.The underlyingprocesses causingthe deviationsare explainedusingselectedcases. The paper is organizedas follows: in section2 the radiative
transfermodelis brieflydescribed. This is followedby a presentationof the ozoneand temperature profilesusedin the study. The sensitivityresultsare presentedanddiscussed next, followed by the conclusion.
•lnstitute ofMeteorology andWater Management, Krakow, Poland. 2. Radiative Transfer Model 2Norut Information Technology, Tromso, Norway. 3Now atNorwegian Polar Institute, Oslo, Norway. The radiativetransfermodel is an upgradedversionof the 4Institute ofMeteorology andWater Management, Legionowo, Poland.
5Norwegian Institute forAirResearch, Kjeller, Norway. Copyright2000 by the AmericanGeophysicalUnion.
modeldescribedby Mayer et al. [1997]. The model is basedon
the multistreamdiscreteordinatesradiativetransferequation solverDISORT by Stamneset al. [1988] with pseudospherical corrections as described by Dahlbackand Stamnes[1991]. The calculations wereperformed for the 280-400nm spectralrange
Papernumber1999JD900417.
with a resolution of 0.05 nm. The ozone cross section was taken
0148-0227/00/1999JD900417509.00
from Molina and Molina [1986], wherenot otherwisenoted.We 5001
5002
LAPETA
ET AL.: SENSITIVITY
OF UV RADIATION
TO OZONE PROFILES
useda tropospheric urbanaerosolmodelwith a spring/summer summer/winter,tropical, and U.S. standardconditions All proprofile and a Henyey-Greenstein phasefunction[Shettle,1989]. files in this data set were scaled to a total ozone column of 350 The aerosolopticaldepthwasscaledusingtheAngstromturbid- Dobsonunits (DU). The actualtotal columnsof the ozoneproity formulawith alphaandbetasetto 0.11 and 1.3, respectively. files consideredherewere in the range274-402 DU. The Rayleighscatteringcrosssectionswerecalculatedaccording 3.2. Data Set 2 to the parameterization givenby Nicolet [ 1984].The extraterresTo studythe underlyingprocesses of the influenceof realistic trial irradianceswere taken from measurementmadeby the Solar ozone profiles on UV radiation, pairs of ozoneprofileswith difUltraviolet SpectralIrradianceMonitor (SUSIM) onboardthe spaceshuttleduringthe ATLAS-3 missionin November1994. Ozoneand temperature profileswere eitherstandardprofilesor measured profilesdescribed below.To estimatethe sensitivityof surfaceUV radiationto ozoneand temperatureprofiles,the UV erythemal[McKinlayand Diffey, 1987] and DNA damage[Setlow, 1974] doseswere calculatedtogetherwith the UV index (UVI). The latteris definedas the daily maximumeffectiveery-
ferent vertical distribution but similar total ozone, within the range of +3%, were selectedfrom the data series.The basis for
the selectionwas the differencein troposphericozone amount and in the heightsof the tropopause[14/MO,1957] and the ozonopause[Chrgian, 1967]. The tropopauseheightis oftenusedfor studyingthe changesof total ozonein midlatitudes[Schubertand Munteanu, 1988; Hoinka et al., 1996]. The reasonfor this was themally weighted irradiance (W/m2)averaged overa duration of formulatedby Steinbrechte! al. [1998] as follows:"The tropo30 min and multiplied by 40 [WMO, 1997]. These radiation pauseformsa boundarybetweenthe well-mixed,ozone-poortroWhen quantitieswere calculatedfor varioussolar zenith anglesand posphereand the stratifiedozone-richlower stratosphere." groundalbedovalues.In addition,irradiances at 305 and340 nm studyingthe temperatureand ozone profiles, one can observe easily the differencesin the heightsof tropopauseand ozonowere calculatedfor use in the ozonecolumnsensitivitystudy. pause.It is really the ozonopausethat separatesthe ozone-poor tropospherefrom the ozone-rich stratosphere;Chrgian [1967] 3. Atmospheric Profiles formulatedthe ozonopauseas "the heightabovewhich the ozone partial pressureis growing rapidly." After Ivanova [1972] the The conservationof realism when performinga sensitivity heightsof tropopauseand ozonopausediffer in 85% by + 1 km, studyon the influenceof ozoneprofileson surfaceUV radiation is not a trivial task. UV radiation is influencedby molecular, but in several cases,the difference can be 4-9 km. Performed seaerosoland cloud scattering,and absorptionand surfacereflec- lectionof ozoneprofile pairsaimedat separatingsuchcases.For tion. Furthermore,the ozone crosssectiondependson tempera- detailedstudies,threepairsof ozoneprofileswerechosen: 3.2.1. Case 1, August3, 1994 and July 7, 1993, Figuresla ture,andhencethe ozoneabsorption is temperature dependent. In and lb. The two profileshave major differencesin the tropoa strictsense,a fully realisticsensitivitystudyshouldonly comsphericand stratospheric ozone amountswith the first profile pareresultswhenall atmospheric andsurface variables areidenhaving 34 DU more of ozone belowthe tropopause and 35 DU tical, exceptfor the variablesof interest.Obtainingan adequately less above, hence providing a good example of ozone redistribusized data set from surface radiation measurementsusing this tion from the stratosphere to the troposphere. The differenceof criterionis generallyimpossible.In our radiativetransfermodelozone amount below and above the ozonopauseis even larger ing, all surfaceand atmospheric parameters exceptfor the ozone (Table 1). The secondfeatureis the differencein the tropopause andtemperature profileswerefixed to typicalsurfaceandatmosandthe ozonopauseheights. phericconditions. In orderto performcomparative studieswhere 3.2.2. Case 2, February 2, 1997 and March 6, 1996, Figonly the effectof verticaldistributions of ozoneandtemperature ures 1½and ld. The profilesdiffer only in the heightsof the trois observed,we were obligedto manipulatethe atmosphericsce- popauseandthe ozonopause. narios.Firstly,the full ozoneprofileswere multipliedby a scal3.2.3. Case 3, March 15 and 1, 1996. Figures le and lf. ing factorto havethe sametotal columnvalue.Secondly,the air There are largedifferencesin the troposphericand stratospheric moleculenumber densityprofile of the U.S. StandardAtmos- ozone amounts.In this casehowever, ozone is redistributedmore phere[/tndersonet al., 1986] wasartificiallyappliedthroughout evenlyin the troposphere then for case1. In addition,largedifthe study.Thereis a risk that thosemanipulations may produce ferencesin the tropopauseandozonopause heightsareobserved. somewhatunrealisticozoneandRayleighextinctionprofiles.The More detailsabout the ozone profiles are providedin Table 1. latter manipulationignoresthe effect of temperature on the air Ozoneprofilesfor eachpair were scaledto the meantotal ozone moleculenumberdensity[Schwandere! al., 1997]. We also ig- amountof the pair. The ozonesoundings are reliableonly below norethe influenceof temperatureon relativehumiditywhichmay 30 km, so abovethis altitude,constantmixing ratioswere usedin havean impacton aerosolopticalproperties. The numberof at- both data sets.The secondprofile in each pair is the reference mospheric homogeneous layersandthustheverticalresolution of one, so in all calculationsthe differenceis obtainedby subtractthe radiativetransfermodel was 50. Two separatesetsof profiles ing the valuecalculatedfor the secondprofilefrom the one calmeasuredin Legionowo,Poland,from measurement madein the culatedfor the first profile. In Figure 1 the referenceprofilesare plottedwith a solid line. period1979-1997,wereprepared for thesensitivity study. 4. Results and Discussion 3.1.
Data
Set 1
Data set 1 consistsof a pool of profilesfrom which extreme influences on surface UV radiation and ozone column retrievals
are explored.In total, 50 temperatureand ozone profile pairs were analyzedfor their influenceon the surfaceerythemaldose rate and total ozone column retrieval. In addition, six standard
atmosphericprofilesobtainedfrom Andersonet al. [1986] were used.They correspondto midlatitudesummer/winter,subarctic
4.1.
Surface
UV Radiation
For convenience and/or due to lack of measurements,model-
ersoftenusestandardozoneandtemperatureprofilesfor their radiative transfer calculations.Using the 55 profiles from data set1, we computedsurfaceerythemaldosesfor varioussolarzenith anglesand comparedthe resultsto corresponding results obtainedby usingthe U.S. StandardAtmosphere.
LAPETA ET AL.' SENSITIVITY
OF UV RADIATION
TO OZONE PROFILES
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