Deposition of ozone to tundra

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Oct 30, 1992 - We identify periods for which MO similarity is applicable by comparing the ..... Cowan [1968] near Barrow, Alaska, in August 1966. These au-.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. D15, PAGES 16,473-16,479, OCTOBER 30, 1992

Depositionof Ozoneto Tundra D. J. Jacob,S.-M. Fan, S.C. Wofsy,P. A. Spiro,andP.S. Bakwin Departmentof EarthandPlanetarySciences andDivisionof AppliedSciences, HarvardUniversity, Cambridge, Massachusetts

J. A. Ritter, E. V. Browell, and G. L. Gregory NASALangleyResearchCenter,Hampton,Virginia

D. R. FitzjarraldandK. E. Moore Atmospheric Sciences ResearchCenter,StateUniversity ofNew Yorkat Albany

Verticalturbulentfluxesof 03 weremeasured by eddycorrelationfrom a 12-m high towererectedover mixedtundraterrain(dryuplandtundra,wetmeadowtundra,andsmalllakes)in westernAlaskaduringtheArctic BoundaryLayerExpedition(ABLE 3A). The measurements weremadecontinuously for 30 daysin July-

August 1988.Themean03 deposition fluxwas1.3x l0TM molecules cm-2 s-1. Themean 03 deposition velocitywas0.24cms-1 inthedaytime and0.12cms-1 atnight.Theday-to-night difference in deposition velocity wasdrivenby bothatmospheric stabilityandsurfacereactivity.The meansurfaceresistance to 03 deposition

was2.6s cm-1 in thedaytime and3.4 s cm-1 atnight.Therelatively lowsurface resistance atnightis attributedto light-insensitive uptakeof 03 at dry uplandtundrasurfaces (mosses, lichens).The smallday-to-night differencein surfaceresistance is attributedto additionalstomataluptakeby wet meadowtundraplantsin the daytime.Flux measurements from the ABLE 3A aircraftflyingoverthe towerarein agreement with the tower

data.ThemeanO3deposition fluxto theworldnorthof 60øNin July-August is estimated at 8.2x 10x0

molecules cm-2 s-1, comparable in magnitude tothe03 photochemical lossratein theregion derived fromthe

ABLE 3A aircraftdata. Suppression of photochemical lossby smallanthropogenic inputsof nitrogenoxides couldhavea majoreffecton 03 concentrations in the summertimeArctictroposphere.

1. INTRODUCTION

photochemistry as a sinkfor 03 in the Arctic,thensuppression of thephotochemical sinkis of little consequence. Concentrations of 03 in the Arctic troposphere have increased We reporthereeddycorrelationmeasurements of 03 deposition

by -- 1%yr-1 overthepasttwodecades, withthelargestincreasesfluxesto tundraduringtheABLE 3A expedition.Tundraoccupies observedin summer[Logan, 1985; Oltmansand Komhyr, 1986]. Anthropogenicinfluencewould be a logical explanationfor these increases. However, aircraft measurementsduring the Arctic BoundaryLayer Expedition(ABLE 3A) in July-August1988 showedthat03 in theregionwasdominantlyof stratospheric rather than of pollutionorigin [Browellet al., this issue;Gregory et al., thisissue].Concentrations of nitrogenoxides(NOx) in ABLE 3A were 10-50 ppt [Sandholmet al., this issue],sufficientlylow thatphotochemistry shouldprovidea net sink for 03 [Jacobet al., this issue]. The budgetof 03 in the summertimeArctic troposphereappearsto be regulatedmainly by input from the stratosphere,andlossesfromphotochemistry anddeposition. One possiblemechanismfor anthropogenic perturbationto 03 levelsin the Arctic troposphere is by partialsuppression of the photochemical sinkdueto smallenhancements of NOx. The presenceof NOx at the low levelsobservedin ABLE 3A may have sloweddownthephotochemical lossrate of 03 by a factorof 2.5 relativeto a NO,,-freeatmosphere [Jacobet al., thisissue]. A sub-

stantialfractionof theNO,,appeared tobeanthropogenic [Singhet al., this issue]. The sensitivityof the regional03 budgetto changes in thephotochemical lossratedepends howevercritically on the rate of loss by deposition.If depositiondominatesover

42% of total land north of 60øN [Matthews,1983]; the efficiency of 03 uptakeat tundrasurfacesis thereforean importantelement in constructinga tropospheric03 budgetfor the Arctic. Experimentalmethodsaredescribedin section2. Surfaceresistances for 03 depositionto tundraare derivedin section3. Flux measurementsfrom tower and aircraftare comparedin section4. The 03 depositionflux to the Arctic in summeris estimatedin section5, and is comparedto the photochemical lossrate of 03 computed from the ABLE 3A aircraftdata. Conclusionsare in section6. 2. EXPERIMENTAL METHODS

The measurements were made from the top of a 12-m high towererected40 km northof Bethel,Alaska,in theYukonDelta National Wildlife Refuge (61ø05.41 ' N, 162000.92 ' W). The measurements were madecontinuously for 30 daysfrom July 14 to August12, 1988. Flat tundraterrainextendedfor severaltens of kilometersin all directionsaroundthe towerand consistedof a finemosaicof dry uplandtundra(lichen-moss), wetmeadowtundra(watersedge), andsmalllakes. The heightof the tundracanopy was5-15 cm, with shrubsup to 1 rn in heightin someof the wetterareas. The footprint(or fetch) sampledby the tower at 12rn altitudeextendedfrom 50 to 1000 rn upwindaccordingto Gaus-

Copyright 1992 bythe American Geophysical Union.

sian plume calculations [Fan etal.,this issue]. Thedistribution of

surfacetypesin the towerfootprintwas inhomogeneous, as shown in Figure1. Two lakesoccupiedabouthalf of the footprintin the

Paper number 91JD02696.

NE sector, whiletheSEsector wasrelatively dry. Wetmeadow

0148-0227/92/91JD-02696505.00

tundrawas most abundantto the west. The most frequentwind 16,473

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JACOB •r A•.: Diwosmo•TOFOzo•m To TLr•RA

DAT

NIGHT E

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Lake Wel Meadow Tundra I

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BLANK= Upland Tundra

Fig. 1. Map of surfacetypesaroundthe ABLE 3A tower,constructed by D. Bartlett(Universityof New Hampshire)from satellite

datawith20x20m2 resolution. Thetower islocated attheorigin.Winddirection frequencies areshown fortheO3fluxmeasurementperiods,withtheradiusof each45ø-widewindsectorproportional to thenumberof hourlymeanobservations in thatsector.

directionwas SSW in the daytimeand WNW at night,but the cenceinsmmaent with a 90% response time of 0.8 s [Gregoryet variancewaslarge(Figure1). al., 1983,1988]. The hnstrument gainwasobtained by cornparisTheverticalturbulent fluxof O3,F, wascomputed fromtheco- on with a Dasibi 1003-AHUV photometer.Data for w andC

varianceof fast-response measurements of verticalwindvelocity wereacquired at 8 Hz. Timedelaysin theO3concentration meas(w) andO3 concentrations (C) over an averaging time At = 60 urements weredetermined by turningon andoff an Oa generator rain: (Hg vaporlamp)placedneartheinlet,andby analyzing thecrossAt

F:•tlw'C'dt

correlationfunctionbetweenw andC [Fan et al., 1990].

(1)notContributions tothe O,flux from frequencies >0.6 Hz could be resolveddue to the responsetime of the Oz insmunent.

The magnitudeof the associated error was assessed usingmeaswhere the primesrepresentdeviationsfrom the mean. The values urementsof the sensibleheat flux, for which the instrument

of w' andC' werecomputed relativeto 4-rainrunningmeanseentcredon the point of calculation.The 60-min averagingtime allowssamplingover a largenumberof eddieswhileprovidingteasonable resolution of thechangein fluxwithtime. Verticalwindvelocitiesweremeasured witha fast-response (10 Hz) three-axissonicanemometer(AppliedTechnologies, Inc.) mountedat theendof a beamextending1 m horizontally fromthe top of the tower. The Oz samplinginlet wasa Teflonrobe0.3 em internaldiameter,locatedon the beam 0.5 m away from the

bandpass exceeded10 Hz. The heatflux dataweresmoothed using a filter with the samebandpass as the Oz instrument[Hicks andMcMillen,1988];on averagelessthan5% of theflux waslost in thedaytime,andlessthan10% waslostat nightunderneutral to moderately stableconditions [Fan et al., thisissue;Fitzjarrald andMoore,thisissue].Lossesundervery stableconditions could not be evaluatedproperlybecausethe fluxeswere small. Additional testswere made that showednegligibleerrorsassociated with selectionof averaginginterval or insmental high-

anemometerandpointingdownwards.The sonicanemometerand frequencynoise[Fan, 1991]. Densitycorrectionswerenotneeded thegasinletwererotatedusinga remotelyoperatedelectricmotor becauseOz mixing ratios (not densities)were measured.The to keep the anemometerupwind of the tower. The coordinate overalluncertaintyon the measurement of F is estimatedto be +

frameforw' wasrotated asdescribed byMcMillen [1988].Con- 15%,andthedetection limitis estimated tobe3x10 • molecules centrations ofOzweremeasured using amodified C2I-I4 lumines-cm-2s-• [Fan,1991].

JACOS m' •t,.: Dva,osrrlo•ov Ozom•xo Ttm•o,

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Wind speed,wind direction,temperature, momentumflux, and sensible heat flux weremeasured continuously at the top of the tower[FitzjarraldandMoore,thisissue].Concentrations of Os andNO,, weremeasured continuously in sequence at eightaltitude levels (11.0, 8.4, 6.0, 4.3, 3.1, 1.4, 0.5, and 0.05 m), with a 30minute time intervalbetweenmeasurements at the highestand lowestlevels. The Os andNOx concentration dataare discussed in detailby Balewinet al. [thisissue].Concentrations of O3 were typicallyin the range 10-30 ppb, while concentrations of NOx weretypicallylessthan0.02 ppb;we canthereforeneglectperturbationsto theOs fluxcaused by adjustment of theOs/NO2photochemicalequilibrium[Lenschow,1982].

All timeswill be givenassolartime (ST), definedby a maximumsolarelevationat noon.Sunrise wasat 0310ST on July14 andat 0415 ST on August12. Solartimelagged3 hoursbehind

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local time.

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3. RESULTS 0.1

:

General Observations o

o

A totalof 673 hourlyaverage fluxesweremeasured duringthe

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sow'•[', houri

expedition. The fluxes are given aspositive when pointing up-Fig. 2.Diumal variations ofthe 03vertical turbulent flux F,the Os con-

wards, following usual convention; thus F isingeneral negative. centration ½,and the Osdeposition velocity Vdat12-m altitude onthe Figure 2 shows themean diurnal variation ofF forthe30-day ABLE 3Atower. Values are means and representative standard deviations

period from July14toAugust 12.Nosignificant secular variation forthe30-day period from July 14toAugust 12,1988. Solar time (STin

of F wasobserved overthatperiod.Themeanvaluewasthetext)isdefined bya maximum solar elevation atnoon. Deposition -1.3x10 n molecules crn -2 s4 . Thestrongest fluxes werein the velocities inthebottom panel arefor(1)thefulldata set(solid line), (2) daytime.

windsfromtheSSWsector only(dotted line),and(3) thereduced dataset

Figure2 alsoshows themeandiurnal variations of the03 con- satisfying criteria ofhomogeneous andstationary turbulence (dashed line). centration at 12-maltitude,C, andof thedeposition velocity,Va - F/C. The 03 concentrations wereminimumin earlymorning

andmaximum inlateafternoon, reflecting theentrainment of03 thetower footprint. Deposition of03 towater surfaces isknown fromaloftduring daytime growth ofthemixed layer[Balavin et tobeslow [Wesely etal.,1981]. Thehighest values ofVdwere in al.,thisissue]. Means andstandard deviations ofVawere 0.24_+therelatively drySEsectors. TheSSW sector contained thelarg0.10cms-• inthedaytime and0.12+_0.10cms-xatnight; the estnumber ofobservations; thediurnal variation ofVaforthat day-to-night difference is significant at the 99% level of sector (dotted lineinFigure 2) issimilar tothatinthefulldataset,

confidence.

indicating thatwinddirection wasnotamajor factor determining

ßhedependence ofV•onwind direction isshown inFigure 3. thediumal variation ofV•. Atmospheric stability andsurface

Values intheNNEsector werelow,possibly duetothelakes in reactivity were more important inthat regard, asdiscussed below.

E

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Fig.3. Dependence oftheOs deposition velocity onwinddirection. Valuesaremeans andstandard errors onthemeans in each 45ø-wide wind sectorfor the full data set.

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JACOB •r AL.: Dm,osmoNor Ozo•m TOTtnvDaA

Selection ofPeriodsof Homogeneous andStationary Turbulence

Interpretation of Va in termsof surfaceproperties is facilitated

100 -

if turbulencein the 0-12 m columnis homogeneous and station-

ary. In thatcase,Va is dependent onlyontheverticalresistance to masstransferbelow 12 m (aerodynamicresistance)and on the

reactivity of O3at thesurface.Theaerodynarm'c resistance canbe computed usingMonin-Obukhov (MO) similarity[Weselyand Hicks,1977]. The residualsurfaceresistance is a characteristic of

50-

thetundraterrainin thetowerfootprint,andcanbe extrapolated to other tundra surfaces.

0

We identifyperiodsfor whichMO similarityis applicable by comparing themeasured windspeedUonsat h = 12 m altitudeto thevalueUMOcomputedfrom similarity:

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Uoes//U140 Fig. 4. Frequencydistribution of the ratio ItOBS/UMO, whereuo]•sis the observed windspeedat 12-maltitudeandUMOis thewindspeedcomputed

u'i

(2a) from Monin-Obukhov similarity using equation (2).Data arefrom the536 hourlyperiodswhenconcurrent measurements of F, u*, andL wereavailable. Ratioshigherthan2 werefoundfor 21 of theperiods(notshown).

with

z-d

(2b)umalvariation of Vaforthisreduced datasetisshown asthe dashed linein Figure2. Valuesarehigherthanin thefull dataset,

particularly atnight,because theMO similarity criterion excludes periods of strongstratification. Themeanday-to-night difference

and

•)M(•)= (1--15•) -1/4

• 0

(2d)atmosphere atnight was animportant factor contributing tothediurnalvariationof Va in thefull dataset.

Hereu* is thefriction velocity, k = 0.4 is thevonKarman con- Surface Resistance toOzone Deposition stant,Zois theroughness height,d is thedisplacement height, L is

theMOlength, and(•M(•)isthestability correction function for Thesurface resistance R•to03deposition was derived from the momentum [Businger etal.,1971]. Values ofu*andL were corn-reduced data setsatisfying MOsimilarity (n= 229)bysubtracting puted from hourly mean tower data formomentum andsensible aerodynamic contributions from thetotal resistance todeposition heatfluxes [Fitzjarrald andMoore, thisissue]; median values ofL R= 1/Va:

were -10minthedaytime and30m atnight. Theroughness

heightmeasured at the towerwas0.5 cm, independent of wind

R•=R-Ra-Rb

(4)

direction [Fitzjarrald andMoore, thisissue]. Thedisplacement Theaerodynamic resistance Rabetween h= 12mand z0was comheight distypically 70-80% ofcanopy height [Brutsaert, 1982],puted from MOsimilarity: and we assme here d = 0.1 m; resultsare insensitiveto the exact

I i ½H(•)az R•=-•Z

value. Figure4 showsthefrequency distribution of theratio/lOB S/UMo

Zo

(5a)

for all hourlyperiods whenconcurrent dataforF, u*, andL were

available (n= 536).Highratios arefound in anumber ofcases,where •h•(•)isthestability correction function forheat[Businger representing strongly stratified conditions. Werequire a 20%fit etal.,1971]: to MO similarity, i.e., a ratioin therange0.8 to 1.2,andareleft

with n---357 hourly periods forwhich MOsimilarity isconsidered

•i(•)=0.74(1 - 9•)-•/2

• 0

(5C)

verified.

Stationarity ofthe03fluxinthe0-12 mcolurn isverified by

comparing F to theaccumulation rateF of 03 in thecolurn:

F=

•t dz i•C(z)

Theboundary resistance Rbaccounts for thetransferof 03 fromZo

(3)to the deposition surfaces, and iscomputed following Wesely and Hicks [1977]:

where C(z) the 03concentration ataltitude z.the We compute F fromthe 03is concentration profilesmeasured at tower at 30-

Rb =•2,(_E_r)2/3 Ds

(6)

minintervals.Adopting ascriterionIFI< 0.2 IFI,we reject40 of

the357hourly periods thatsatisfy theMOsimilarity criterion. where •c= 0.2cm 2sq isthethermal diffusivity ofairand Ds= Another 88hourly periods arerejected because vertical profiles of 0.13cm 2 s-1isthemolecular diffusivity of03. Thecumulative 03 concentrations werenotmeasured (andhence F could notbe contributions ofRa,Rb,andR•tothetotalresistance R areshown computed). inFigure 5 asafunction oftimeofday.Weseethatdeposition is Wearefinallyleftwithn = 229hourlyaverage measurements limited bythesurface resistance atalltimes.Thenighttime values of F representing theactual surface fluxof 03 tothetowerfoot- of R, arerelatively lowbecause strongly stratified periods were printunderconditions whenMO similarity is applicable. Thedi- excluded fromthereduced dataset.

J'ACOB I•TAL.:Daposrno•Ol,Ozo• ro Ttn•,

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Mossesand lichens,which cover most of the dry upland tundra surface,have little internalcontrolover water vapor or CO2 exchange [Oechel, 1976; Lechowicz, 1981, 1982; Chapin and Shaver,1985]. The surfaceconductance for O3 depositionto dry uplandtundramay thereforevary little with time of day. In contrast, the surfaceconductancefor O3 depositionto wet meadow tundrashouldvary stronglybetweenday andnight due to stomatal closureof the wet meadow tundra plants at night. Stoner and

ß

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Miller [1975] measured stomatal and cuticular conductancesfor

water vapor exchangein a numberof wet meadowtundraplants;

R• 2

K.....-: ............... •.•..T...-•-- .--.'T...'T..• .......

.,.•"' ...... ..-...• ..... -"..

0 0

$

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theyfoundstomatal conductances gi in therange0.3-1cms-1 per cm2 of leaf depending on theplant,andverylow cuticular conductances (typically0.025cm s-1 percm2 of leaf). The day-tonight variation of g• resultingfrom the stomatalactivity of wet meadowtundraplantscanbe estimatedsimplyasfollows: zxg•=f^ • g•

SOLAR TIME. hours

(7)

Fig. 5. Cumulativecontributions of the individualresistances R,, Rb, and

Rctothetotalresistance todeposition R-- 1/Va= R, + Rb+ Re.Valueswhere f = 0.3is thefractional areaof wetmeadow tundra in the aremeans foreachhourof theday,forthereduced datasetsatisfying cri- towerfootprint[Fan et al., thisissue],A = 1 is theleaf areaindex teriaof homogeneous andstationary turbulence.

of wet meadowtundra[Miller et al., 1976], and a = 0.6 is the ratio

of themoleculardiffusivities of 03 andH•O neededto scalegi [Hickset al., 1987]. Equation(7) yieldsvaluesfor Ag• in the

Forfurther dataanalysis wereplace R•bythesurface conducrange 0.05-0.18 cms-1,consistent withobservations. Some furthrance g•= lIRa,which provides better statistics (statistics onRc erevidence forastomatal influence on03uptake isoffered bythe areaffected by occasional highvalues). Figure 6 shows the larger values ofAg•intherelatively wetwestern sectors than in dependence ofg•onwind direction, separately fordayand night.therelatively dryeastern sectors (Figure 6). Low valuesare found in the two northernsectors,due perhapsto

the adjacentlake but corresponding to only a smallnumberof

4. COMPARISON WITHAIRCRAFT OBSERVATIONS

points(n = 9). Valuesin the easternsectorsaresomewhat higher

than inthe western sectors, possibly reflecting the drier terrain. 3AVertical turbulent fluxes of03were measured from theABLE aircraftflyingabovethetoweronJuly28 andAugust9 [Ritter We view

the data taken outside the two northern

sectors as

representative ofthemixed tundra terrain inthearea. Means andetal.,this issue]. TheJuly 28measurements consist ofavertical standard deviations ofg•forthat ensemble are0.38ñ 0.16 cms-1 profile of03fluxes inthemixed layer at0945-1040 ST,averaged inthedaytime (n-- 149)and 0.29ñ 0.12cms-1atnight (n= 71).horizontally over a100krnflight track centered atthetower (FigThemean day-to-night difference Ag•is0.09ñ 0.02crns-1, smallure7). Theincrease ofthedownward fluxwithaltitude inFigure

but significant atthe 99% level The relatively high value of of gcconfidence. at night, and the small day-to-

1000

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nightdifferenceAg•, suggestthat 03 depositiontookplacemostly at dry upland tundrasurfaces(either vegetationor the ground). 800 0.6

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-2 o -8 -6 -4 Fig. 6. Surfaceconductmaces for 03 depositionto the ABLE 3A tower footprint, as a function of wind directionand separatelyfor day (open squares) andnight (blacksquares).Valuesare meansand standard errors Fig.7. Verticalturbulentfluxesof 03 measured abovetheABLE 3A tower onthemeansin each45ø-widewindsectorfor thereduced datasetsatisfy- at 0945-1040ST on July28. Measurements fromthe aircraftat fouraltiing criteriaof homogeneous andstationary turbulence.The 00-45ø sector tudes(crosses)are comparedto the concurrent measurement at the tower in the daytimemadthe 315ø-360ø sectorat nighteachcontainonly oneob- (square).Errorbarsindicatemeasurement uncertainties.The line is a fit to servation.Therewereno observations in the 00-45ø sectorat night. the aircraftdataignoringtheanomalous pointat 730 m altitude.

03 FLUX,101•molecules cm-2 s-•

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JACOSETAL.:D•osrnoN OFOZOSSTOTUNDRA

7 reflectsthe entrainmentof 03 at the top of the rapidlygrowing TABLE 1. AverageDepositionVelocityof Oa North of 60øNin Summer

mixedlayer. Linearextrapolation of the aircraftdatato thesurface,ignoringthe anomalous pointat 730 m altitude,suggests a SurfaceType downwardflux 20% higherthan concurrently measuredat the Tundra tower. The differenceis within the uncertaintyin the extrapolation.

The August9 aircraftmeasurements weremadeat 10-15ST duringa seriesof flightlegsat 150 m altitudecriss-crossing the tundra terrain around the tower. The mean 03 flux measured

abovethetowerwas-2.3x10n molecules cm-2 s-1, consistent with concurrenttower measurementsindicating a mean flux of -2.0x10 n moleculescm-2 s-1 .

Deciduous forest Coniferous forest Shrub Grassland Cultivated

Ocean/ice World north of 60øN

ToudArea,% 17.5 10.6 11.2 1.7 0.2 0.1

58.7 100

Deposition Velocity, cms-• 0.20 0.26 0.23 '0.31 0.42 0.43

0.025 0.11

The depositionvelocitiesover each surfacetype are averagesfor July-

Theagreement between thefluxes measured fromtower andAugust referenced to200-m altitude. aircraftsuggests that the surfaceresistances computedin the previoussectionarerepresentative of themixedtundraterrainaround

thetower over ascale ofseveral tens ofkilometers. Weattempt in molecules cm -2s-1forthe0-6kmcolumn during ABLE 3A.It thenext section toextrapolate oursurface resistance data globally thus appears that deposition and photochemistry provide sinks of to all tundra surfaces.

5. OZONE DEPOSITION TO THE ARCTIC

comparablemagnitudefor O3 in the summertimeArctic troposphere.The average0-6 kanO3 columnconcentration in ABLE

3A was6.4x1017 molecules cm-2 s-1 [Gregory et al., thisissue],

from which we deducean O3 columnlifetime of 46 days. This lifetimeis sufficientlyshortthatO3 concentrations shouldbe highTo our knowledge,the only measurements previouslyreported of thephotochemical sinkby smallanfor O3 depositionto tundraare thoseof KelleyandMcTaggart- ly sensitiveto perturbation thropogenic enhancements of NO,,. Cowan [1968] near Barrow, Alaska, in August 1966. These authorsmeasuredverticalprofilesof O3 concentrations and wind 6. CONCLUSIONS speedsin the 0-4 m column,and derivedO3 fluxeswhen conditions were presumedneutral (as diagnosedfrom the temperature Depositionfluxes of O3 were measuredat 12-m altitudeover profiles). Ten flux measurements were reportedwhich ranged mixed tundra terrain in western Alaska during ABLE 3A. The

from -7x10n to 8x10n molecules cm-2 s-1, corresponding to fluxwas1.3x10 I1 molecules cm-2 s-a for a 30deposition velocities in therange-0.4 to 0.6 cms-1. Threeof the meandeposition

ten flux values were positive (upward), stronglysuggestiveof a measurementproblem. We estimatehere the mean O3 depositionflux to the world north of 60øN in July-Augustby extrapolatingour surfaceresistancedata to all tundrasurfaces,and usingliteraturedata to estimate O3 depositionto othersurfaces.Table 1 showsthe distribution of surfacetypesnorthof 60øN [Matthews,1983]. Tundraaccountsfor 18% of the total surface(42% of the land). Surface resistancesto tundra are taken from section3 as 2.6 s cm-1 in the

day period in July-August1988. The mean depositionvelocity

was0.24 cm s-1 in the daytimeand0.12 cm s-1 at night. The day-to-nightdifferencein depositionvelocity was driven by both atmosphericstabilityandsurfacereactivity. A reduceddataset wascompiledfor periodssatisfyingcriteria of homogeneousand stationaryturbulence. From this reduced data set we derivedaveragesurfaceresistances for O3 deposition

to tundraof 0.26s cm-1 in thedaytime and0.34s cm-x atnight.

Most of the O3 depositionappearedto take place at dry upland diurnal variation in surface daytimeand3.4 s cm-x at night. Surfaceresistances to other tundra surfaces,with no 'xtgnifw•at resistance. The small day-to-night difference of surfaceresistance vegetatedland surfacesare computedfollowingWesely[1989] as is attributed to additional stomatal uptake of O3 by wet meadow the sum of stomatal,cuticular,and groundresistancesplacedin parallel;the formulationsgiven by Wesely[1989] for theseresis- tundraplantsin thedaytime.

tartees depend onlocal temperature and solar irradiance, which are The03fluxes measured atthetower were consistent withflux obtained fromageneral circulation model simulation with4øx5 ø measurements fromtheABLE3Aaircraft flying inthemixed resolution andfulldiurnal cycle [Hansen etal.,1983].Aero-layer above thetower. Thesurface resistances to03deposition dynamic resistances over allland surfaces arealso computed fromcomputed fromthetower datamaytherefore be viewed as thegeneral circulation model using equations (5)and(6).Fixedrepresentative oftheterrain surrounding thetower overascale of deposition velocities of 0.025cms-1 areassumed overoceans tens ofkilometers. [Kawa and Pearson, 1989] and over ice[Wesely etal.,1981]. Anaverage 03deposition fluxof8.2x10 lømolecules cm -2s-1 Theregional average deposition velocities computed foreachisestimated fortheworld north of60øN insummer. This value is surface typearelisted in Table1. Deposition velocities overofcomparable magnitude tothe24-hour average 03photochemiforests andtundra areofcomparable magnitude, because themorecallossratein the0-6kancolumn derived bymodeling ofthe lifetimeof 03 efficientstomataluptakeby forestsin the daytimeis balancedat ABLE 3A aircraftdata. The resultingatmospheric

night byhigh cuticular resistances and bythestability ofthecano-inthe0-6kmcolumn over theArctic insummer isestimated at46 pywhich inhibits transfer totheground. Deposition velocities days. Partial suppression ofthephotochemical sink bysmall anover shrub, grassland, and cultivated land arerelatively high but thropogenic inputs ofNOx could possibly explain thesecular in-

observedin the Arctic troposphere the corresponding areasare small. The averagedeposition veloci- creaseof 03 concentrations over the past two decades. ty northof 60øNis estimated to be 0.11 cm s-1 (referenced to 200 m altitude).Assumingan 03 concentration of 30 ppbat thataltiAcknowledgments.This work was supportedby National Science

tude, based ondata fromABLE 3Asurvey flights [Gregory etal., Foundation grants NSF-ATM-8858074 and NSF-ATM-8921119 toHat-

thisissue], weobtain anaverage deposition fluxof8.2x10 lø yard University, byaPackard Foundation Fellowship toD.J.J., byanAlex-

molecules cm-2sq . Bycomparison, Jacob etal. [thisissue] cal- ander Host Foundation Fellowship toP.S.B., and bygrants from theTroculated a 24-hour average 03 photochemical lossrateof8.0x10 lø posphedc Chemistry Program oftheNational Aeronautics andSpace Ad-

JACOB ETAL.:DEPOSlTIO•OFOzo•m TOTUNDRA

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ministrationto participatingscientists.Helpful discussions with K. Lechowicz,M. J., The effectof climaticpattemon lichenproductivity: Bartlett(University of New Hampshire),and field and logisticalsupport Cetraria cucullata(Bell.) Ach. in the arctictundraof northernAlaska,

from J. Hoell and J. Drewry (NASA-Langley), are gratefullyack-

nowledged.

Oecologia(Berlin),50,210-216,1981.

Lechowicz, M. J.,Ecological trends in lichenphotosynthesis, Oecologia (Berlin), 53, 330-336, 1982.

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