Dec 20, 1994 - K.J. Davis, D.H. Lenschow, and P.R. Zimmerman ..... Davis [1992] shows plots of the mean diurnal ...... We thank William Blumen, Jim Green-.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 99, NO. D12, PAGES 25,587-25,598, DECEMBER 20, 1994
Biogenicnonmethanehydrocarbon emissionsestimatedfrom tethered
balloon
observations
K.J. Davis, D.H. Lenschow, and P.R. Zimmerman NationalCenterfor Atmospheric Research, Boulder,Colorado
Abstract. A new techniquefor estimatingsurfacefluxesof tracegases,the mixed-layer gradienttechnique,is usedto calculateisopreneand terpeneemissionsfrom forests.The techniqueis appliedto tetheredballoonmeasurements madeoverthe Amazonforestand a pine-oakforestin Alabamaat altitudesup to 300 m. The observations were madeduring thedry seasonAmazonBoundaryLayer Experiment(ABLE 2A) andthe Rural Oxidantsin the SouthernEnvironment1990 experiment(ROSE I). Resultsfrom largeeddy simulations of scalartransportin the clear convectiveboundarylayer are usedto infer fluxesfrom the balloonprofiles. Profilesfrom the Amazongive a meandaytimeemissionof 3630 q-
1400/•gisoprene m-2 h-i , wheretheuncertainty represents thestandard deviation of the meanof eight flux estimates.Twentyprofilesfrom Alabamagive emissionsof 4470 q-
3300/•gisoprene m-2 h-i , 1740q- 1060/•galpha-pinene m-2 h-1, and790q-560/•g beta-pinene m-2 h-i , respectively. Theseresults arein agreement withemissions derived from chemicalbudgets.The emissionsmay be overestimated becauseof uncertaintyabout how to incorporatethe effectsof the canopyon the mixed-layergradients. The large variabilityin theseemissionestimatesis probablydueto therelativelyshortsamplingtimes of the balloonprofiles,thoughspatiallyheterogeneous emissionsmay also play a role. Fluxesderivedusingthis techniqueare representativeof an upwindfootprintof several kilometersand are independent of hydrocarbonoxidationrate andmeanadvection.
transported longdistances.Partiallyoxidizedhydrocarbons arefurtheroxidizedby OH, andorganicnitrogencompounds Isoprene (C5H8)andthecompounds calledmonoterpenesmay contributeto OH production,sotheseNMHC oxidation (C10H16)are chemicallyreactivetrace gasesemittedin by-productsmay control OH concentrationsfar from the largequantities by trees.Thesenonmethane hydrocarbonsNMHC sourceregions[Madronichand Calvert, 1990]. OH oxidizing (NMHCs)are primarilyoxidizedby the hydroxylradical is the dominantcomponentof the atmosphere's capacity and is the primary oxidant of many trace gasesin (OH). Forsomemonoterpenes, ozone(03) oxidation is also the troposphere, including CO and CH4. important. In thepresence of sunlight andsufficient concenBiogenicNMHC emissionsalsoconstitutea significant trationsof nitrogenoxides(NOx) thisoxidationcanresult portion of the atmosphericcarbonbudget. They are the intheproduction of ozone[Finlayson-Pitts andPitts,1986]. source of at leastas muchatmospheric carbonas emissions Biogenic NMHC emissions areoneprecursor toregional 03
Introduction
pollution eventsin theeastern UnitedStates[Sillmanetal., of methane [Lenschowand Hicks, 1989]. Zimmerman et
1990;McKeenet al., 1991;Traineret al., 1991;Chameides al., [1988] estimatethatisopreneemissionsrepresent2% of etal., 1992].Oxidationof NMHCs byOH inheavilyforested the netprimaryproductivity(the amountof carbonfixedby areascontrolsOH concentrations in the boundarylayer and vegetation)of the Amazon forest. Despitethis, global and regionalemissionsof NMHCs mayproduce a significant amount of carbon monoxide [Zimare not accurately known. Laboratory-based emissionalgomermanet al., 1988;Jacoband Wofsy,1988]. rithms combined with vegetation surveys and climatological Globally,biogenicNMHC emissions mayplayanimpor-
tant role in determiningthe oxidizingcapacityof the at-
data have been used to construct emission inventories for the
mosphere. WhileNMHCsthemselves areshort-lived with United States[Guentheret al., 1994;Lamb et al., 1993]). A
greatdealof uncertainty existsin boththevegetation surveys and the extrapolation of the emission algorithms to treesof products, partiallyoxidized hydrocarbons andorganic nitrovaryingspecies,ages,and growingconditions.Direct meagencompounds, havelifetimes of several daysandcanbe surements of NMHC emissionsover a regionalscalerepresentativeof anecosystem areneededto verify theseemission lifetimesof the order of hours,someof their oxidationby-
inventories.
Copyright1994by the AmericanGeophysicalUnion.
Papernumber94JD02009. 0148-0227/94/94JD-02009505.00
MeasuringNMHC emissions on a regionalscale,however, is difficult. Eddy correlation,the mostdirect flux measurementtechnique,requiresinstrumentation which canmeasure
mixingratioswithgoodprecision at sampling rates>_1 s-• . 25,587
25,588
DAVIS
ET AL.:
BIOGENIC
Suchinstrumentation for NMHCs hasyet to be appliedsuccessfullytoambientconditions.Inferringsurfacefluxesfrom mixingratio gradientsin the surfacelayer (the lowesttens of metersof atmosphere)is difficultin forestedterrainandis representative of afootprintof afractionof a square kilometer in convectiveconditions[Horst and Weil, 1992]. Enclosure techniques may alterthe wind, temperature, andhumidity withintheenclosure and,sinceemissions varygreatlyamong plantspecies, mustbe coupledwith painstaking surveysof regionalflora in order to determineregionalfluxes. It is
NMHC
EMISSIONS
filesarevery nearlyconstant,thatis, "well mixed." The ML makesup mostof theCBL andis of theorderof 1 km deep. Wyngaardand Brost [1984] proposedthat in a quasisteady,Cloud-free,horizontallyhomogeneous CBL, scalar profilesin theML will bedetermined solelybythescalarsur-
faceandentrainment fluxes,(WC)o and(wc)z•,respectively, thedepthof theCBL, zi, andtheconvective velocityscale,
w, = (g(wO,)oZi/O,0) •/3,where (wO,)o isthesurface vir-
tualtemperature flux;19,0is thesurfacevirtualtemperature; andg is gravitational acceleration. Theyalsoproposed that difficult to account for horizontal advection and to measure transportof scalarsfrom thetopof theCBL downward,initiOH mixingratioswhenestimating NMHC emissions using atedby entrainmentfluxes,andfrom thebottomof the CBL regionalchemicalbudgets. upward,initiatedby surfacefluxes,couldbe treatedsepaThispaperpresents themixed-layergradienttechnique for rately and a flux-profilerelationshipfoundfor each. This calculating surfacefluxesof scalarsbasedon meanmixing hypothesiswas written, ratiodifferencesin the well-mixedportionof theconvective boundarylayer. We applythe techniqueto tetheredballoon profilesof NMHCs over forests. Isopreneemissionsare Oz-- --gb zi ziw, zi ziw, estimatedfromprofilestakenovertheDuckeForestReserve, about10 km northof the city of Manaus,Amazonas,Brazil, in JulyandAugust1985 as partof the AmazonBoundary whereC is the meanscalarmixingratio,c represents deLayerExperiment 2A (ABLE 2A) (dryseason) [Zimmerman parturesfrom the mean,andgbandgt are the dimensionless etal., 1988;Harrissetal., 1988].Isoprene andmonoterpenebottom-upand top-downgradientfunctionsof normalized emissions (alpha-pinene andbeta-pinene) areestimated from CBL height. Moeng and Wyngaard[1984, 1989] useda profiles collected overawestcentral Alabama pine-oak forest largeeddysimulation(LES) of scalartransport in the CBL as part of the Rural Oxidantsin the SouthernEnvironments to calculatethesegradientfunctions. (ROSEI) experiment duringJuneandJuly1990. Wecanintegrate theflux-profilerelationship (1) oversome Mixingratiomeasurements ashighasseveral hundred me- verticalintervaland, givenmeasurements of zi, w,, and at ters above the Earth's surface can be used to make surface leasttwo mixing ratio differencesin the ML, solvefor both
OC
z__ (WC)O gt -- •,
(1)
flux estimates, differentiating thistechnique from surface the surface and the entrainment fluxes. If we know the rate of layergradient techniques. Theadvantages ofthemixed-layer growthof the convectivelayer and the meanverticalwind, gradienttechnique arethatfastresponse chemicalmeasure- W, we canestimate theentrainment usinga simplejump mentsarenotrequired,thefootprintof a flux estimatemade model [Lilly, 1968],
usingthetechnique is of theorderof 10 km2, andthealtitudeof themeasurements meansthatthecomplications of (we),,--kat - w ac, (2) a forested surface layerareminimized. Thedisadvantages, inherentin usinghigh-altitude mixingratiomeasurements, arethesmallverticalmixingratiodifferences andlargecon- whereAC is thejumpin mixingratioacrosstheentrainment vectiveeddysizesthatoccurin thewell-mixed layerwhich zone. Then only onemixingratio differencewithin the ML require,respectively, extremely accurate mixingratiodiffer- is needed to estimate a surface flux. We use the jump model in this paperto estimateenencemeasurements andlongmeasurement averagingtimes to accurately determine fluxes.Despitethe apparent oxy- trainment flux because the tethered balloon observations are moron("mixed-layergradient"),it hasbeenshown[Mahrt, mostlyin thelowerpartof themixedlayer,givinglittle in-
ioz, )
flux of theNMHCs. Also, 1976; Wyngaardand Brost, 1984] that measurablevertical formationaboutthe entrainment gradients do existin themixedlayer(ML). Becauseof their becausetheir lifetimesare of the orderof hoursor less,we very low backgroundvalues,NMHCs are ideal candidates canassumethattheirmixingratiosareroughlyzeroabove the CBL. for reliabledetection of thesegradients. Davisand Lenschow[1994] discussthe rangeof atmoSection 2 describes theboundary layermeteorology which whichsatisfytheassumptions necessary is thefoundation of thetechnique.Section3 presents the sphericconditions chemicaland meteorological observations which are used for (1) and the LES resultsto be valid. They showthat aresynonymous withthecommon assumphereto calculate emissions of NMHCs;section4 presents theseconditions theresultsof thesecalculations,andsection5 is a discussion tion of a linearturbulent flux profilein the CBL. During of these results. middayunderclear to fair weathercumulusconditions,in an environmentwhichis fairly homogeneous on scalesof a few kilometers, and for scalars with lifetimes somewhat
Mixed Layer--Flux Gradient Relationship
longerthantheconvective eddyturnovertime(zi/w, • 15 minutes),flux estimates madeusingthistechnique should The mixed layer (ML) is that portion of the convec- bevalid.Theypresent observations whichshowagreement tive boundarylayer(CBL) dominated by convectiveeddies withtheLES gradient functions to aboutq- 50% accuracy; whosescalesarecomparable to thedepthof theentireCBL. scatter in theaircraftdataprevents a moreprecisecompariThenamemixedlayercomesfromthefactthatverticalpro- son.
DAVIS ET AL.: BIOGENIC NMHC EMISSIONS
Observations
Amazon Boundary Layer Experiment 2A (ABLE 2A)
25,589
Meteorologicalobservations. Convectivelayerheight andsurfacevirtualpotentialtemperature fluxbotharenecessaryto derivefluxesfromtheNMHC profiles(seeequation (1)). Theentrainment velocity,equalto theconvective layer
growthrate in the absenceof mean vertical wind, is needed Nonmethane hydrocarbon profiles. Twenty-five teth- to parameterize the entrainment usingthejump model(2). eredballoonprofilesof hydrocarbons werecollected,18 dur- Surfacemomentumflux measurements (•-•0) areneededto ing the day and 7 at night, over the Ducke ForestReserve determine theminimumappropriate samplingaltitudes(see as part of the dry seasonABLE 2A conductedby the Na- section4). tional Aeronauticsand SpaceAdministration.Zimmerman The convectivelayerheightswereestimatedfromthetemet al., [1988] describe the tethered balloon data in detail.
perature,humidityandwindprofilesgatheredby thetethered balloonin betweenthe hydrocarbon profiles. The tethersonderegularlytook measurements up to 1000 m. When the CBL height grew above 1000 m, radiosondeobservationsandaverageCBL growthratesduringABLE 2A were balloon was then lowered to the next altitude and another usedto extrapolatemixed-layerheight[Martinet al., 1988]. 10-L samplecollectedin a differentTeflonbag. Four sam- The CBL height, zi, was chosenas the lowest altitudeat pleswere collectedfor mostprofiles,typicallyat 305, 152, whichthe temperature profileshoweda persistent change 61, and30 m aboveground, thoughtwoprofilesreplacedthe from well-mixed to subadiabatic conditions. The convective 305-m samplewith oneat 915 m. The forestcanopyheight cloudlayer, whichoften developedin the afternoons,was wasabout30 m. The entireprofilecollectionsequence took not consideredpart of the CBL. about30 min. At the groundthe samplesweretransferred The CBL growthrate was determinedfrom the change into stainless steel canisters. in CBL heightobservedin soundings beforeand after the NMHCs, CH4, andCO wereanalyzedat theNationalCen- hydrocarbon profilewascollected.If appropriate soundings terfor Atmospheric Research in Boulder,Coloradousinggas werenotavailable,meanmorningandafternoon growthrates chromatography with flameionizationdetection.The analy- calculatedby Martin et al., [1988] were used. sisprocedures aredescribedby Greenbergand Zimmerman The surfacetemperature flux wasestimatedfrom the work [1984] and Heidt [1978]. The upperboundsin the uncer- of FitzjarraMandStormwind [1988].Eddycorrelation meatainty of the NMHC measurements are reportedto be 0.09 surements were madeat 39 m aboveground, roughly5 m partsper billion by volume (ppbv) at concentrations of 2 abovethe forestcanopy,usinga sonicanemometerand a ppbv,0.018 ppbv at concentrations of 0.2 ppbv,and 0.004 fine-wirethermocouple.Latentheat fluxesweremeasured ppbvat concentrations of 0.02 ppbv.The absolutedetection usinga kryptonhygrometer andmomentum fluxesusingthe limit of the NMHC analysiswas0.002 ppbv isopreneand sonicanemometer. Thesefluxesweremeasured duringsix 0.001 ppbvterprenesfor a 1-L sample. of theeightballoonprofilesusedto estimateisoprene emisIsopreneprofilesused for flux calculationsare listed in sions. Mean diurnalfluxesfor the periodfrom July 30 to the work of Davis [1992]. Only daytimeprofileswhere August5 wereusedfor missingflux measurements. two or more mixing ratio measurementswere within the Thesetowermeasurements arerepresentative of a signifML were usedfor flux calculations.Two daytimeprofiles icantly smallersurfaceareathan are tetheredballoonobserwere excludedbecauseof thunderstorm activity (July 27) vations.The experiment area,however,wasfairlyhomogewhichdisruptedthe CBL. Ten otherdaytimeprofileswere neouson a large scaleandthe NMHC flux estimatesare not excludedbecausetheydid not containat leasttwo mixing extremelysensitive to thetowerfluxes,sowe do notexpect ratio measurements within the ML. Our definition of the ML themismatchin footprintto posea seriousproblem. will be discussed in section 4. Meteorological variableswhich were used for the flux Terpenefluxeswerenotestimated.Terpeneprofileswere calculations are listed in Table 1. Those entries which are frequentlypoorlymixed;theirmixingratiosvariedasmuch notdirectobservations andaretakenfromseveraldaymeans as an orderof magnitudewithin the convectivelayer. The are noted. terpenefluctuations appearedto be closelycorrelatedwith compounds suchas tolueneand ethyl-benzene.Greenberg Rural Oxidants in the Southern Environment (ROSE) A balloonwith a radio-controlled air-samplingsystemand meteorologicalinstrumentationwas raised to its maximum altitudeabovetheforestcanopy.A 10-L air samplewasthen collectedin a Mylar-covered Teflonbagin about2 min. The
and Zimmerman [1984] note similar variations in the con-
centrationof aromatics from airbornesamplesaboveBrazil and conjecturethat high concentrations may be the result of biomass burning.WithintheCBL a highlyspace-and/or time-dependent terpenesource,suchasbiomass burning,too closeto the samplinglocationto be mixedby the time it is advected to thesamplebags,couldexplainsuchprofiles.Terpenes,whicharevolatilecompounds storedin wood,might be boiledout as a functionof temperature, while emissions of isoprene, linkedmoredirectlyto plantmetabolism, might not be greatlyenhanced by burning.The isopreneprofiles show little or noneof the large fluctuationsevidentin the terpenedata.
Nonmethane hydrocarbon profiles. The ROSE I experiment,sponsored by theNationalOceanographic andAtmosphericAdministration(NOAA), took place duringthe summerof 1990 in westcentralAlabamanear the juncture of Sumter,Choctaw,and Marengocountiesand the Mississippiborderat a loblollypineplantation.Grab samplesof air werecollectedfrom a tetheredballoonsystem.Threeimportantdifferences distinguished ROSE I hydrocarbon samplingfrom ABLE 2A. DuringROSE I, Teflonsamplebags andpumpswereplacedon thetetherat multiplealtitudesand the pumpswere controlledwith automatictimers. The samples were collectedsimultaneously,so vertical correlation
25,590
DAVIS ET AL.'
BIOGENIC
NMHC EMISSIONS
Table 1. ABLE 2A MeteorologicalMeasurements
Time, Date
LST
July25
0917
July26
1415
July 27
0832
July29 July29 August 2
1040 1448 1250
August4 August5
0820 0831
zi, a
19Zi/19t,
(W0•)0 ' b,c
m
cm s-•
K m s-•
b,d
0•,0•a
ms -!
K
m
611
5.1
0.065
303
1.09
0.38 g
65
1000 f
1.Of
0.067
306
1.29
0.36
54
315
4.4
0.017
301
0.56
0.20
36
950f 1360 f 1110 f
6.2f 1.0f 1.0f
0.076g 0.052g 0.043
307
1.32
0.43 g
82
308
1.31
0.45 g
137
307
1.15
3.3 5.9
0.019 0.047
300
0.56 0.84
0.40 g 0.24 g 0.32 g
116
277 387
302
54 54
ABLE 2A, AmazonBoundaryLayer Experiment2A. LST, Local StandardTime. azi and 0•0 data from tethersondemeasurements.
t'uoand(w0•)0datafromtowermeasurements byFitzjarrald etal.,[1988]. Cl Kms-• = 1156W m-:.
%. =friction velocity --(-•"•o)«. eL= Obukhov length = -zi•3./l•w3.,where k isthevonKaxman constant. fValues fromMartinetal., [1988]r •Mean valuesfrom Fitzjarrald eta!., [1988].
shouldreducethe error variancein the mixingratio differences.Second,the hydrocarboncontentof the sampleswas analyzedin the field [Greenberget al., 1993]. Finally, the 10- to 15-L sampleswerecollectedusinglow volumepumps over about15 min insteadof the 2-min samplingtimesfor theAmazon. Horizontaladvectionduringtheextendedsampling time shouldprovide significantlygreaterhorizontal averagingand lesserror variancein the mean mixing ratio measurements.
Forty-oneprofiles were collectedduring daylight from June29 to July 19, 1990, one at night. Collectionaltitudes variedfrom the surfaceto 350 m abovethe ground. Seventeenof the daytime profilesdid not containat least two pointswithin the ML andhencewereunsuitablefor flux estimates.Four profileswere eliminatedbasedon the absence of a developedCBL. The forest canopyheight was about 15 m and the treeswere primarily pine and oak with some sweetgum. Davis [ 1992] containsthe20 profilesof isoprene, alpha-pinene andbeta-pineneusedin flux calculations.
theboundarylayer. On July 17, radarobservations indicated a CBL neverdeveloped,sono NMHC profilesfromthisday wereusedfor flux calculations.Stratiformrain persistedon thisday. Surfacetemperature, temperature flux, latentheatflux,and momentumfluxeswere measuredfrom a 15-m-hightower usingthe eddycorrelationtechniqueby R. T. McMillen of NOAA's AtmosphericTurbulenceand Diffusion Division from June30 throughJuly 7. The tower, locatedabout75 m from the tethersonde,also contained instrumentationfor
measuringwind speed,wind direction,net radiation,ozone mixing ratio, and ozone, carbondioxide, and latent heat fluxes. Table 2 containsthe meteorologicalobservations which enteredinto flux estimatesfor the isopreneand terpeneprofiles.Davis [1992] showsplotsof themeandiurnal sensibleheatflux andfrictionvelocityduringa week in Alabama derived from R. T. McMillen's
observations.
Analysis
Meteorological observations. NOAA's Environmental Assessmentof Basic Assumptions Technologies Laboratoryoperateda 915-MHz Dopplerradar near Ward, Alabama duringROSE I to monitorboundary The conditionsderivedby Davis and Lenschow[1994] layer developmentfrom June8 throughJuly 20. The radar describewhen(1) canbe usedfor estimatingfluxesbasedon
waspointedto zenithandrecordedreflectivityandDoppler dataup to 4 km with a 3-min temporalresolution.A 4-kHz sodarprobedthe surfacelayer up to 200 m. WhiteandFairall [ 1991] showthatthesignalto noiseratio fromtheradarreflectivityshouldbeproportionalto theatmosphere'srefractiveindex structurefunction.A maximumin the refractiveindex structurefunctionis causedby the temperatureinversionandwatervapordecreaseacrossthe CBL top. The radardatathereforecanbe usedto observethe CBL growth.Whiteand Fairall [ 1991] showplotsof radarsignal to noiseratio whichillustratethe patternof convectivelayer growthoverthe courseof the day. As well as themaximum signalto noise(SNR) ratioat CBL top,a secondpeakin SNR
Variationsin the virtual potentialtemperatureflux and convectivelayerheightovertimewill modifyscalarprofiles. If the timescalefor changesin theseCBL parameters is very short, where short means less than or comparableto the convectiveturnovertime, convectivemixing may not be rapid enoughto maintaina quasi-steadyprofile. Martin et al. [1988] and Fitzjarrald and Stormwind[1988] show the typical diurnal evolutionof CBL height and sensible heat flux, respectively,from ABLE 2A. White and Fairall
at about 2500 m altitude is sometimes observed in the after-
local standardtime (LST), the timescales,T, for variations
noonandmay be dueto convectivecloudspenetratingabove
in theseparametersare certainlylongerthanthe convective
mixingratio profiles.We now assess the applicationof (1) to the Amazon and Alabama forests.
[1991] andDavis [1992] showthe samevariablesfor ROSE
I. Inspectionof theseplotsshowsthatbetween0900and1500
DAVIS ET AL.: BIOGENIC NMHC EMISSIONS
25,591
Table 2. ROSE I MeteorologicalMeasurements
Time,
Zi, a
Date
LST
m
cm s- •
K m s- •
K
m s- •
m s- •
m
June29 June30 June30 July 3 July 3
1210 0910 1235 0840 0925
1600 637 1700 457 560
0.0 7.8 5.6 4.4 3.3
0.167d 0.135d 0.154d 0.117 0.143
306d 303d 306d 303 304
2.05 1.41 2.03 1.20 1.37
0.46d 0.44d 0.46d 0.24 0.31
46 48 49 9 16
July7 July8 July9 July9 July10 July10 July11 July11 July13 July15 July15 July16 July16 July18 July19
1200 0950 1100 1400 1015 1315 0910 1200 1215 0900 1500 0930 1500 1100 1100
1050 390 640 775 775 1185 635d 1425 d 1390 610 2230 600 2050 690 1280
6.7 3.3 1.9 0.0 6.1 0.6 5.4d 4.0d 4.4 6.7 0.0 4.7 2.2 2.8 4.2
0.167d 0.148 0.181d 0.130 d 0.162 d 0.155d 0.135d 0.167 d 0.167d 0.135d 0.090d 0.147d 0.090d 0.181d 0.181d
306d 304d 305d 307d 304d 307d 303d 306d 306d 303d 307d 304d 307d 305d 305d
1.78 1.23 1.55 1.48 1.59 1.80 1.41 1.97 1.95 1.39 1.86 1.42 1.81 1.59 1.95
0.46d 0.41 0.48d 0.49d 0.47d 0.50d 0.44d 0.46d 0.46d 0.44d 0.49d 0.43d 0.49d 0.48d 0.48d
46 36 46 71 49 63 48 46 45 49 103 43 103 46 46
ROSE, Rural Oxidants in the SouthernEnvironment azi data from radarmeasurements,White and Fairall [ 1991].
b0•,0, uo,and(wO,)odatafromtowermeasurements byR.T.McMillen, National Oceanographic andAtmospheric Administration AtmosphericTurbulenceandDiffusionDivision,Oak Ridge,Tennessee. c1Kms-l_1156Wm -2.
dValues arediurnal means, dailymeasurements notavailable.
precursors andavailablesunlight.Table turnovertime,zi/w,, andthiscondition will be satisfied. ratiosof chemical 4 lists the chemical lifetimes of isoprene, alpha-pinene, and Outsideof thesehoursit is possible thatscalarprofilesmay beta-pinene as well as the reaction constants for their oxidabe perturbed morerapidlythancanbecompensated for by tionby 03 andOH. In theAmazonthelifetimeof roughly turbulentmixing. cansafelybeconsidered a conThe chemicallifetimesof isoprene,alpha-pinene,and 3 hoursmeansthatisoprene turnover beta-pinene are sufficiently shortthattheymustbe evalu- servedscalarwithrespectto thetypicalconvective atedto seeif theycanbe treatedasconserved scalars.OH timeof about15 min. In Alabama,higherlevelsof oxidants hydrocarbon lifetimes, about80min. and03 aretheprimarysinksof thesehydrocarbons with implyshorter
isoprene oxidation beingdominated by OH. Thechemical
Assuming thata givenmixingratioof hydrocarbon is
lifetimeof a hydrocarbon oxidizedby OH and03 is given advectedfrom the bottomto the top of the mixed layer at roughly w, -- 1.5m s-1andthatthehydrocarbon reactivity by 1
r - ko.[OHl +ko•[O31'
is described by a simpledecaywith lifetime(3), we can
a crude estimateof the effect of chemicalreactions (3) make on the calculatedsurfaceflux. Given an initial isoprene
we wherekoI•andkO3aretherateconstants for thehydrocar- mixingratioof 2.5ppbv,typicalfor theseobservations, calculate thechange in mixingratioatz/zi - 0.5,a typical tively. Table3 showsrelatively high03 andOH mixing maximum altitudefor theseobservations.We then compare tothedifference predicted dueto ratiosmeasuredandestimated,respectively, duringABLE thismixingratiodifference
bon reactionswith concentrations[OH] and [O31, respec-
(1)givenatypical isoprene surface fluxof0.5ppbv 2A andROSE I. The OH concentrations arequiteuncertain transport sincetheywerenotmeasured butestimated fromthemixing m s-1 . We find that chemicalreactionswould increasethe Table3. Boundary LayerOxidant Concentrations DuringABLE2A andROSEI
Daytime Concentrations, molecules m-3 Oxidant 03
ABLE 2A
4.8 x 1017[Gregory etal., 1988] 1 x 10•2[JacobandWofsy, 1988]
ROSEI
1.2 x 10•8(R.T. McMillen,unpublished data,1990) 2 x 1012[Montzkaetal., 1993]
25,592
DAVIS ET AL.: BIOGENIC NMHC EMISSIONS
Table 4. ReactionRatesandLifetimesfor IsopreneandPinenes -1
Reaction Constants, m3 molecule-•s
Species
ko.a
Isoprene 1.01x 10-•6 Alpha-pinene 5.32x 10-l? Beta-pinene 7.82x 10-1?
ko3 b 1.4x 10-23 8.4X 10-23 2.1x 10-23
Lifetimes, hours 7'Amazon
7'Alabama
2.6
1.3
3.0
1.3
3.1
1.5
aAtkinson,[ 1985].
bAtkinson and Carter,[1984].
verticalgradientby anorderof 20% givenan80-minreaction time and 10% given a 3-hour reactiontime. Systematic errorsin theresultingflux calculationswouldbe of the same magnitude. Gao et al., [1993] have done a more detailed analysisof the surfacelayer abovea forestcanopy. They showresultswhichindicatethattheeffectof OH on isoprene profilesdecreasessignificantlywith heightabovethe forest canopy,andtransportprocesses dominatetheverticalprofile. Heterogeneityin surfaceemissionsmay also violate the mixed-layerconditionsnecessaryfor (1) to be valid. One problemariseswhen wind shearcreatesvertically varying advectionof a horizontalmixing ratio gradient. Surface roughnesslengthsof 1 to 2 m typical of temperateconiferous forests and the Amazon [Shuttleworth, 1989] could
causesignificantwind shearup to 50 and 100 m, respectively [Davis and Lenschow,1994]. Large changesin surface emissionsupwind of the samplinglocation therefore couldperturbthelower altitudesin the balloonprofilesfrom the convectivelymixed assumption.Sinceno observations of variability in surfaceemissionsor upwind mean mixing ratio differencesin thesecasesare available,it is impossible to concludewhetheror not this effect will be significantin
thanunity on scalesof severalkilometers. This analysis givesa similarresultfor theAmazonduringABLE 2A. Timbercompanymapsanda satellitesurveyusingLandsat5 multispectralscannerdata(C. Ennis,personalcommunication,1992) showkilometer-scale patchesof coniferous anddeciduoustreesin thevicinityof theROSE I site. Since coniferous treesaregenerallyterpeneemittersanddeciduous treesisopreneemitters,this speciesvariabilitycouldcreate significantheterogeneityin surfaceemissions.In the Amazonthespecies diversityis muchgreater,butthesespecies are randomlydistributedthroughoutthe forestand that largerscaleheterogeneitydoesnot exist. A majorproblemwith theprofilesfrombothexperiments is samplingtime. The Amazonsamplinglength(2 minute
samplex 5 m s-•, or about600m) is onlycomparable to thetypicallengthscalesof turbulencein theCBL. Lenschow and Stankov [ 1986] found from aircraft observationsthat the
integral scale of scalarfluctuationsin the ML is a few to
several hundred meters, increasing as(z/zi)•/2.Since inte-
gratingacrossmanyintegralscalesis necessary to determine a meanmixing ratio accurately,the Amazonmeasurements sufferfrom relativelypoorsamplingstatistics.An individual either the Amazon or Alabama. ABLE 2A balloonprofileshouldnotbe expectedto produce The flux gradientrelation(1) canbe violatedif a horizon- an accuratesurfaceflux estimate. The ROSE I sampling (15 minx 2 to 3 m s-• equals2 to 3 km) arealso tal differencein the verticalmixingratiogradientof a scalar lengths is advectedbeforeconvectioncan verticallyredistributethe insufficient.Thesesamplingproblems,however,shouldbe scalar. Heterogeneityin surfacefluxescan createsuchhor- randomandaverageoutwithrepeatedprofiles.An ensemble a total izontal differencesin scalarprofiles. This effect is difficult of 10 profilesfrom ROSE I, for example,represent to evaluatedueto a lack of informationregardingthe spatial sampleof 20 to 30 km of advectedboundarylayer, many variabilityof emissions or mixingratioprofiles.It ispossible, timesthe integralscalefor scalarfluctuations.We proceed however, to set limits on tolerable levels of surface emission with the expectationthatthe ensemblemeanfluxeswill conheterogeneity.The limit where surfaceflux heterogeneityis tain statisticallysignificantinformation. We notethataveraging overmanyprofilesintroduces varinot importantis [Davis and Lenschow,1994] abilitydueto thetemperature andlightdependence of NMHC a(wc)o L•w, ranges 1 unless notedotherwise.1000ttg m- h- isoprene-- Zimmermanet al. [1988]. Theypredictedaverageisoprene emissions of3170/•gm- 2h- 1from0600to 1800LST.This 0.118ppbvm s-• atPair -- 1 kgm-3.
DAVIS ET AL.: BIOGENIC NMHC EMISSIONS
Table 6. ROSE I NMHC
25,595
Surface Fluxes
Time,
Fluxes, t•gm-2 h-•
LST
Isoprene
June29 June30 June30
1210 0910 1235
4530 - 1050 -11450
10530 -3100 -740
-2220 - 1050 -250
July3 July3 July7 July8 July9 July9 July10 July10 July11 July11 July13 July15 July15 July16 July16 July18 July19
0840 0925 1200 0950 1100 1400 1015 1315 0910 1200 1215 0900 1500 0930 1500 1100 1100
-320 2430 - 10070 50330 10980 10530 800 -23050 6140 -2090 1710 3360 - 1000 5190 17260 24400 710
740 7520 -3700 3180 12630 -1830 -3120 -3160 760 - 1370 650 -200 1840 790 4000 9030 260
370 4750 -4190 2240 6650 3200 1100 -910 540 -760 180 160 0 210 1040 4680 0
Date
Alpha-pinene
Beta-pinene
Mean fluxes
4470 3300
1740 q- 1060
790 q- 560
Mean fluxes,
3120 2460
1340 q- 850
560 q- 440
including10-m displacement height NMHC, Non-MethaneHydrocarbon.
Calculated using nodisplacement height andonlypoints where-z/œ >_1unless notedotherwise.
1000t•gm-2 h-• isoprene -- 0.118 ppbv ms-• atPair -- 1 kgm-3. 1000t•gm-2 h-• alphaor beta-pinene --0.059ppbv ms-• at/:>air ---1kgm-3. estimate has the same uncertainties as that of Zimmerman
emissionestimatesof Zimmerman et al. [1988] and Jacob
described by theirestiet al. [1988] thoughthe chemistryis moresophisticated. andWofsy[1988],theareaemissions It shouldbe notedthat this daily flux estimateis actually matesshouldbe a functionof the lifetime of isoprene,about about50% higherthanthat of Zimmermanet al. [1988] 2.6 hours,timesthe mean advectionvelocity,roughly5 m fetchof about45kmis a potential source sinceit encompasses 12hoursof emission compared to eight s-• . Theresulting of differences between the emission estimates derived using for Zimmermanet al. [1988]. Sinceisopreneemissionis these techniques. stronglydependent on temperature andsunlight[Guenther The fluxes derivedusingthe convectivelayer gradient etal., 1991,and 1993;Tingeyet al., 1979],theearlymorning showlittleto nodiurnalpatternin emissions preandearlyeveninghours(0600-0800and1600-1800)should technique and sunlightdependence of isocontribute anegligibleamounttothedailyisoprene emission. dictedby the temperature Theballoonprofiles,however, donotcover JacobandWofsy'sdailyflux,distributed from0800 to 1600, preneemissions. thehoursof thedayevenly,beingconcentrated-at 0800-0900 wouldbe4750•g m-2 h-•. and 1400-1500 hours, and the uncertainty in the individual JacobandWofsy's [ 1988]estimateof approximately 4750 flux estimates makes detecting a diurnal pattern unlikely. tzgm-2 h-• fallsat theupperrangeof theuncertainty in our results. Zimmerman's et al.
[1988] result falls well
within the rangeof our estimates.Generalagreementof thisresultwith twoindependent fluxestimates lendsvalidity to the expectation thatthe meanfluxesfrom manyprofiles computed usingtheconvective layergradienttechnique can providea reasonable fluxestimate, evenif thefluxesderived fromindividualprofilesshowedlargerandomvariability. This flux estimateshouldbe representative of emissions over a regionextendingseveralkilometersupwindof the samplingsite [Weiland Horst, 1992]. Sincethe oxidation rateof isoprene is thephysicalprocess whichdetermines the
ROSE I Isopreneand TerpeneFluxes The flux estimatesfrom ROSE I profiles,counterto our
expectations basedonthesignificantly longersample collection timesandsimultaneous samplecollection,showmuch morevariabilitythanthoseestimated fromABLE 2A data. Severaleffectscouldberesponsible for thisvariability.First, windconditions werenearlycalm,sotheadvected horizontal
sampling lengthwasnotgreat(thoughstilllongerthanduringABLE 2A). Second, theObukhov lengthwasgenerally
25,596
DAVIS ET AL.: BIOGENIC NMHC EMISSIONS
11 July, 0910 LST - ROSE 1990 ''1
.........
I .........
I .........
I .........
I .........
theuncertaintyaboutdisplacementheightis not soimportant an issue for ROSE I fluxes as for ABLE
2A results.
Alpha and beta-pineneprofiles,in contrastto ABLE 2A, seembetterbehavedand more consistentthan the isoprene profiles.Mean derivedemissionfluxesare 1740 4- 1060/•g
0.8
m-2 h-• for alpha-pinene and790 4- 560/•g m-2 h-• for beta-pinene,with 77 and 71% of those valuesthe results, respectively,whenthe displacement heightis used. Montzka et al. [1993] have usedthe one-dimensionalCBL model of Trainer et al. [1987] and the ROSE I data set to
I
studythe oxidationby-productsof isoprene.They adjusted NOx and isoprenesurfacefluxes so that observeddaytime surfacelayerNOx andisoprenemixing ratiosmatchedthose predictedby the model. Montzka et al. [1993] found that
0.4 Surface
Flux
= 6140 gg m'2h'•
0.2
0
I
2
3
4
5
a middaymaximum isoprene emission rateof 4880/•gm-2 h-• providedreasonable agreement with typicalobserved 6
Isoprene Mixing Ratio (ppbv) Figure 1. Observedisopreneprofile and fit from the Rural Oxidantsin the SouthernEnvironmentstudybasedon the flux-gradientrelationship(1). The fit is calculatedfrom the mixed layer gradientderivedfluxesand the large eddy simulation(LES) generatedgradientfunctions. The fluxes
werecalculatedusingonly observations where-z/L
>_ 1
and neglectingthe displacementheight. The observedmixing ratiosare markedwith crosses.Observationsnot used in the flux calculation are marked with asterisks. The hor-
izontal bars throughthesepointsrepresenttheoreticalerror variancesin the mixing ratio measurements calculatedfrom LES predictionsof scalarvarianceas a functionof surface and entrainmentflux [Moengand Wyngaard,1989], observationsof integrallengthscalesin the convectiveboundary layer (CBL) [Lenschowand Stankov,1986] and a statistical descriptionof CBL turbulence[Lenschowand Kristensen,
isoprenemixingratios. Their assumeddiurnalcyclein isoprene emissionis describedby Trainer et al. [1991] and follows the temperaturedependencedescribedby Lamb et al. [1987] andthe light dependence of Tingeyet al. [1979]. This isopreneemissionestimateis in good agreementwith the resultsof the ML gradienttechnique. As with the Amazon emission estimates,the ROSE I fluxes
shownodiscernible diurnalcycle,contraryto theexpectation that middayfluxesshouldbe greatestdue to high temperaturesandmaximumsunlight.Temperaturedatado showthe expecteddiurnalcycle. Any patternis probablyhiddenby the uncertaintyin the individualflux estimates.The ROSE I isopreneemissionestimatesare somewhatgreaterthanthe ABLE 2A emissions,althoughtheuncertaintiesin the mean fluxes as well as the limited and irregular samplingtimes imply thatthe emissionestimatesfrom the two experiments arenot significantlydifferent.As mentionedpreviously,the higherOH mixingratiosandshorterNMHC lifetimesin Alabamamay be increasingthe observedverticalgradientin
1985].
29 July, 1448 LST - ABLE 2A larger during ABLE 2A due to smaller sensibleheat fluxes andslightlyhigherwind speeds,so the increasedvariability maybe dueto larger-scale convectiveturbulence beingmore dominant in ROSE
I over smaller-scale
mechanical
Entrainment
Flux
= 320 gg m'2h'•
0.8
turbu-
lence. Third, it is possiblethat the emissioncharacteristics of the landscape aroundthe ROSE I samplingsitearemore heterogeneous on thefootprintscale(a few kilometers)than the DuckeForestPreserveandsurroundings in theAmazon. The shorterNMHC lifetimesduringROSE I mightbe expectedto causea systematicemissionoverestimatein the ROSE I resultsbut would probablynot add randomvariability. Anotherdifferencebetweenthe two experimentsis thatsamplingaltitudesduringROSE I werequitevariablein contrastto ABLE 2A. It is unlikelythatthiswouldadversely affect the flux estimates.
The meanisopreneemissioncalculatedfrom 20 profiles spanning0830 to 1500 LST, usingonly observations where
.
0.4
Surface Flux h'•
0.2
0.0
ß
0
ß
gm' .
I
2
4
6
Isoprene Mixing Ratio (ppbv) -z/L >_ 1 andno displacement height,is 4470/•g m-2 h-• witha standard deviation of themeanof 3300/•gm-2 Figure 2. Same as Figure 1 but a sampleprofile from the h-•. Including a displacement heightof 10m [Shuttleworth,AmazonBoundaryLayerExperiment2A. The dottedvertical 1989] yields emissionswhich are 70% of this value. The smallerdisplacementheightdue to smallertreesmeansthat
line is addedto highlightwherethe fitted profile increases slightlywith heightindicatinga countergradient flux.
DAVIS ET AL.:
BIOGENIC
NMHC
EMISSIONS
25,597
ROSE I slightly,causingthe emissionto be systematically discerniblediurnal pattern,probablydue to the large varioverestimated by an orderof 20%, comparedto an orderof ability in individual flux estimates.The primary sourceof 10% for ABLE 2A emissions. this variabilityis thoughtto be relatively shortsamplecolOther measurements of NMHC emissions have been made lectiontimesemployedin the tetheredballoonobservations, in the southeasternUnited States. [Zimmerman, 1979; Kno- thoughthe increasein variability in the flux estimatesdeerr and Mowry, 1981; Arnts et al., 1982]. Zimmerman, rived from ROSE I profiles, where the samplingtime was for example,usedenclosures to measureNMHC emissions muchlongerthanduringABLE 2A, contradictsthishypothin the Tampa Bay-St. Petersburg,Florida, area. He esti- esis. Heterogeneityin the surfaceemissionson the scale matedmediandaytimeemissionsfrom a deciduous forestto of a few kilometersmay be contributingsignificantlyto the be4600/•gm-2 h-1 forisoprene and350/•gm-2 h-1 for variability in the fluxes in the caseof the ROSE I results. alpha-pinene. A mixed hardwood-pineforest in the same This hypothesisseemsreasonablebasedon observationsof kilometerscalespeciesheterogeneityaroundthe ROSE I site areaproduced 3500/•g m-2 h-1 of isoprene and700 m-2 h-1 of alpha-pinene. Theseresultsarequitesimilar andthe assumedkilometer-scalehomogeneityof the ABLE to the emissionestimatesderivedabove,with the exception 2A site. Measurementsof upwind variability in emissions thatZimmerman'sresultsshowlower alpha-pineneemission or CBL mixing ratiosare neededto evaluatethis hypothesis rates. and are not availablefrom theseexperiments. Theseresultsshowthatthemixed-layergradienttechnique is a viableway to makeemissionestimatesrepresentative of Conclusion fairly large areas,independentof observationsor estimates of OH mixing ratio, unaffectedby uniform horizontal adThe mixed-layer gradienttechniquehas been appliedto vection,and requiringmuch lesseffort than enclosuremeatetheredballoon NMHC profile measurements rangingup surementsencompassing largeareas.This techniqueshould to 300 m over the Amazon forest and a mixed pine-oak provevaluablefor validatingregionalemissionbudgetsdeforestin Alabama. Estimatesof meandaytimesurfacefluxes rived from emissionalgorithmsbasedon light and temperafrom 8 Amazon and 20 Alabama profilesshowfairly good ture [Guenther et al., 1994; Lamb et al., 1993]. agreement with independent estimates basedonthesamedata using chemical budget arguments,though uncertaintiesin Acknowledgments. K. Davis was supportedby the National how to treatthe issueof displacement heightcouldmeanthat Centerfor AtmosphericResearch'sAdvancedStudiesProgramand ourestimatesrepresenta significantoverestimate,especially theNationalAeronauticsandSpaceAdministration'sGraduateStufor the Amazon data. There is not sufficienthigh-altitude dentResearchersProgram. We thankWilliam Blumen, Jim Greendata to eliminate this uncertaintyby using only data above berg,Alex Guenther,Michael Trainer,Chin-HohMoeng,Jeff Weil, the altitudeof a few canopyheights. Useful entrainment Kent Goodrich, Dave Fitzjarrald, and SashaMadronich and an anonymousreviewerfor their assistance.The National Centerfor Atmospheric Researchis supportedby the National ScienceFounrelativelylow samplingaltitudes,thereforethe entrainment flux estimates
cannot be made with these data due to the
flux is parameterizedusinga jump model. Uncertaintyabout whetheror not to use a displacement height meansthat the Amazon emissionsmay be overestimated by at most a factor of 2 and the ROSE I emissionsby at most about 35%. These are the upper limits of the possibleerror, sincetheseestimatesdo not account for the enhancedtransportobservedimmediatelyaboveforestcanopies.Given thattheLES gradientfunctionshavenot beengeneratedfor a forestedenvironment,thattheenhanced scalartransportjust abovea canopyappearsto nearlycancel the effect of adding a displacementheight, and that these profileswere collectedover small clearings,the estimates madewithouta displacement heightare believedto be more representative of the true emissions. In the future, either higheraltitudeballoonsamplesor gradientfunctionswhich describethe ML abovea forestcanopyare neededto avoid thisuncertainty. ROSE I mean isopreneemissionsare found to be nearly 25% largerthanABLE 2A emissions,thoughthe difference may not be significantin the light of the large standarddeviations of the mean emission estimates. Chemical
lifetimes
dation.
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acceptedAugust2, 1994.)
