Vertical profiling and determination of ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 103, NO. D19, PAGES 25,519-25,532, OCTOBER 20, 1998

Vertical profiling and determinationof landscapefluxes of biogenicnonmethanehydrocarbonswithin the planetary boundary layer in the Peruvian Amazon DetlevHelmig,• BenBalsley, •KennethDavis,2LauraR. Kuck,• Mike Jensen, • JohnBognar, • TyrrelSmithJr.,3Rosaura Vasquez Arrieta, 4RodolfoRodr5guez, 4 and John W. Birks •

Abstract.Verticalprofilesof volatileorganiccompounds (VOCs) withinthe convectiveboundary layer (CBL) weremeasuredat a tropicalforestsitein the PeruvianAmazon duringJuly 1996 from a tetheredballoonsamplingplatform.A profiling techniquebasedon the collectionof VOCs onto solidadsorbent cartridgeswasusedto take samplesat altitudesup to 1600m aboveground.VOC analysiswasperformedby thermaldesorptionwith gaschromatographic separationandmass spectrometric andflame ionizationdetection.A total of 26 VOCs were stmcturallyidentified. VOCs were dominatedby biogeniccompounds.Highestconcentrations were observedfor isoprene,followedby c•-pinene,p-cymene,and [3-pinene.Combined,all monoterpenes accountedfor approximately15-20% of the total carbonfrom biogenicVOCs (BVOCs). The isopreneoxidationproductsmethacrolein (MAC), methylvinylketone (MVK), and3-methylfuran were observedthroughoutthe CBL. Besidestheubiquitouschlorofluorocarbons, anthropogenic VOC concentrations were at the lower end of concentrationrangesobservedin rural air. From the verticalprofiles,BVOC surfaceflux estimateswere derived.Emissionrateswere estimatedfrom five verticalprofilesusingthe mixed-layergradientand CBL budgetmethods.Emissionestimates

varieddepending onmethodandchoiceof statistics, butwerewithin3000-8200•tgcompound m-2 h-• forisoprene, 120-370•tgm-2h-•for ct-pinene, 40-75•tgm-2h-•for [3-pinene, about16 •tgm-2 h-•forp-cymene, and40-50•tgm-2h-•for camphene. Thechanges in theratiosof MAC andMVK to isoprenewith altitudewereutilizedto estimatethemixing timesbetweenthe surfacelayer, mixed layer and lower troposphere.

1. Introduction

burning have been investigated thoroughly [Levine, 1991]. Extensiveexperimentalcampaigns[Thompson,1996] have been The importanceof tropical forestsin balancingthe Earth's conductedin tropicalregionsto addresstheseissues.Someof the atmospherehas long been recognizedin balancingthe energy most comprehensivestudies performed include Atmospheric andchemicalcompositionof the Earth'satmosphere. On a global Boundary Layer Experiment [Harriss et al., 1988, 1990], scale the tropical forests'constituteone of the most important Dynamique et Chimie de l'Atmosph•re en For•t Equatoriale biomes for biosphere-atmosphere interactions. Scientific [Fontan et al., 1992], Biomass Burning Airborne and experimentsin the past have focusedon the exchangeof heat, SpaceborneExperimentin the Amazonas[Kaufmanet al., 1992], water, and trace gasesbetween the forest and the atmosphere. SouthernAfricanFire-Atmosphere ResearchInitiative[Lindesay Specialattentionhas been given to the flux of carbondioxide in et al., 1996], Transportand AtmosphericChemistryNear the an effort to understandthe capacityof the tropicsfor mitigating EquatorialAtlantic [Andreaeet al., 1996], and Experimentfor the observedrise of atmosphericcarbon dioxide mixing ratios. RegionalSourcesand Sinks of Oxidants. The rapid conversionof tropical forestsinto alternativeland In an effort to extendour understanding of the role of tropical usageis a major concern.In particular,the effects of biomass forests on atmospheric greenhouse gas concentrations and atmospheric oxidants, a rather small-scale experiment was conductedjointly by the University of Piura, Peru, and the •Department of Chemistryand Biochemistry and Cooperative CooperativeInstitute for Researchin EnvironmentalSciencesat Institute for Research in EnvironmentalSciences,University of the Universityof Coloradoin July 1996 in the PeruvianAmazon. Colorado, Boulder. The emphasisof this experimentwas the measurementof fluxes 2Department of Soil,Water,andClimate,Universityof Minnesota, of greenhousegases and biogenic hydrocarbonsbetween the St. Paul. 3ChemistryTechnology Department,TRW, Redondo Beach, tropical forest and the atmosphere.Besides the analysis of California. volatile organic compounds(VOCs) reported here, additional 4Facultad deIngenieria,Universidad dePiura,Piura,Peru. real-time, in situ measurementswere made for temperature, relative humidity, atmosphericpressure,wind speedand wind Copyright 1998bytheAmerican Geophysical Union. direction, and ozone. Evacuated glass flasks were remotely sampled for analysis of carbon dioxide, methane, carbon Papernumber 98JD01023. 0148-0227/98/98JD-01023509.00

monoxide, nitrous oxide, 8•3CO2, 8C•802, hydrogen and 25,519

25,520

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

sulfurhexafluoride. Resultsof those measurements are reported elsewhere(J. Birks et al., manuscriptin preparation,1998). Verticalprofilesof relevanttracegasesweremeasuredfrom a tethered balloon samplin•gplatform, and surfacefluxes were derivedfrom the profile databy the mixed-layergradient[Davis et al., 1994] and mass balance techniques[Guentheret al., 1996c]. Airbome gas and flux measurements have a numberof advantages over ground-based measurements:They are representative of a largeregion(estimatedfootprintareafor the

2. Experiment 2.1. Site Description

The experiment wasconducted duringJuly7 - 15, 1996,at a research sitein thePeruvian Amazonlocatedapproximately 500 km westof Iquitos(Figure1) andapproximately 50 - 200 km from the eastern Andes mountain ranges (4ø35'15"S/ 77ø28'00"W).Accessto thisregionis only possibleby boator helicopter. The site is within a mainlyprimarytropicalforest

tetheredballoonmeasurements is •, 100 km2) and are better with an averagecanopyheightof approximately 20 - 30 m. The suitedto determinethe overall VOC loading of the atmosphere. degreeof' anthropogenic disturbance to the forestin this areais Furthermore, interferencesand contaminantsfrom the activities low; somesmall clearingswithin a 10 km radiusfrom the site at the experimentalsitearelesssignificant. account for less than an estimated 5% of the total land cover. The Tetheredballoon techniqueshave been used previouslyfor samplingof VOCs in the lower troposphere [Zimmermanet al.,

balloonlaunchsitewasa clearingin the forestextending over

approximately 100 x 300 m. The clearingareahadbeenclear-cut approximately1 - 2 years prior to the experiment,and revegetation wasin progress. Duringthe experimental periodno biomassburningwas incidentwithin the experimental region. This observationwas confirmedby video recordingsof the Teflon bags with small, battery-poweredsampling pumps. surrounding areatakenfrom the balloon. Sampleswere either analyzedupon returnto the groundby gas chromatography on a field instrumentor alternativelytransferred 2.2. Balloon Platform into stainless steelcanistersfor shippingand lateranalysis.Some A 43 m3 tetheredhelium balloon (The Blimp Works, of the disadvantagesof this techniquefor field use are the relatively high costsof canisters,the spacerequirementsand Statesville,North Carolina)wasusedfor verticalprofilingwithin weightof the canisters, costof shipping,andtheamountof labor the convectiveboundary layer (CBL). Only the buoyant lift for the canisterpreparation,sample transfer,and analysis. capacityof the balloonwas used(approximately20 kg) sinceit Furthermore, losses have been observed for a number of was sufficientto lift all analytical equipmentutilized for this chemicalspecies,suchas for heavierand morepolar VOCs in experiment.For a winch we applied our techniquepreviously sampling bags and canistersduring sampling and storage usedfor kites [Balsleyet al., 1992, 1994a,b, 1998;Knappet al., [Andinoand Butler, 1991; Berkley et al., 1991; Wang et al., 1998]: A pickuptruck was rentedlocally, the rear of the truck 1996]. In an effort to circumventthese drawbackswe have was put on jack stands,and a modifiedcapstanattachedto the developeda new approachutilizing a solidadsorption technique rear axle was used as the main winch. A small, 12 V electric which minimizes some of the above listed constraints. take-up reel was used for spooling the tether. This winch

1988; Andronacheet al., 1994; Davis et al., 1994; Guentheret al., 1996a,c] Most methodsthathavebeenappliedin the pastfor the measurement of VOCs from balloonplatformswerebasedon the wholeair samplingprinciple.Air sampleswerecollectedinto

Figure1. Map depicting theresearch siteneartheRS.oMarafi6nin theheadwaters of thePeruvian Amazon.

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

techniquehas the advantageof allowingthe deploymentof tetheredballoonswith a high lift capacitywithout having to provideanappropriate high-powered electrical winch.TheCBL heightwasdeducedfrom the in situmeasurements of relative humidity,temperature, and ozone.Using the data of these

25,521

thesampling packageis illustrated in Figure2. Samplingflow rates

wereapproximately 300mL min-1overa 30 minsampling period

yieldingsamplingvolumesof.approximately 9 L. The vent of the pumpwascollectedintoa Teflonbag.Uponretumto theground,a precisemeasurement of the samplingvolumewas obtainedby parameters, VOC andgreenhouse gassamples werecollected at pumpingthebagvolumethrougha wet gasmeter(GCA/Precision heights of 1.2,0.8,0.4,0.2,and0.1timestheCBLheight.Actual Scientific,Chicago, Illinois). This method proved to be more sampling heightsabovegroundwerededuced fromreadings of reliable than the previoustechniquesused for sample volume and provideda precisionof the orderof betterthan the pressure sondeandare estimated to havean accuracy of + determination 2%. 5%. Up to five of thesepackageswere flown simultaneously. In addition,one groundsamplewas collectedat 2 m heightabout75 m upwind from the balloon launchsite to avoid contamination 2.3. VOC Measurements from the siteoperation.

VOC samplingwas accomplished by pulling sampleair In order to prevent possiblelossesof unsaturated VOCs by through multi-stage solid adsorbent cartridges with reactionwith ozone [Helmig, 1997], air sampleswere collected programmable and battery-powered pumps(BuckI.H. pump, throughan ozonefilter upstreamto the adsorbentcartridge.Ozone BuckInc.,Orlando,Florida)attached to theballoontetherline.In thistechnique the accurate controlandmeasurement of the air

filters were prepared by soaking glass fiber filters (Gelman Sciences,Ann Arbor, Michigan) in a 5% solution of sodium volumesampled through theadsorbent cartridge is decisive forthe thiosulfate(Na2S203)followed by dry purge in a streamof calculation of VOC concentrations. Previous work has nitrogen.Ozone filterswere testedbeforeand after the field trip. demonstrated thatthe precisemaintenance of constant flow rates Ozoneremovalefficiencyat ambientlevels (20 - 60 ppb) was > duringthesampling aloftis ratherdifficultbecause thesampling 95% and did not show any reductionafter the field use of the flow rate is influencedby parameters suchas (1) changesin filters.The overallozonescrubbingcapacitywasmeasuredto be in

temperature, (2)changes in atmospheric pressure, (3) changes in air excessof 1 m3 ambientair. Furthertestsestablished quantitative humidity, (4) different flowresistance of individual cartridges, and VOC recoveriesof standardssampledthrough the filters: No (5) voltage dropof thepumpbattery duringsampling. All these compoundlosseswere observedfor isoprenestandards(low ppb variablesinfluencethe constantflow stability.To overcomethese range) as well as for the componentsof a 55 multicomponent constraints, a newsampling technique wasdeveloped. A scheme of hydrocarbon standard(C2-Ci•, 10 - 15 ppb).This standard contains saturatedhydrocarbons,alkenes,and aromaticcompoundsand is used within the International Hydrocarbon Intercomparison Experiment[Apel et al., 1994]. In previouswork [Helmig et al., 1998a]we have alsodemonstrated that selectedmonoterpenes are notretainedby thisozonescrubbertype.

TL

TT

PPF TT ssu

•tC

IF

....................... ......... $P

TT

TB

While previouslymostof our samplingwasdoneon Carbotrap300 (Supelco,Bellefonte,Pennsylvania)type adsorbentcartridges (a three-layermultibedadsorbent systemconsisting of CarbotrapC, Carbotrap,and CarbosieveS-III [Helmig and Greenberg,1994; Hehnig, 1996]) Air Toxics type (Supelco)adsorbenttubeswere usedin thisstudy.This dual-bedadsorbent trapcontainsCarbotrap andCarboxen1008. We have found similartrappingand recovery propertiesfor the Air Toxicstubesas for Carbotrap-300;however, the flow resistanceof the Air Toxics tubesis significantlylessand morereproducible,which is a major advantagein the application with thebattery-powered samplingpumps.Thesecartridgesallow VOC analysisin the volatility rangeof approximatelypropaneto dodecane.After collection,samplingcartridgeswere sealedwith Swagelokcapsandkept in Teflonjars underpurifiedair. The jars were storedin ice chestsandshippedto the Boulderlaboratoryfor

analysis. Uponarrivalin Boulder,samples werestoredin a freezer at-80øC and analyzedwithin 6 weeks.Numerousstudieshave shownthatunderthesestorageconditions stabilityof the adsorbed VOCs is achieved[Jansonand Kristensson, 1991;Ciccioliet al., TL

1993;Boehleret al., 1995].

Analysiswas performedby thermal desorptionwith gas chromatographic separationand massspectrometric detection (GC/MS) for compoundidentificationand flame ionization detection(GC/FID; PerkinElmer AutomatedThermalDesorber Figure2. Scheme of theadsorbent cartridge sampling package. The GC/MS hasa special,homeIllustrated components are the samplingpump(SP) with line (ATD-400)) for quantification. attachment (LA) and aluminumrod (AR), 1/8" Teflontubing made injectionsystemfor thermal desorptionand cryogenic of VOCs.Thecoreof theinstrument is a HewlettPackard sampling line (TT), adsorbent cartridge (AC), ozonescrubber focusing

inlet filter (IF), pumpprefilter(PPF), streamselectorvalve 5890, SeriesII GC with a Hewlett Packard5970 massselective detector.VOCs covetingthe volatilityandretentionindexrange (SSV),15L Teflonbag(TB), andballoontetherline(TL).

25,522

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

(DB-1) from approximately 300 (propane)to 1200 (dodecane) can be analyzedon both systems.Other analyticalparametersof the thermodesorption/GC/MS systemhave been reportedin detail previously[D. Helmig, Gas concentration andinjectionsystemfor chromatographic analysis of organic trace gases,U.S. Patent //5,563,352,October8, 1996; Helmiget al., 1998b].Identifications from

the

GC/MS

measurements

were

referred

to

the

FID

hasindicated that13-pinene mayundergo rearrangement reactions in solidadsorption/thermal desorption methods. We havenotyet concludedstudyingthis effect under our conditions.Therefore quantitativedata for [•-pinene shouldbe consideredas a lower mixing ratio limit at this time. 2.4. Flux Calculations

chromatogramsby computing and comparison of linear Surfacefluxes of biogenichydrocarbonswere derived from programmed retentionindiceson both GC columns.On both, the FID and the MS instrument,a dry purge [Helmig and Vierling, vertical mixing ratio profiles using the mixed-layer gradient 1995;McClennyet al., 1995] of the cartridgesfor waterremoval (MLG) method[Davis, 1992, Davis et al., 1994] and a simple prior to the thermaldesorptionwas performedin orderto reduce mixed-layer(ML) budgetmethod[Guentheret al., 1996c].The the amount of water transferred onto the GC column. The ATDMLG method has a footprint which is determinedby the 400 GC/FID parameterswere as follows: cartridgedesorption turbulenttransportin the ML. The budgetmethod footprintis time 15 min, desorptiontemperature300øC, desorptionflow 40 roughlythe lifetime of the chemicalspeciestimes the horizontal mL min-•,microtrapair toxics30øCto 325øC,heldfor 5 min, out wind speed (for a reactive chemical). The upwind fetches split3 mL min'•, columnHP-5, 30 m x 0.32mmx 0.25 gm film, representedby thesemethodsrangefrom a few to 10 km for the ovenprogram-60øC for 3 min,4øCmin-• to 125øC,40øCmin-• to MLG method,and from 10 to 30 km for the ML budgetmethod 250øC, 250øC for 10 min. given a typical lifetime for isoprene. Quantified compounds on the FID were isoprene, 2.4.1. MLG method. Six vertical profiles of isopreneand methacrolein(MAC), methylvinylketone(MVK), 3-methylfuran monoterpeneswere collected during daytime, convective (3-MF), toluene,ethylbenzene,o-xylene, ct-pinene,camphene, conditions(July 12, 13, and 14). All profiles were collected [•-pinene, and p-cymene. Calibration was establishedby during late morning to mid-afternoonhours. A balloon flight dynamic dilution of a two-componenthydrocarbonstandard must contain two or more cartridge sampleswithin the ML containing2,2-dimethylbutaneand n-decane (Scott Specialty (above 50 m and below 0.9 times the depthof the CBL) for the

Gases,Longmont,Colorado)and collectionof 7.5 L samplesof standardin the 0.2 - 5 ppb concentrationrange onto adsorbent cartridges.The FID responsefactor determinedfor the two hydrocarbons was usedfor the quantificationof all listedVOCs exceptfor the oxygenatedcompounds.FID responsefactorsfor MAC, MVK, 3-MF were correctedaccordingto theprocedureby

MLG-methodto be applied.The MLG methodwas appliedto eachprofile to derive an estimateof the surfaceflux. The ML

profile is influenced by entrainment and surface fluxes. Entrainmentis the mixing of troposphericair into the ML as it growsover the courseof the day. Profile samplingfocusedon the lower ML in order to best derive surface fluxes, but dorgensenet al. [1990]. The effective carbonnumbersper entrainmentmust be computedas well to solve the ML flux molecule were estimated to 3.2, 3.2, and 4.2 for MAC, MVK, gradientrelationship[Davis et al., 1994] The flux gradient and 3-MF, respectively. Detection limits for quantitative relationship, originallyproposedby WyngaardandBrost[1984], measurements were of the order of 1-5 parts per trillion by FC volume(pptv) compound.The validity of the calibrationmethod

was confirmedby quantitativemeasurements and recovery experimentsusingthe multicomponentstandarddescribedabove. In anotherexperimentwe diluted the standardwith humidified

zero air to obtainabout80% relativehumidityin the sample. Analysis after dry purge revealed a slight deteriorationof precisionbut no systematicdeviationfor the test compounds. Recoveryand calibrationexperimentsfor the isopreneoxidation products using this solid adsorption technique showed quantitativerecoveryand were conductedby J. Greenberget al. (unpublished results,NationalCenterfor Atmospheric Research, 1995). Because of the limited availability of appropriate standards,so far we have not been able to examine the effect of

2i c3C Fo cg6½/2i) z,gt (2/), C•Z W,Zi W,Zi

.....

proposes that,within the ML, the verticalmixingratio gradient withinthe ML of any conserved scalardC/dz,is determined by

the surface andentrainment fluxesFoc andF:ic, theconvective velocity scale w,, the boundary layer depth zi, and nondimensional bottom-up andtop-downgradientfunction,go andgt, which have beencomputedby Moengand Wyngaard [ 1989],usinglarge-eddysimulations of theCBL. Integrating this flux gradientrelationship overthe altitudebetweentwo mixing ratio measurements and computing entrainment flux

our dry purge procedureon the recovery of the isoprene independentlyallowsoneto solvefor the surfaceflux. Entrainmentflux was computedusinga simplejump model oxidationproducts.At thistime we assumethatthesecompounds exhibit a similar behavior as the other VOCs tested and therefore parameterization[Lilly, 1968]. The jump model of entrainment allow quantificationby this procedure. However, for these requiresthe rateof growthof ML depthandthejump in species reasons,datafor thesecompoundsshouldbe considereda lower mixing ratio acrossthe ML top. The ML depthand the rate of

mixingratiolimit anddatadiscussions andderivedconciusionsgrowth of the ML were derived from the tethered balloon The jump in hydrocarbon mixing ratio acrossthe ML as preliminary in nature. More details on the solid adsorbent soundings.

analysistechniquehavebeengivenelsewhere[Helmig,1996]. Consideringall possiblesourcesfor analyticalerror from the sampling,storage,calibrationand analysissteps,accuracyand precisionare estimatedto be of the order of +10% for isoprene and aromatic compoundsand +20% for monoterpenesand isoprene oxidation products (with the limitations discussed above).Recentwork [Cao and Hewitt, 1993;Arntset al., 1995]

top was taken from direct observationswhen a samplewas collectedabove the ML. If no sampleswere available,we assumedzero mixing ratiosin the troposphere sincethe biogenic VOCs (BVOCs) are relatively short-lived. The observations whichdid existabovethe ML (threeof six profiles)showedthat this is a goodassumption. A crudeestimateof the surfacebuoyancy(virtual potential

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

25,523

temperature)flux, the final input neededfor the MLG method, molecules cm-3wasused.No OH measurements wereperformed during this study. Therefore we utilized this value as an on surface energy balance measurementsover the Brazilian approximateestimate.It was deducedas a daytimemeanfrom a Amazon [Fitzjarrald et al., 1988], we estimatedthat sensible series of OH measurements in 11 available literature references heatflux wasonesixthof incomingsolarradiationandthat latent with an emphasison those measurementsthat were obtained heatflux was onehalf of the incomingsolarradiation.The MLG undercomparableambient conditions.We found this the most even methoddependson the one-thirdpower of the virtual potential feasibleandpossibleapproachfor the given circumstances, temperature flux, sothis crudeestimateshouldbe sufficient. thoughit is expectedthat underthe low NO• levelsexpectedat The MLG equation is fit to the data via singular value thisremotesite,OH will stronglydependon the isoprenemixing decomposition(SVD). SVD provides an estimate of the data ratioitself.For O3a numberdensityof 3.7 x 10II molecules cm-3 precisionrequired to achieve a significantsolution given the = 15 parts per billion by volume (ppbv) was used, which was altitudes of the observed data. This measure of solution derivedas the mean of the CBL ozoneprofile measurements in significance(the inverseof the conditionnumber)showedthat this study.Using theseoxidantconcentrations and OH and O3 five of the six profilesprovidedequally good observations for reactionrate constantsfrom the literature[Atkinsonet al., 1986, solvingthe flux gradientrelationship,but one profile did a poor 1990a,b; Atkinson,1989; Corchnoyand Atkinson,1990; Greene job of constraining the ML flux gradientrelationship.This canbe and Atkinson, 1992], the overall atmosphericlifetimes were explainedby the altitudeof the samples.While most profiles calculatedto be 0.61 hour for isoprene,1.0 hour for ct-pinene, includedmeasurementsat about 0.1, 0.2, and 0.4 times the ML 0.78 hour for [3-pinene,1.2 hoursfor camphene,and 4.1 hours depth,providinga goodpictureof theexpected decrease in mixing for p-cymene.The mixing ratiosof the speciesin the middleof ratiowith altitude,this oneprofilehad only two pointsat about0.4 the ML (0.5 times the ML depth)were estimatedas part of the and 0.7 times the ML depth. Since there are only very small singularvalue decomposition donein the MLG flux calculation. verticalgradientsexpectedat thesealtitudesin the ML, the two Thesemixing ratioswere usedas representative of the ML mean observations providevery little informationaboutthe surfaceflux, value,which is substantiallydifferentfrom the observedsurface and very small differencesin mixing ratio betweentheselevels layer mixing ratio. Fluxes were estimatedfor all five ML imply very large and spuriousflux values.Thereforewe have profiles. excludedthis profile(July 14, 10:35- 11:05 localtime) from our We also performed a budget estimatewhere we did not calculations. assumeconstantBVOC mixing ratios over time but computed 2.4.2. ML budget method. Unlike the MLG methodwhich the rate of changefrom successivemixing ratio profiles. The is quite sensitiveto small differencesin mixing ratio between scalarconservation equationfor this setof assumptions is was derived from the tethered balloon solar radiation data. Based

profilepoints,a ML budget[Guentheret al., 1996c;Denmeade! al., 1996]calculationis morerobust.It is basedon the ML-mean scalarconservationequation[e.g., Garratt, 1992], which can be simplifiedto

OC OC Foc - FC --=-U--+ -ko. OHC-ko• O• C,

ac Foc

-•- = •-zi

koHOH C- ko3 O3C,

and we treat the differential in time as a difference in time

betweenthe two observations. Profile pairs on July 13 and 14 wereusedin thisway. The time intervalsbetweenprofileswere

2, 1.5, and 5 hours. wheredC/dt is the local derivativein the BVOC mixing ratio, U is the mean horizontalwind, and dC/dx is the local gradient alongthe directionof the meanwind. The third termrepresents 3. Results the verticalflux divergence, whichis assumed to be linearwith heightwithin the ML (a good assumption underwell-mixed A total of 15 sampleswere analyzedby GC/MS. Identified

conditions), and the last two termsare chemicallossesdue to oxidationwith OH and 03. ko. and k03are the reactionrate coefficients. Thisapproach suffersfroman inabilityto accountfor horizontal advection, anddepends on an accurate description of the atmospheric lifetimesof the species. For BVOCs,thismeansOH

compounds are listedin Table 1. Additionalinformation given includethe compound linearprogrammed retentionindex[Van denDool and Kratz, 1963] and massspectrum fragmentation. Thirty one balloonprofile sampleswere quantifiedon the FID instrument. A typicalFID chromatogram of a samplecollected

rcmrentraticm.q rnn•qt he well

within the CBIJ at 91 rn altitude above eround is shown in Fieure

known.

which

is rarely the case.

Nevertheless, this methodprovidesa simpleorderof magnitude 3. An additional20 field sampleswere collectedfor blanksand flux estimate.Since entrainmentfluxes are estimatedto be quite

analyticaltests.

small,their effecton the ML budgetis neglectedfor simplicity. Medianmixingratiosfor quantifiedcompounds including Giventhatsimplification andneglect,by necessity, of horizontal biogenic emissions, isoprene oxidation products, and advection, we are left balancingsurfacefluxes,oxidationby OH anthropogenic hydrocarbons are given in Table 2. The dataare and03, andthetimerateof change of thespecies mixingratio.One brokenup intosamples collected at groundlevel(2 m sampling furthersimplification is the assumption of a steadystatespecies height),withinthe CBL (91 - 1167m abovegroundlevel),and mixingratio.Theseassumptions reducethe scalarconservationabovethe CBL (1481 - 1554m abovegroundlevel). equationto

Results from the flux calculations are shown in Table 3. The

FøC =koaOH C+ko, O3C

dataresultedfrom the remainingfive profilesobtainedby the criteriadiscussed above.Meanandmedianisoprene, ct-pinene, [3-

pinene,camphene,and p-cymenesurfaceand entrainmentfluxes calculatedby the MLG method and mean and median surface OH andO3mixingratioestimates: For[OH],a valueof 4.5 x 106 fluxescalculated by the budgetmethodare given. Also included

This allows estimationof surfacefluxes for each profile, given

25,524

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

Table 1. VolatileOrganicCompounds Identifiedin Surface andBalloonSamples at thePeruvian AmazonResearch Site Retention

Retention*

MassSpectrum Fragment Abundance #

Time, min

Index

(% Relative Intensity)

Propane

15.89

301.0

Chlorodifluoromethane

16.38

310.5

29( 100),27( 58),28(40),39( 30),43(23 ) 51(100),31 (19),67(18),35(12),32(11)

Identification

Dichlorodifluororomethane (F-12)

16.95

321.6

85(100),87(31),50(18),35(16),31(14)

Chloromethane

18.09

343.8

50(100)52(10)

1,2-Dichloro-1,1,2,2-tetrafluoroethane

19.74

375.9

85(100),135(50),31(33),39(27),87(26)

1-Butene/2-Methylpropene

20.62

393.0

41( 100),39(67),56(33),40(29),55(23)

n-Butane

21.09

402.2

43( 100),44(53),41(53),39(19),40(8)

Trichlorofluoromethane (F-11)

25.07

482.1

101(100),103(63),35(25),31 (20),47(16)

Acetone

25.28

486.3

43(100),31(31),58(18),44(17),42(16)

Isoprene

26.18

505.0

67(100),39(90),68(83),53(77),40(58)

Dichloromethane

26.85

520.2

49(100),84(51),51(30),86(27)47(24)

1,1,2-Trichloro-1,2,2-trifluoroethane (F-113)

27.40

532.7

101(100),151(77),103(64),85(48),31(45)

Methacrolein

28.35

554.3

41(100),39(94),70(57),38(33),42(22)

Methylvinylketone 3-Methylfuran

29.03 30.46

569.8 602.5

43( 100),55(95)70(29),42(25),39(19)

1,1,1-Trichloroethane

31.71

634.2

97(100),99(62),61(57),32(45),35(30)

Benzene

32.31

649.4

78(100),50(34),32(34),77(33),52(13)

Tetrachloromethane

32.51

654.4

117(100),119(98),35(39),47(33),121(30)

Toluene

36.29

755.5

91(100),92(46),39(28),38(23),63(19)

•x-Pinene

42.27

937.5

93(100),91(59),92(44),77(44),39(40)

et-Fenchene

42.64

949.8

93(100),41(68),91(58),79(56),77(50)

Camphene [3-Pinene p-Cymene

42.71 43.52 44.64

952.2 979.1 1017.6

93(100),41(53),91(51),79(46), 121(44)

Limonene

44.94

1028.3

93(100),68(98),39(94),67(70),41(58)

p-Cymenene

46.38

1079.9

117(100),132(90),91(53),115(51),39(40)

82(100),53(93),39(80),81(70),50(45)

93(100),91(51),41(79),77(53),79(44)

119(100),91(34),134(26),117(18),39(16)

*Linear programmed retention indexonDB-1(6 degC min4). #Mass spectra weredetermined byaveraging approximately fivescans around thepeakmaximum andbackground substraction. Fragments with relativeintensities< 5% are not reported.

are the estimatesderived from the non-steadystate budget deviation.The convectivevelocityscale,computedusingour calculations. estimateof the virtual temperatureflux, was 1.7 +/-0.15 rn/s. The mean computedconvectiveturnovertime was 13.2 +/- 2.5 min. SimilarisopreneverticalCBL profileshavebeenobserved 4. Discussion by otherresearchers in temperateregions[Blake et al., 1992; Isoprene wasthedominant hydrocarbon in samples collected Davis et al., 1994; Guentheret al., 1996a,b] as well as in the at groundandwithinthe CBL (Figure3 andTable2). Near- tropics[Zimmermanet al., 1988; Davis et al., 1994]. Table 4 surface mixingratios(2 m) wereon the orderof 1 - 7 ppbv. showsa comparisonof the isoprenedata from this studywith

Mixingratiosdropthroughout the CBL with increasing height measurements from other sites in the Amazon. It becomes (Figure4). Abovethe CBL isoprenemixingratioswereabout evidentthat isoprenemixingratioswere quitecomparable at the 10% of CBL values. The observedisopreneprofile differentsitesstudiedandgenerallyarein thelowerppbrange. characteristics (gradualdecreasewith height) were consistent with expectedprofiles of a scalarwith a surfacesourceand entrainmentof isoprene-poorair from above. Photochemical oxidationof a species with a lifetimeof approximately 30 min to 1 hourshouldhavea relativelysmalleffecton the shapeof the verticalprofile in a vigorouslymixed CBL, wherethe convective turnovertime (zi/w.) is about 10 min but would tendto enhance the decreasein mixing ratio with altitudeabovethe source.In this experiment,6 tetheredballoon flights duringwell-mixed conditions penetrated theCBL top andtheheightof theinversion

Amongthe monoterpenes observed,the highestmixingratios werefoundfor c•-pinene,followedby p-cymeneandcamphene (Tables 2 and 4 and Figure 3). Other monoterpenes identified were [3-pinene, limonene,c•-fenchene, limoneneandp-cymenene (Table 1). Monoterpenemixing ratioswere about> 20 timesless thanfor isoprene.Combined,all monoterpenes identifiedaccount for approximately15-20% of the total carbonfrom BVOCs in the atmosphere. Vertical CBL profilesfor monoterpenes (Figure5) showsimilarcharacteristics asfor isoprene. MedianCBL mixing ratios are approximately1/3 of ground levels. Mixing ratios was found at 1230 +/- 80 m. These measurements were collected above the CBL frequently were below the method detection over 3 consecutivedays, and the error given is the standard limit. Only few dataon monoterpenes in the Amazonatmosphere

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

o

•

o

10

(-

I

I

=

•

O

25,525

(-

•

.E 0

30

Retention

O

Time

Figure 3. GC/FID chromatogram of a 7.8 L samplecollectedon July 13, 1996,from 12:00to 12.30hourslocal time at an altitudeof 91 m. Identifiedandquantifiedpeaksare labeledwith the compoundidentification.Due to contamination problems,signalsfor n-pentaneand benzeneare larger than representative for the sample composition. Theretbreneitherof theseVOCs werequantified.

[Greenberg and Zimmerman, 1984;Zimmerman et al., 1988] contrast,our sitewas far awayfrom largecities,andtrajectory have been published in the literature and do not allow a conclusivecomparison. In contrast to BVOCs, anthropogenicVOCs show little

analysis showed thatthe•itewasnotimpacted by airfromthe populatedregionswest of the Andes.Instead,air parcelshad traveledlong distancesacrossthe Amazon basin.Thereforeour

differences in mixingratiosnearthe ground,in the CBL and lowermixingratiosfor the alkylatedbenzenes appearreasonable above the CBL (Table 2). The absenceof distinct altitude comparedto the otherresults.

profilesfor thesecompounds infersthatthereareno significant The atmosphericoxidation of isoprenehas been studied emission sources in thisareaandthatsurfacedeposition of these thoroughly [Paulsonand Seinfeld,1992;Grosjeanet al., 1993]. Aftor

mnnxz

xzonrc J

the current understandingof sources and sinks of these hydrocarbons. Emissionsarisemainly from the combustionof fossilfuel, and atmospheric depletionis dominated by reaction with the OH radical. Atmospheric lifetimes of toluene, ethylbenzene ando-xylenefrom reactionswith OH are estimated to be of theorderof 1 - 3 days[Atkinson,1989]Therefore,with a transport time of lessthana weekfrom emissionregions,these

J

nf

lahnratnrxz

ovnor•monfc J

nn

fhsa

nvi4.'•f•nn

'""1'

kineticsand productformation,the presenceof the isoprene oxidationproductsMAC, MVK, and 3-MF has been confirmed

in ambientair. Their ambientmixing ratiosand diurnalmixing ratiotrendshavebeenmeasured in severallocations[Pierottiet al., 1990; Martin et al., 1991' Montzka et al., 1993, 1995; Riemer et al., 1994; Yokouchi, 1994; Goldan et al., 1995; Biesenthalet al., 1997;Helmig et al., 1998a].It hasbeenshown compounds haveatmospheric mixingratioswhichare strongly that ratiosof the two major isopreneoxidationproductsMAC depletedandwell mixedwithinthe troposphere. This is reflected andMVK showa regulardiurnalpatternwhich can be relatedto by the overall low levels observedat this remote site. Our data the differentproductratiosfrom reactionsof isoprenewith OH for the alkyl-benzenes toluene,ethylbenzeneand o-xylene is and ozone and the differences in the rate of the MAC and MVK about1 orderof magnitudelowerthanthe datafrom Greenberg removal processesby OH and ozone during day and night and Zimmerman[1984] andZimmermanet al. [1988]. From the [Montzkaet al., 1993, 1995;Biesenthalet al., 1997,'Helmig et sitedescriptions given in thesereferences,it appearsthat mostof al., 1998a]. Recently,it has also been shownthat during rain th_eirdata were obtained in the vidnity of u_rbanareas. In events,wet depositionis an importantremoval processthat

25,526

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

Table2. MedianMixingRatiosObserved in Surface Layer transport time. This effectis expected to be applicable to the Samples (Sampling Height2 m),Mixed-Layer Samples (Samplingverticaltransport withinthe CBL sinceisoprene arisessolely Height91-1167m) andAbovetheBoundaryLayer(Sampling from surfaceemissions. The verticalprofile data of isoprene, Height1481-1554m) andRatiosof theMediansfor theBiogenic MAC andMVK indeeddisplaythisbehavior.Figure6 showsthe OxidationProductsMethacrolein(MAC) and vertical profiles of the ratios of the MAC/isopreneand MVK/isoprenemixing ratios.Both data setsshow increasesin Methylvinylketone(MVK) to Isoprene the compoundratios with altitude confirmingthe above considerations of the formationroute,atmospheric oxidationand Median Mixing Ratio,ppbv transportof isoprene,MAC and MVK. This trend is further

confirmed by theratiosof medianconcentrations in surfacelayer

Above

Compound

Ground Mixed 'Layer MixedLayer

Isoprene

3.31.

1.39

0.16

•t-Pinene

0.15

0.043

0.014

air, ML air and above ML given in Table 2: The ratios of the

medianconcentations of MAC to isoprene andMVK to isoprene

[5-Pinene

0.013

0.008

< 0.001

Camphene

0.020

0.005

< 0.001

p-Cymene

0.032

0.009

0.007

Methacrolein

0.50

0.27

0.077

Methylvinylketone

0.74

0.60

0.24

3-Methylfuran

0.093

0.080

0.081

Toluene

0.016

0.016

0.018

Ethylbenzene

0.004

0.007

0.005

o-Xylene

0.007

0.010

0.007

MAC/Isoprene

0.15

0.19

0.48

MVK/Isoprene

0.22

0.43

MVK/MAC

1.5

2.2

(andMVK to MAC) steadilyincreasewith altitude. This analysiscanbe takena stepfurtherif we assumethatthe lifetimesof thesecompoundsare constant,and we assumethat

whenisopreneis oxidized,a set fractionof the oxidationbyproductsare MAC or MVK. The evolutionof theseratioswith altitudecanbeusedto estimate thetransport time,or "age"of the chemicalconstituents in theair mass.Thisis of particularinterest in the caseof transportabovethe mixedlayer.Transportwithin the mixedlayeris betterunderstood andcanbe usedto checkthis

technique. The assumptions aboutthemixingratiosare

I(t)=Ioexp(-t/ri)

M(t)=Mo exp(-t/rM)+fMIoO-exp (-t/ri))exp(-t/rM),

1.5

V(t)=Voexp(-t/r•,)+f•,IoO-exp(-t/rl))exp(-t/r•,),

3.1

where I, M, andV aretheisoprene, MAC,andMVKmixing ratios, respectively, Io,Mo,andVoarethesurface layermixing affectsthe atmospheric ratio of MAC/MVK [Helrniget al.,

ratios,x•,M, v are the isoprene,MAC and MVK lifetimeswith

1998a]. Estimateddaytimeatmosphericlifetimesof MAC and MVK

respect tooxidation, t istime,andf•varethefraction ofisoprene

from reactions with OH and ozone are estimated to be 1.9 and

r•,• = 0.6, 1.9,and3.2 hours, respectively, takingtheground levelmedians fromTable2 astheground levelmixing ratios,

oxidized whichbecomes MACandMVK,respectively. Using

3.2 hours,respectively (for [OH] = 4.5 x 106molecules cm'3and [03] = 15 ppb), and thereforeare approximately 3 to 6 times longerthanfor isoprene (0.6 hourunderthe sameconditions). In rural forested regions, where isoprene is the dominant atmospheric VOC, MAC andMVK predominantly arisefromthe oxidationof thissoleprecursor compound. In a givenair plume loadedwith isoprenefrom a primaryemissionsource,the ratios of MAC and MVK to isoprenethereforeshouldincreasewith

andassuming thatfM = f• = a fixed value,the ML andabove mixed-layerratiosfrom Table 2 can be usedto solvethe above equations for thetimet requiredto reachthatratio.

Thevalueforf•,• wasvariedbetween 0.1and0.5(butkept equal)yieldingthe estimates of mediantransport time for isoprenefrom the canopyinto the ML and to abovethe ML

shown in Table5. Values forMVK/isoprene andMAC/isoprene

Table3. Mean,Standard Error(1cff•]n), andMedian Fluxes Calculated bytheMixed-Layer Gradient Method (MLG),Steady State Budget(ss),andNon-SteadyStateBudget(nss)Methods MLG SurfaceFlux

BVOC Isoprene

Mean

•l•]n

MLG EntrainmentFlux

Median Mean

•/•]n

BudgetMethod

Median

Mean(ss) •/,]n(ss) Median (ss) Mean(nss)

7400

7600

3000

370

120

300

8100

•t-Pinene

370

140

120

20

10

10

270

[5-Pinene Camphene p-Cymene

40 10 - 15

65 70 110

45 5 -35

6 2 5

4 1 5

3 2 2

75 50 16

2600 70

35 20 4

8100

8200

220

220

50 40 16

50 -

All fluxesaregivenin gg/(m2h). Fiveprofilesproducing five fluxestimates wereusedfortheMLG andsteadystatebudgetestimate. Three profilepairswereusedfor thenon-steady statebudgetestimates.

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

25,527

1600 o

1400

ß 7-12, 13.43-14.13 a 7-12, 17.15-17.45 •, 7-13, 12.00-12.30

-

x 7-13,14.00-14.30 x 7-13, o 7-14, + 7-14, - 7-14, - 7-15, o 7-15,

1200

1000

8OO

+

15.40-16.10 10.35-11.05 15.10-15.40 20.45-21.15 11.10-11.40 12.53-13.23

x

6OO

4OO

A

+

x

2OO

+

x

• A

0.00

1.00

O

X

2.00

3.00

4.00

5.00

6.00

7.00

Mixing Ratio (ppbv) Figure 4. Cumulativeplot of groundandballoonprofiledatafor isoprene.Flight datesand samplecollection timesaregivenin the insert.

differ, indicatingthatthe choicesof constants may not be correct, eddies, which have an order of 10 min turnover time. This but both ratiosdo predictrealisticmixed-layerair mass"ages," reactivity probably does increasethe vertical gradientto some since convective turnover times in the ML are of the order of 10 extent(chemicallifetimes of 30 rain to 1 hour), hencethe MLG It is obviousfrom the MLG min. We concludethereforethat the estimatesof the age of the fluxesmay be a modestoverestimate. above-ML air are reasonable order-of-magnitudeestimates, flux resultsthatcampheneandp-cymenemixingratioprofileswere despitethe coarseness of our assumptions. These air massages, too small to show any evidenceof surfaceemissions. Any or transport times for ground level air to reach the free emissions werebelowthe level of sensitivityof thismethod.It is troposphere,are rather short, suggestingthat ML air has been difficultto say,giventhe limitednumberof samples,whetherthe ventedrecentlyinto the free troposphere, perhapsby convective meanor medianisopreneand ct-pinenefluxesare more accurate of typicalflux rates. clouds.The samplesalso might be contaminatedby occasional representations thermalswhich extendabovethe meanML depthand reachthe Since the BVOC sampleswere collectedover 30 rain they above-ML cartridge,resultingin a falsely shortestimateof the shouldrepresent,at ambientwind speedsof order 5 m s'l, an transporttime. Suchprofile observations, in combinationwith a integration over several convective eddies, hence a decent of the ML mean profiles.The altitudedistribution model of hydrocarbonchemistry, are a promising tool for representation examiningthe vigor of exchangebetweenthe ML and the free of the sampleswas alsoexcellentfor solvingthe MLG equation troposphere. for a surfaceflux estimate.The precisionin the mean profile, CalculatedBVOC surfaceand entrainmentfluxes using the which is requiredto solve the MLG equation,however,is high, uncertainty experimental approaches detailedaboveare given in Table 3. andsamplinguncertaintywill still leadto substantial Resultsderivedby the threedifferentapproaches detailedabove in the MLG surface flux estimates. Additional sources of (ML gradient,steadystateML budgetandnon-steady stateML uncertaintyare the validity of the nondimensionalgradient budget)agreereasonably well. Isoprene andct-pinene meanfluxes functions,which to date have not beenconclusivelyverified via and deviationsfrom the meteorologicalconditions differsignificantly fromthemedian.This differenceis because of observations, thesmallsamplesize,andonelargesurfaceflux valuecaused by a whichunderliethe largeeddy simulation(LES)-derivedgradient very steepmixing ratio gradient.This profile, however,was functions.Processesnot accountedfor in the MLG theory and collectedduringstronglyconvective, well-mixedconditions. The LES-derived functions include vertically varying horizontal data are distributedin the vertical in a manner appropriatefor advection(horizontaladvectionwhich is uniformwith heightis solvingthe MLG equations, so we retainthis flux estimate.The not a problem), scalar sourcesor sinks within the ML (e.g., BVOCs are assumed to be conserved on the timescales of ML BVOC oxidation),andmodificationsto the ML profile due to the

25,528

HELMIG ET AL.' BVOC FLUXES IN PERUVIAN AMAZON

Table 4. MedianandInterquartile MixingRatiosof Biogenic andAnthropogenic Hydrocarbons (All GroundandCBL Data)Compared With Other Measurements in the Amazon

ThisStudy

Greenberg and

Zimmerman etal.

Zimmerman[ 1984]

[ 1988]a

[1988]b

Khalil[1988]

Median

Median

Range

Median

Mean

[InterquartileRange]

Compound

Isopren e

[Range]

1.69

2.40

2.04

[1.00-5.24]

[1.14-2.72]

0.048

0.10

[0.033-0.088]

[0.05-0.15

13-Pinene

0.008

0.27

0.03

[0.006-0.014]

[0.07-0.54]

[0.01-0.04]

Camphene

0.006

0.03

[0.004-0.012]

[0-0.08]

p-Cymene

et al.

Rasmussen and

Donoso et al.

[1996] ' Mean

[InterquartileRange]

[1.21-2.81] •x-Pinene

Zimmerman

2-4

5.45

1.06; 1.31

0.20

0.10 0.04

0.012 [0.006-0.023]

Toluene

0.016

0.12

0.74

[0.008-0.024]

[0.04-0.19]

[0.35-2.55]

Ethylbenzene

0.006

0.08

0.07

[0.004-0.008]

[0.51-1.01]

[0.02-0.25]

o-Xylene

0.009

0.08

0.04

[0.007-0.013]

[0.04-0.14]

[0.01-0.12]

0.27

0.01 0.04

References are GreenbergandZimmerman[1984], surfacedata,nearManausandHumaita,Brazil;Rasmussen andKhalil [1988],boundarylayer

concentrations, Brazil;Zimmerman et al. [1988]aboundary layerdatato305m,nearManaus, Brazil;Zimmerman et al. [1988]bBrazil,nofurther informationgiven;Donosoet al. [1996] surfacedatafromtwo ruralVenezuelansites.

1600

ß a-Pinene

1400

ß Camphene ß b-Pinene

1200

x p-Cymene 1000

800

600

400

200

0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Mixing Ratio (ppbv) Figure 5. Cumulativeplot of groundand balloonprofile data for the monoterpenes oc-pinene,p-cymene, campheneand [3-pinene. All displayeddatawere collectedduringdaytimebetween10.35 and 17.45 hours localtimeon a totalof eightflights.

HELMIG ET AL.: BVOC'FLUXES IN PERUVIAN AMAZON 1600

25,529

........................................................................................................................................................................................................................................................................ •

, Methacrolein/Isoprene

[] Methylvinylketone/Isoprene

14oo

1200

lOOO

E J:: 800

600

400

200

•C]

0.00

1

0.80

0.40

1.20

1.60

Ratio

Figure6. Verticalprofiles of theratiosfor methacrolein (ppbv)/isoprene (ppbv)andmethylvinylketone (ppbv)/isoprene (ppbv)in thesamedatasetdisplayed in Figure4.

forestcanopy. Untilthegradient functions havebeenvalidated BVOC emissionsare closely tied to tree speciesvariability and via long-termprofileand eddy-covariance flux measurements,that a favored wind direction exists). Again, the standard andtheimpactof forestcanopies andscalaroxidation hasbeen deviationof the individual flux estimatesprovidesan upper limit more completelyevaluated,systematicerrors in BVOC on random error due to sampling. Since determininga mixedemissions estimated from this method could conceivably layer mean is significantlyeasierthan determiningthe vertical approach 50%. The standard deviationof the derivedflux gradientin the meanprofile,the randomvariabilityis smallerfor estimates areanupperlimit(.since naturalvariabilityin emissions this methodthan the MLG method. Systematicerrors,however, is alsocaptured) for the randomerrordueto limitedprofile aremore of a challenge. Improvements in theselarge-scaleemissionestimatescan be sampling. obtained via more profilescollectedover longerperiodsof time. Theuncertainties in thebudgetmethodof derivingthesurface flux, asnotedearlier,aredominated by ourinabilityto compute Collectingfour to six 30-min profiles over 2-3 hours, and on horizontaladvection,andthe uncertainty in our estimateof the severaldays, for instance,would provide a much more precise OH radicalmixingratio.Uncertainty in OH is relatedlinearlyto estimate of surface emissionswithin the limits of possible

systematic errors.Work is underwayto characterize theinfluence of scalar reactivity and forest canopies in the MLG method. probably within50 - 100%of the actualvalue.Horizontal

theuncertainty of thesurface flux estimate. OurOH estimate is

advection is typicallyof similarmagnitude as verticalflux Direct measurements of horizontal advection and OH would divergences in boundary layerbudgets, hencewill causeorder greatlyimproveML budgetflux estimates. For a comparison of the surfaceflux datawith someselected 100% uncertainty for any one budget-based surfaceflux estimate.Both the OH estimateand horizontal advectionerrors

data from the literature, it has to be consideredthat our data

arelikelyto be systematic for a givenlocation (assuming that representdaytime-onlyflux estimates.Daytimefluxes are the Table 5: Estimatesof TimesRequiredfor Air to ReachFrom SurfaceLayer(SL) to the Mixing Layer(ML), and SurfaceLayer to Abovethe ML (AML), Assumingthe SimpleEquations Describedin the Text for the Time Evolutionof Isoprene(ISO), methacrolein(MAC), and methylvinylketone(MVK), and SolvingthoseEquationsto Match the Median Mixing Ratio Data from Table 2.

Ratio

SL to ML TransportTime,

SL to AML TransportTime,

hours

hours

MAC/ISO

0.04 to 0.12

0.29 to 0.69

MVK/ISO

0.17 to 0.36

0.77 to 1.21

Therange oftimes represent estimates forfM=fv= 0.5to0.1.

25,530

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

Table6. DaytimeFluxEstimates forBiogenic VolatileOrganic Compounds FromVerticalProfileMeasurements in theConvective BoundaryLayerin thisStudyandfromLiterature References.

BVOC

ThisStudy*

tx-Pinene

[3-Pinene Camphene p-Cymene

3000-8200 120-370

40-75 40-50 16

Entrainment Flux,

•tgcompound m-2h 4

ggcompound m-2h 4

RasmussenJacoband Zimmerman etal. andKhalil [1988]

Isoprene

SurfaceFlux,

1660

Davisetal.

WoJ•y [1988]

[1988]

[1994]

4060

2670

3630+/- 1400

120

ThisStudy

Davis et al.

[19941

370

420 +/- 110

20

total monoterpenes:

6 2 5

320

References areRasmussen andKhalil[1988],deduced froma dailymeanof 648ggm'2h4;nofurther details available; JacobandWoJ•y [1988]

deduced froma dailymean of 1580•tgm-2h4;Zimmerman etal.[1988]deduced froma dailymean of 1040•tgm-2h4 forisoprene and230•tgm-2h4 terpenes, respectively; DuckeForest Researve, nearManuas, Brazil,tethered balloon measurements to300m;Davisetal. [1994],DuckeForest Reserve, nearManaus,Brazil,tethered t-alloonmeasurements to 300m aboveground.All datawereadjusted to daytimeconditions (seetext). Notethat Zimmerman etal. [1988],JacobandWofsy[1988]andDavisetal. [1994]usethesameobservational database. *Rangeofmeanandmedian valueg determined bymixed-layer gradient and mixed-layer budget method.

maximum fluxes observed.During nighttime, isopreneand monoterpene emissions declinesignificantly because of the light and temperature dependency of these BVOC emissions [Guentheret al., 1991, 1993]. Isopreneemissionshave been shownto ceaseduring night. Therefore24-hourdaily mean fluxesfor isoprene aresignificantly lowerthanobserved daytime fluxesduringfull light intensitiesand high temperatures. In a recentstudythe ratio of normalized(30øCtemperature and 1000

atmospheric watervaporon the adsorbent cartridges occuringat high ambientrelativehumidities, whichmay requiredry purge measures priorto analysis by thermaldesorption. The researchsiteinvestigated in the headwaters of the Amazon displayed verylow impactby humanactivities asindicated by low anthropogenic hydrocarbonconcentrations. Isoprenewas the dominant hydrocarbon in the CBL. OtherBVOCsidentifiedwere the monoterpenes cz-pinene,p-cymene, camphene,13-pinene,

•tmolm-2s-•photosynthetically activeradiation) to actualdiurnal limonene, cz-fenchene, limonene, and p-cymenene. BVOC fluxes for isopreneand monoterpenewere estimatedusingan ambientsummertimerecordof temperature andlight [Helmiget al., 1998c].The averagedaily fluxeswere calculatedto be 39% and 72% of the valuesundernormalizedconditionsfor isoprene and monoterpenes, respectively.These correctionfactorswere usedhereto scalethe 24 hourdaytimedatagivenin someof the referencesto maximum daytime values for a comparisonwith our data (Table 6). Consideringthesecorrections,BVOC fluxes measuredin this studycomparereasonablywell with the other BVOC flux estimates for tropical forests reported in the

surfacefluxesdeterminedby the mixed-layergradientand ML

budget methods were3000to 8200gg m'2h-• for isoprene, 120370[tgm-2h-•forct-pinene, 40-75[tgm-2h4 for 13-pinene, about 16 [tgm-2h-• forp-cymene and40-50[tgm-2h4 for camphene.

AmbientBVOC mixing ratiosand flux estimatesare in good agreement withpreviousstudies in theAmazonregion. IsopreneoxidationproductsMAC and MVK and 3-MF were observedthroughoutthe CBL. The observations show that the ratiosMAC/isopreneand MVK/isopreneincreasewith altitude. Theseprofilesappearto confirmour currentunderstanding of the literature. Note that the previous estimateswere derived from atmosphericsources and sinks, relative kinetic data and only two collectionsof data, gatheredfrom the sameexperiment atmospheric transportfor thesecompounds. in 1985. Therefore,this studyrepresents a substantial increasein Observations of isopreneand its oxidationby-products, MAC our dataon AmazonBVOC profiles. andMVK, may providea meansfor derivingsimpleestimates of exchangeratesbetweenthe ML and the free troposphere. The 5. Conclusion

A newanalytical methodbasedonsolidadsorbent sampling was adaptedfor tetheredballoonverticalprofilemeasurements of

times derived from these observations and very crude assumptions aboutthe chemistryof thesecompounds imply that the air just abovethe ML was agedapproximately1 hour since isoprene wasemittedinto theforestcanopyair.

VOCs and BVOCs in the CBL. The methodhas advantages over Acknowledgments.We like to thank the following colleaguesand previously usedwhole air samplingmethods with respectto robustness, lower cost of samplingequipmentand for sample institutionsfor providinghelp andresourcesessentialto the conductance of this study:A. Buck from A.P. Buck Inc., Orlando,Florida, for lending shipping,easierfield application, and lesstime consumption.us the sampling pumps; R. Monson and his group, EPO Biology,

Furthermore, it allowsthe sensitiveanalysisof heavierBVOCs suchasmonoterpenes andtheisoprene oxidation products MAC, MVK, and 3-MF. A limitationis the partial collectionof

University of Colorado, Boulder, for letting us use their Perkin Elmer ATD-400 instrumentand the calibrationstandard;E. Apel, A. Guenther, P. Zimmerman from the National Center of Atmospheric Research,

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

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Boulder, Colorado, for making available to us the multicomponent Cao, X.-L., and C.N. Hewitt, Thermal desorptionefficiencies for hydrocarbon standardandthe line attachment techniqueandparts,Petro different adsorbate/adsorbent systems typically used in air Perufor providingtransportation, accommodation andlogisticsupportat monitoringprograms,Chemosphere, 27, 695-705, 1993. the site,the City of BoulderWastewaterTreatmentPlant for making Ciccioli,P., E. Brancaleoni,A. Cecinato,R. Sparapini,and M. Frattoni, available to us the GC/MS instrumentand M. Trainer, National Oceanic Identificationand determinationof biogenic and anthropogenic andAtmosphericAdministration,Boulder,CO, for helpfulcommentson VOCs in forestareasof Northernand SouthernEuropeand a remote a draft manuscript.All other participantsof the field experimentare site of the Himalaya region by high-resolution GC-MS, J. thankedfor helpat thefield site.Thisstudywaspartiallyfundedthrough Chromatogr.,643, 55-69, 1993. a grantby the CHEMRAWN VII Follow Up Program,the Dreyfus Corchnoy,S.B., and R. Atkinson,Kineticsof the gas-phase reactionsof Foundation,andU.S. Environmental ProtectionAgencygrantR825379. OH and NO3 radicalswith 2-carene, 1,8-tincole,p-cymene, and This researchhas not been subjectedto EPA review and does not terpinolene,Environ. Sci. Technol.,24, 1497-1502, 1990. necessarily reflectthe views of this agency. Davis,K.J., Surfacefluxesof tracegasesderivedfrom convective-layer profiles,NCAR, National Centerfor AtmosphericResearch,Boulder, Colorado,CT-139, 281 pp, 1992. References Davis K.J., D.H., Lenschow, and P.R. Zimmerman, Biogenic nonmethanehydrocarbonemissionsestimatedfrom tetheredballoon Andino, J. M., and J. W. Butler., A study of the stability of methanolobservations, J. Geophys.Res., 99, 25587-25598, 1994. fueled vehicle emissionsin Tedlar bags,Environ. $ci. Technol.,25, Denmead, O.T., M.R. Raupach, F.X. Dunin, H.A. Cleugh, and R. 1644-1645, 1991. Leuning, Boundary layer budgets for regional estimatesof scalar Andteac, M. O., J. Fishman,and J. Lindesay, The SouthernTropical fluxes,Global ChangeBiol., 2, 255-264, 1996. Atlantic RegionalExperiment(STARE): Transportand Atmospheric Donoso, L., R. Romero, A..Rondon, E. Fernandez,P. Oyola, and E. ChemistryNear the Equatorial-Atlantic(TRACE A) and Southern Sanhueza,Natural and anthropogenicC-2 to C-6 hydrocarbons in the African Fire-Atmosphere Research Initiative (SAFARI): An central-easternVenezuelan atmosphereduring the rainy season,J. introduction,`/. Geophys.Res., 101, 23519-23520, 1996. Atmos. Chem., 25, 201-214, 1996. Andronache,C., W. L. Chameides,M. O. Rodgers, J. Martinez, P. Fitzjarrald, D.R., B.L. Stormwind, G. Fisch, and O.M.R. Cabral, Zimmerman,and J. Greenberg,Vertical distributionof isoprenein the Turbulent transport observed just above the Amazon forest, d. lower boundarylayer of the rural and urbansouthernUnited States,ar. Geophys.Res.,93, 1551-1563, 1988. Geophys. Res.,99, 16989-16999,1994. Fontan,J., A. Druilhet, B. Benech, R. Lyra, and B. Cros, The DECAFE Apel, E.C., J.G. Calvert, 'and F.C. Fehsenfeld, The nonmethane experiments: Overview and meteorology,`/. Geophys.Res., 97, 6123hydrocarbonintercomparison experiment(NOMHICE): Tasks 1 and 6136, 1992. 2,,/. Geophys.Res.,99, 16651-16664, 1994. Garratt, J.R., The Atmospheric Boundary Layer, 316 pp. Cambridge Arnts, R.R., D.F. Smith, and T.E. Kleindienst, Developmentof multiUniv. Press,New York, 1992. bed solid adsorbentmethod for samplingand analysisof polar and Goldan, P.D., W.C. Kuster, F.C. Fehsenfeld, and S.A. Montzka, non-polar biogenic

volatile

organics, paper

presented at

Measurement of Toxic and Related Air Pollutants Conference, Air

and Waste ManagementAssociation,ResearchTriangle Park, N.C., May 16-18, 1995. Atkinson,R., Kineticsand mechanismsof the gas-phasereactionsof the hydroxylradical with organiccompounds, ,/. Phys. Chem.Ref Data Monogr.,1, 1-246, 1989. Atkinson,R., S.M. Aschmann,andJ.N. PittsJr., Rateconstants for the gasphasereactionsof the OH radicalwith a seriesof monoterpenes at 294 + 1 K, d. Chem.Kinet., 18, 287-299, 1986. Atkinson, R., D. Hasegawa,and S.M. Aschmann,Rate constantsfor the

gas-phase reactionsof 03 with a seriesof monoterpenes and related compounds at 296 _+2 K,J. Chem.Kinet., 22, 871-887, 1990a.

Atkinson,R., S.M. Aschmann,and J. Arey, Rate constants for the gasphasereactionsof OH and NO3 radicalsand 03 with sabineneand campheneat 296 + 2 K, Atmos.Environ.,24A, 2647-2654, 1990b.

Hydrocarbonmeasurementsin the southeasternUnited States:The rural oxidantsin the SouthernEnvironment(ROSE) program 1990, Geophys.Res.,100, 25945-25963, 1995. Greenberg,J., and P. Zimmerman,Nonmethanehydrocarbons in remote,

tropical,continental, and marineatmospheres, ,/. Geophys.Res.,89, 4767-4778, 1984.

Greene,C.R., andR. Atkinson,Rateconstants for the gas-phase reactions of 03 with a seriesof alkenesat 296 + 2 K, ,/. Chem.Kin., 24, 803-811, 1992.

Grosjean, D., E.L.WilliamsII, andE. Grosjean, Atmospheric chemistry of isoprene anditscarbonyl p.roducts, Environ.Sci.Technol., 27, 830-840, 1993.

Guenther,A.B., R.K. Mortson,and R. Fall, Isopreneand monoterpene emissionrate variability:Observations with eucalyptusand emission rate algorithm development,,/. Geophys.Res., 96, 10799-10808, 1991.

Balsley,B.B., J.B.Williams,.G.W.Tyrrell,andC.L Balsley,Atmospheric Guenther, A., P. Zimmerman, P. Harley, R. Mortson, and R. Fall, research usingkites' Herewe go again!,Bull. Am. Meteor.Soc.,73, 17Isoprene and monoterpene emission rate variability: Model 29, 1992. evaluationand sensitivityanalysis,d. Geophys.Res., 98, 12609Balsley,B.B., J.W Birks, M.L Jensen,K.G. Knapp, J.B. Williams, and G.W. Tyrrell,Ozoneprofilingusingkites,Nature,369, 23, 1994a.

12617, 1993.

Guenther,A., P. Zimmerman,L. Klinger, J. Greenberg,C. Ennis, K. Davis, W. Pollock, H. Westberg,E. Allwine, and C. Geron,Estimates of regionalnaturalvolatile organiccompoundfluxesfrom enclosure kites, Environ. Sci. Technol., 28, 422A-427A, 1994b. and ambient measurements,`/. Geophys.Res., 101, 1345-1359, Balsley,B.B., M.L. Jensen,and R.G. Frehlich,The use of state-of-the-art 1996a. kitesfor profilingthe lower atmosphere. Boundary-Layer Meteorol., Guenther, A., J. Greenberg, P. Harley, D. Helmig, L. Klinger, L. 87, 1-25, 1998. Viefling, P. Zimmerman, and C. Geron, Leaf, branch, stand, and Berkley, R.E., J.L. Varns, and J. Pleil, Comparisonof portable gas landscapescalemeasurements of volatile organiccompoundfluxes chromatographs and passivatedcanistersfor field samplingairborne from U.S. woodlands,Tree Physiol.,16, 17-24, 1996b. toxic organicvaporsin the United Statesand the USSR, Environ.Sci. Guenther,A., et al., Isoprene fluxes measuredby enclosure,relaxed Technol.,2.5, 1439-1444 1991. eddy accumulation,surface-layergradient, and mixed layer mass Biesenthal,T.A., Q. Wu, P.B. Shepson,H.A. Wiebe, K.G. Anlauf, andG.I. balancetechniques,,/.Geophys.Res., 101, 18555-18567, 1996c. Mackay, A study of relationshipsbetween isoprene,its oxidation products,and ozone,in the lower FraserValley, BC, Atmos.Environ., Harriss,R.C., et al., The Amazon boundarylayer experiment(ABLE 2A): Dry season1985,`/. Geophys.Res.,93, 1351-1360, 1988. 31, 2049-2058, 1997. Harriss, R.C., et al., The Amazon boundary layer experiment:Wet Blake, D.R., D.F. Hurst,T.W. SmithJr., W.J. Whipple, T.-Y. Chen, N.J. season1987,,/. Geophys.Res., 95, 16721-16736, 1990. Blake, and F.S. Rowland, Summertime measurements of selected Helmig, D., Artifact-free preparation,storage and analysis of solid nonmethane hydrocarbons in the Arcticand Subarcticduringthe 1988 adsorbentsamplingcartridgesusedin the analysisof volatile organic Arctic BoundaryLayer Experiment(ABLE 3A), J. Geophys.Res.,97, 16559-16588, 1992. compoundsin air, `/. Chromatogr.,732, 414-417, 1996. Boehler, W.F., J.C. Pavelka, K.M. Hill, and P.R. Ames, Improved Helmig, D., Review article: Ozone removal techniquesin the sampling sotbentbasedmethodologyfor the analysisof CAAA hazardousair of atmosphericvolatile organic trace gases,Atmos. Environ., 31, 3635-3651, 1997. pollutant,in Measurementof Toxic and Related Air Pollutants,pp. 397-405, Air and WasteManage. Assoc.,Pittsburgh,Pa, 1995. Helmig,D., andJ. Greenberg,Automatedin-situgaschromatography/mass

Balsley,B.B.,J.W. Birks.,M.L. Jensen, K.G. Knapp,J. B. Williams,and G.W. Tyrrell, Verticalprofilingof the Atmosphere for ozoneusing

25,532

HELMIG ET AL.: BVOC FLUXES IN PERUVIAN AMAZON

spectrometric analysisof ppt level volatile organictrace gasesusing multistagesolid-adsorbent trapping,d. Chromatogr.,677, 123-132, 1994.

Helmig, D., and L. Vierling, Water adsorptioncapacity of the solid adsorbents Tenax TA, Tenax GR, Carbotrap, Carbotrap C, Carbosieve SIII and Carboxen 569 and water management techniques, Anal. Chem.,67, 4380-4386, 1995. Helmig, D., J. Greenberg,A. Guenther,P. Zimmerman, and C. Geron, Anthropogenicand biogenic volatile organic compoundsand their oxidationproductsat a forestsitein Oak Ridge/TN,d. Geophys.Res., in press,1998a. Helmig D., L.F. Klinger, A. Guenther,L. Vierling, C. Geron, and P. Zimmerman, Biogenic volatile organic compound emissions (BVOCs) I. Identificationsfrom three continentalsitesin the U.S., Chemosphere, in press,1998b. Helmig D., L.F. Klinger, A.. Guenther,L. Vierling, C. Geron, and P. Zimmerman, Biogenic volatile organic compound emissions (BVOCs) II. Landscapeflux potentialsfrom threecontinentalsitesin the U.S., Chemosphere, in press,1998c. Jacob,D.J., and S.C. Wofsy, Photochemistryof biogenicemissionsover the AmazonForest,or. Geophys.Res.,93, 1477-1486, 1988. Janson,R., and J. Kristensson,Sampling and analysisof atmospheric monoterpenes,Rep. CM-79, Dep. of Meteorol., Univ., Stockholm, 1991.

dissipation closures in second-order modeling,or.Atmos.Sci., 46, 2311-2330, 1989.

Montzka, S.A., M. Trainer, P.D. Goldan, W.C. Kuster, and F.C.

Fehsenfeld, !soprene andits oxidation products, methylvinylketone andmethacrolein, in theruraltroposphere, J. Geophys. Res.,98, 1101. 1111, 1993.

Montzka,S.A., M. Trainer,W.M. Angevine,and F.C. Fehsenfeld, Measurements of 3-methyl furan, methyl vinyl ketone, and methacrolein at a ruralforested sitein thesoutheastem UnitedStates, J. Geophys.Res., 100, 11393-11401, 1995.

Paulson,S.E., and J.H. Seihfeld,Developmentand evaluationof a photooxidation mechanism forisoprene, or.Geophys. Res.,97, 2070320715, 1992.

Pierotti, D., S.C.Wofsy,D. Jacob, andR.A.Rasmussen, Isoprene andits oxidationproducts:Methacroleinand methyl vinyl ketone,J. Geophys.Res.,95, 1871-1881.1990.

Rasmussen, R.A., andM.A.K. Khalil, Isopreneoverthe Amazonbasin, J. Geophys.Res.,93, 1417-1421,1988. Riemer,D.D., P.J. Milne, C.T. Farmer,and R.G. Zika, Determination of

terpeneand relatedcompounds in semi-urban air by GC-MSD, Chemosphere, 28, 837-850, 1994.

Thompson, A.M., Newdirections: Biomass burning andtheatmosphere Accomplishments andresearch opportunities, Atmos.Environ.,30, i-ii, 1996.

Jorgensen,A.D., K.C. Picel, and V.C. Stamoudis,Predictionof gas chromaotography flame ionization detectorresponsefactors from

Van den Dool, H., and P.D. Kratz, A generalization of the retention

molecular structures,Anal. Chem., 62, 683-689, 1990. Kaufman, Y.J., A. Setzer, D. Ward, D. Tanre, B.N. Holben, P. Menzel, M.C. Pereira, and R. Rasmussen,Biomass burning airborne and

partitionchromatography, J. Chromatogr, I1,463-471, 1963. Wang, Y., T.S. Raihala,A.P. Jackman,and R. St. John,Use of Tedlar bagsin VOC testingand storage:Evidenceof significantVOC

spaceborneexperimentin the Amazonas(BASE-A), d. Geophys.

indexsystemincludinglineartemperature programmed gas-liquid

losses,Environ.Sci. Technol,30, 3115-3117, 1996.

Res., 97, 14581-14599, 1992.

Wyngaard, J.C.,andR.A. Brost,Top-downandbottom-up diffusionin KnappK.G.,M.L. Jensen, B.B.Balsley,J.A.Bognar,S.J.Oltmans, T.W. theconvective boundarylayer,or.Atmos.Sci.,41,102-112, 1984. Smith,andJ.W.Birks,Verticalprofiling usinga complementary kite Yokouchi,Y., Seasonal anddiurnalvariationof isoprene anditsreaction and tethered balloonplatformat FerrylandDows,Newfoundland, productsin a semi-ruralarea.Atmos.Environ,28, 2651-2658,1994. Canada:Observation of a dry, ozone-richplume in the free Zimmerman,P.R., J.P.Greenberg, andC.E. Westberg,Measurements of troposphere. d. Geophys. Res.,103, 13389-13397,1998. atmospherichydrocarbonsand biogenic emissionfluxes in the Amazonboundarylayer,or.Geophys.Res.,93, 1407-1416,1988. Levine,J.S.(Ed.),GlobalBiomass Burning:Atmospheric, Climaticand Biospheric Implications, MIT Press,Cambridge, Mass.,1991.

Lilly, D.K., Modelsof cloud-topped mixedlayersundera strong inversion,Q. d. R. Meteorol.Soc.,94, 292-309, 1968.

R. V. ArrietaandR. Rodriguez,Facultadde Ingenieria,Universidad Lindesay,J.A., M.O. Andreae,J.G. Goldammer,G. Harris, H.J. de Piura,Apartado353, Piura, Peru. Annegam,M. Garstang,R.J. Scholes,and B.W. van Wilgen, B. Balsley,J. W. Birks,J. Bognar,D. Helmig,M. Jensen,andL. R. International geosphere-biosphere programme/international global of Chemistryand Biochemistry and Cooperative atmospheric chemistry SAFAR-92fieldexperiment: Background and Kuck, Department Institutefor Researchin EnvironmentalSciences,Universityof overview,J. Geophys.Res.,101, 23521-23530, 1996. Colorado, Boulder, CO 80309-0215. Martin,R.S.,H. Westberg,E. Allwine,L. Ashman,J.C. Farmer,andB. K. Davis, Departmentof Soil, Water, and Climate,Universityof Lamb, Measurements of isopreneand its atmospheric oxidation productsin a centralPennsylvania deciduous forest,J. Atmos.Chem. 13, 1-32, 1991.

McClenny,W.A., K.D. Oliver,and E.H. Daughtrey Jr., Analysisof VOCs in ambient air using multisorbentpackingsfor VOC accumulation andsampledrying,or.Air WasteManage.Assoc.,45, 792-800, 1995.

Moeng, C.-H., and J.C. Wyngaard, Evaluation of turbulent and

Minnesota, St. Paul, MN 55108-6028.

T. Smith Jr., ChemistryTechnologyDepartment,TRW, One Space Park, 01/2020, Redondo Beach, CA 90278.

(ReceivedNovember26, 1997;revisedMarch 18, 1998; acceptedMarch 20, 1998.)