Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based ...

25 Pages·2007·1.23 MB·English

Atmos. Chem. Phys.,7,2855–2879,2007
Atmospheric
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Chemistry
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underaCreativeCommonsLicense. and Physics
Isoprene and monoterpene fluxes from Central Amazonian
rainforest inferred from tower-based and airborne measurements,
and implications on the atmospheric chemistry and the local carbon
budget
U.Kuhn1,M.O.Andreae1,C.Ammann2,A.C.Arau´jo3,E.Brancaleoni4,P.Ciccioli4,T.Dindorf1,M.Frattoni4,
L.V.Gatti5,L.Ganzeveld6,B.Kruijt7,J.Lelieveld6,J.Lloyd8,*,F.X.Meixner1,A.D.Nobre3,U.Po¨schl1,C.Spirig2,
P.Stefani9,A.Thielmann1,R.Valentini9,andJ.Kesselmeier1
1MaxPlanckInstituteforChemistry,BiogeochemistryDept.,Mainz,Germany
2FederalResearchStationforAgroecologyandAgriculture,Zu¨rich,Switzerland
3InstitutoNacionaldePesquisasdaAmazoˆnia(INPA),Manaus,Brazil
4IstitutodiMetodologieChimiche,AreadelleRicercadiRoma,Monterot. Scalo,Italy
5InstitutodePesquisasEnergeticaseNucleares(IPEN),Sa˜oPaulo,Brazil
6MaxPlanckInstituteforChemistry,AtmosphericChemistryDept.,Mainz,Germany
7Alterra,WageningenUniversityandResearchCentre,Wageningen,Netherlands
8MaxPlanckInstituteforBiogeochemistry,Jena,Germany
9UniversityofTuscia,DepartmentofForestScienceandEnvironment,Viterbo,Italy
*nowat: EarthandBiosphereInstitute,SchoolofGeography,UniversityofLeeds,UK
Received: 1December2006–PublishedinAtmos. Chem. Phys.Discuss.: 16January2007
Revised: 14May2007–Accepted: 22May2007–Published: 11June2007
Abstract. We estimated the isoprene and monoterpene were by far higher than observed, indicating that chem-
source strengths of a pristine tropical forest north of Man- ical processes may not be adequately represented in the
aus in the central Amazon Basin using three different mi- model. The observed vertical gradients of isoprene and its
crometeorologicalfluxmeasurementapproaches. Duringthe primary degradation products methyl vinyl ketone (MVK)
early dry season campaign of the Cooperative LBA Air- and methacrolein (MACR) suggest that the oxidation ca-
borneRegionalExperiment(LBA-CLAIRE-2001), atower- pacity in the tropical CBL is much higher than previ-
based surface layer gradient (SLG) technique was applied ously assumed. A simple chemical kinetics model was
simultaneously with a relaxed eddy accumulation (REA) used to infer OH radical concentrations from the verti-
system. Airborne measurements of vertical profiles within cal gradients of (MVK+MACR)/isoprene. The estimated
and above the convective boundary layer (CBL) were used range of OH concentrations during the daytime was 3–
to estimate fluxes on a landscape scale by application of 8×106moleculescm−3, i.e., an order of magnitude higher
the mixed layer gradient (MLG) technique. The mean thanisestimatedforthetropicalCBLbycurrentstate-of-the-
daytime fluxes of organic carbon measured by REA were artatmosphericchemistryandtransportmodels.Theremark-
2.1mgCm−2h−1 for isoprene, 0.20mgCm−2h−1 for α- ablyhighOHconcentrationswerealsosupportedbyresults
pinene,and0.39mgCm−2h−1forthesumofmonoterpenes. ofasimplebudgetanalysis,basedontheflux-to-lifetimere-
Thesevaluesareinreasonableagreementwithfluxesdeter- lationship of isoprene within the CBL. Furthermore, VOC
minedwiththeSLGapproach,whichexhibitedahigherscat- fluxes determined with the airborne MLG approach were
ter,asexpectedforthecomplexterraininvestigated. Theob- onlyinreasonableagreementwiththoseofthetower-based
served VOC fluxes are in good agreement with simulations REA and SLGapproaches after correctionfor chemical de-
usingasingle-columnchemistryandclimatemodel(SCM). cay by OH radicals, applying a best estimate OH concen-
In contrast, the model-derived mixing ratios of VOCs tration of 5.5×106moleculescm−3. The SCM model cal-
culationssupportrelativelyhighOHconcentrationestimates
Correspondenceto: U.Kuhn after specifically being constrained by the mixing ratios of
([email protected]) chemicalconstituentsobservedduringthecampaign.
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
2856 U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest
TherelevanceoftheVOCfluxesforthelocalcarbonbud- Furthermore,tropicalforestecosystemsplayamajorrole
get of the tropical rainforest site during the measurements in global carbon sequestration (Tian et al., 1998; Botta et
campaign was assessed by comparison with the concurrent al., 2002). A fraction of the assimilated carbon is reemit-
CO fluxes,estimatedbythreedifferentmethods(eddycor- ted as VOCs by terrestrial vegetation. While VOC emis-
2
relation,Lagrangiandispersion,andmassbudgetapproach). sions may be small in relation to net primary productivity
Depending on the CO flux estimate, 1–6% or more of the (NPP), the amount of carbon lost as VOC emissions can be
2
carbongainedbynetecosystemproductivityappearedtobe significant relative to the net ecosystem productivity (NEP)
re-emittedthroughVOCemissions. or net biome productivity (NBP), respectively (Guenther et
al., 1995; Guenther, 2002; Kesselmeier et al., 2002). On a
global scale VOCs are emitted at an estimated 1.2PgCa−1
(Guenther et al., 1995), a number of the same magnitude
1 Introduction as the mean annual increase of CO in the earth’s atmo-
2
sphere, and the carbon sink by the terrestrial biosphere (3.2
The Amazon Basin, one of the most productive terrestrial and 2.4PgCa−1, respectively; ICPP 2001). Isoprene and
ecosystems of the Earth, plays an important role in global monoterpenes are estimated to account for the major share
atmospheric chemistry and physics, and any change in the of VOC emissions, and tropical vegetation constitutes their
atmospheric chemical processes in this area can have a largestsinglesource(Guentheretal.,1995).
profound impact on global climate (Andreae and Crutzen, Consideringtheimportanceoftropicalecosystemsonat-
1997). Photochemical reactions of volatile organic com- mosphericprocesses,globalcarbonsequestration,andVOC
pounds (VOCs) have a significant influence on atmospheric emission strength, available data on the exchange of VOCs
ozone(O3),thehydroxylradical(OH),precipitationacidity, intheseregionsarestillscarce. Inviewofthevastbiodiver-
andaerosolformation,amongothers. Previousstudieshave sity of tropical forest ecosystems, with an estimated 35000
indicatedthattheinfluenceofisopreneandmonoterpenere- species of angiosperms in the Amazon Basin alone, the ex-
actions on important aspects of atmospheric chemistry can trapolationofsinglespecies-specificexchangestudiesbased
be substantial, as these compounds play an important role ontheleafandbranchlevelcanhardlybesufficienttoade-
in shaping the odd hydrogen photochemistry and regional quatelycharacterizelandscapeexchangerates. Tower-based
ozoneproductionofremoteareas(Goldanetal.,2000). The andairbornemulti-scalemicrometeorologicalfluxmeasure-
dominating role of biogenic VOCs in the chemistry of the menttechniquescanquantitativelyintegrateemissionsatthe
lowertroposphereisdueto(i)theirgreaterabundanceinre- landscapescaleinamorerepresentativemanner. Severalre-
moteareas,especiallyinthetropics,and(ii)theirhighatmo- centstudieshaveconsideredVOCfluxes,butstillthesignif-
sphericreactivitycomparedtothemajorityofanthropogenic icanceofVOCexchangebetweentropicalterrestrialecosys-
VOCs(Fuentesetal.,2000). tems and the atmosphere in regional and global carbon cy-
Theprincipaloxidizingagentinthetropospherethatdom- clingremainspoorlyunderstood(Zimmermannetal.,1988;
inates the daytime removal of most gaseous pollutants is Rasmussen and Khalil, 1988; Helmig et al., 1998; Serca et
the OH radical, which can be conceived as a measure for al., 2001; Geron et al., 2002; Rinne et al., 2002; Greenberg
the self-cleansing capacity of the atmosphere, and is there- etal.,2004;Karletal.,2004).
foredubbedthe“detergent”oftheatmosphere(Andreaeand The Cooperative LBA Airborne Regional Experiment
Crutzen,1997).ItgovernstheatmosphericlifetimeofVOCs, (LBA-CLAIRE) project forms part of a series of inte-
carbon monoxide (CO), as well as of the greenhouse gases grated airborne and ground-based campaigns, as part of the
methane(CH ),HCFCs,andCH Br,andthusmaintainsthe Large-scale Biosphere-Atmosphere Experiment in Amazo-
4 3
chemical composition of the atmosphere. The production nia (LBA). During the intensive LBA-CLAIRE 2001 cam-
of OH radicals is believed to occur largely via photolysis paign, continuous tower-based relaxed eddy accumulation
of ozone followed by subsequent reaction of the emerging (REA)observationsofcanopy-scaleVOCfluxesinaremote
O(1D)withwatervapourandbyphotolysisofformaldehyde tropical rainforest site north of Manaus were run simulta-
(HCHO)(Tanetal.,2001). TheproductionofOHradicalsis neously with measurements using a surface layer gradient
at the maximum in the tropics, as the ingredients (UV radi- (SLG) flux approach. Both methods integrate over an up-
ationandwatervapour)areatahighlevel. Hencethetrop- wind footprint area of typically 2–3km2, but do not inte-
ical troposphere is assumed to be responsible for the major gratethelarge-scalespatialdiversityofthetropicalrainfor-
shareoftheglobalatmosphericoxidationoflong-livedgases. est,withclearings,riversandthevastbiodiversityofvegeta-
However,currentatmosphericchemistrymodelstendtopre- tionspeciescomposition. Inaddition,VOCverticalprofiles
dictthathighmixingratiosofVOCscausesubstantialreduc- withinandabovetheconvectiveboundarylayer(CBL)were
tions in OH radical concentration in the lower troposphere investigated by aircraft measurements. These were used to
oftropicalareas(e.g.Warnekeetal., 2001; Lelieveldetal., infer VOC fluxes on a landscape scale by application of a
2004), andrealmeasurementsofOHradicalconcentrations mixed layer gradient (MLG) approach, which, however, in-
inthe“GreatTropicalReactor”arenotyetavailable. volvesotheruncertainties,suchasanincreasedinfluenceof
Atmos. Chem. Phys.,7,2855–2879,2007 www.atmos-chem-phys.net/7/2855/2007/
U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest 2857
theassumptionsmadeonboundarylayerturbulence,dynam-

ics,andatmosphericchemistry. Forreactivescalarslikebio- NN
genicVOCsthetimescaleofatmosphericchemistrymaybe
similartotheconvectiveeddyturnovertime.
Our results seek to contribute to a better understanding
of the source strength, the persistence, and fate of biogenic
VOCsintheatmosphereoftheAmazonBasin, andtherole KK3344
ofthisregionintheglobalatmosphericchemistryandcarbon
cycle.Themainquestionsaddressedinthisworkare(i)what
is the magnitude of the VOC emission strength, (ii) what is
the relative contribution of the VOC emission with respect
toecosystemcarbonexchange,and(iii)docurrentchemistry
models accurately predict VOC emission fluxes, mixing ra-
MMaannaauuss
5500kkmm
tios, andtheirfateinthetropicalatmospherewithregardto
theoxidationcapacity?
Fig.1. Overviewofthemeasuringsite,withpristinerainforestin
themajorityofthefootprintareaofthetower-based(K34)andair-
2 Materialandmethods bornemeasurements,respectively. Thesolidlineswithdotsrepre-
sentthe6-hback-trajectoriescalculatedfortheindividualtimepe-
2.1 Sitecharacterization riodsoftheflightmeasurements,for100m(red)and500m(green)
heightsabovetheK34towersiteusingtheoutputoftheHYSPLIT
model(NOAAAirResourcesLaboratory)asanoverlayonGoogle
ToassesstheregionalVOCsandcarbonbalance,directflux
Earth(GoogleEarth™mappingservice). Thecyanlinerepresents
and profile measurements were carried out in the Reserva
thetypicalflightpattern,whichconsistedofacontinuousspiralup
Biologica do Cuieiras, an undisturbed mature lowland rain
from>50mto>3000minaltitudeinthevicinityoftheK34tower,
forest reserve of the Instituto Nacional de Pesquisas da
followedbyastepwisedescendingpatternwith6flightlegs,oneon
Amazoˆnia (INPA) (Andreae et al., 2002). Tower-based flux
topoftheother.
data were collected from a 52 m walk-up scaffolding tower
(K34), located on a medium sized plateau at 2◦3503300S;
60◦1202700W, about 60km NNW of the city of Manaus, in ian classification) or oxisol (U.S. classification), with clay
central Amazonia (Fig. 1). The K34 tower has been oper- andsandcontentsof80%and10%,respectively. Hodnettet
ational for eddy correlation CO2 flux estimates since July al. (1996) calculate the available water capacity to be about
1999. 70mmm−1 intheuppermeterandabout30mmm−1 below
Ingeneralthisareaexhibitsasmall-scalereliefofplateaus 2m. Theyestimatedthatthemaximumwateruptakebelow
and lowlands that has favoured a pattern of dense vegeta- 2mdepthbyvegetationcanreach250mminadryyear. The
tionwithhighertreeslocatedontheplateausandapalm-rich climate at this site is characterized by little seasonal varia-
openforestinthelowlands(Ribeiroetal.,1999). Asdemon- tion in temperature and solar radiation and a large seasonal
stratedbyArau´joetal.(2002), thefootprintoftheCuieiras variationinrainfall. AshortdryseasonfromJulytoOctober
site shows a relatively low fraction of plateaus (40% within occurs when the Intertropical Convergence Zone (ITCZ) is
1kmradius). Theold-growthtropicalwetforestcanopyhas at its northern extreme. Details of the site are described in
a height of approximately 35m (Roberts et al., 1996), a to- Malhietal.(1998,2002),Arau´joetal.(2002),andAndreae
talsinglesidedleafareaindex(LAI)of4.6,andamaximum etal.(2002).
leaf area density of 0.65 at mean canopy height (Simon et
al.,2005). ThesevaluesareingoodagreementwithLAIval- 2.2 Tracegascollectionandanalysis
ues of 4–6 estimated by other authors for terra firme forest
ecosystems(e.g.McWilliametal.,1993;Kruijtetal.,2000; Threedifferentsampling/analysissystemswereusedtomea-
Andreae et al., 2002). Even though tree demography is ex- sureVOCs. Formostofthemeasurements,ambientairsam-
tremely complex due to the vast biodiversity, the Lecythi- ples were collected on solid adsorbents for off-line analy-
daceae, Sapotaceae, Euphorbiaceae and Caesalpinaceae sis in the lab. The majority of airborne and all ground-
families are found most frequently (Jardim and Hosokawa, based SLG samples were collected on 2-bed graphitic car-
1987). Above-ground dry phytomass has been estimated bonadsorbentsandanalyzedusingathermaldesorptiongas
at 344–393Mgha−1 (Klinge et al., 1975) in this forest. A chromatograph with a flame ionization detector (GC-FID)
morerecentpaperreportsfreshabove-groundphytomassof as described in Kuhn et al. (2002; 2004a). Samples were
561Mgha−1,abasalareaof29m2ha−1,andphytomassvol- collected by drawing air through fused silica-lined stain-
umes of 438m3ha−1 (Higuchi et al., 1998). The soils in less steel cartridges (89mm length, 5.33mm I.D., Silicos-
this area are characterized as a yellow clay latosol (Brazil- teel, Restek, USA) packed with sequential adsorbent beds
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2858 U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest
tention features and selective ions of biogenic VOCs, their
6
b] degradationproducts,calibrationandqualityassurancepro-
p
p ceduresaredescribedindetailinCicciolietal.(2002). The
C) [ 5 detectionlimitswereinthesamerangeasforGC-FIDanal-
MPI 4 ysis. Dominantmonoterpenespeciesdetectedbybothsolid
s ( adsorbentanalyticalsystems(CG-FIDandGC-MS)wereα-
e
mpl 3 pinene,β-pinene,myrcene,limonene,andρ-cymene.
Forairbornemeasurements,stainlesssteelcanistersof6L
a
s
ge 2 volume were used as an additional backup system for iso-
d preneanalysis. Thesecanistersampleswereanalyzedwitha
artri 1 yR 2= = 1 .00.46x2 gaschromatographequippedwithaFIDandcoupledMSac-
c cordingtoTrostdorfetal.(2004).Reasonableagreementwas
e, achievedforisoprenemixingratiosderivedbycartridgesan-
n
e 0
r alyzedwithGC-FIDandcanistersamples(Fig.2). Thecor-
p
o relation analysis reveals a relatively high scatter, but an ex-
s
I -1 cellentagreementinthemeanofbothdatasets(slope=1.04;
-1 0 1 2 3 4 5 6 R2=0.62). In general a high scatter is expected from the
snap-shot sampling characteristic of the canister technique
Isoprene, canister samples (IPEN) [ppb]
comparedtocartridge(15min,atflowratesof200mlmin−1)
sampling,withshortsamplingintervalsbeinglessrepresen-
Fig.2. Comparisonofisoprenemixingratiosobservedduringair-
tative for mean CBL mixing ratios. Individual downdrafts
borne measurements by solid sorbent cartridge sample collection
and updrafts may contain substantially different mixing ra-
(MPIC)andbycanistersamplecollections(IPEN),analyzedbydif-
ferentGC-FIDsystems. tios. Deviationsfromanidealatmosphericgradientmaybe
observed if the sample collection period is shorter than the
average convective turnover time, i.e., if sampling times do
notintegrateoverseverallargeeddies. Withtypicalhorizon-
of130mgCarbograph1(90m2g−1,Laras.r.l.,Rome,Italy) talwindspeedsof5ms−1,andthescaleofsomeconvective
followed by 130 mg Carbograph 5 (560m2g−1). For the
eddies being as large as the CBL depth, minimum sample
GC-FID technique, calibration was accomplished by use
timesof15–30min,orlengthofseveralhundredmetersare
of different gaseous standards containing isoprene, several
neededtointegrateoverarepresentativeairmass(Lenschow
n-alkanes, methyl vinyl ketone (MVK), and methacrolein
et al., 1980; Lenschow and Stankov 1986). The cartridges
(MACR).Thedetectionlimitofthemethodwasestimatedas
analyzedwithGC-MS,whichwerecollectedsimultaneously
the greater of the variability in the blank levels (at the 95%
with cartridges analyzed by GC-FID (2 of 8 flights), also
confidence level, i.e., 1.96 times the standard deviation of
showedexcellentagreementformonoterpenemixingratios.
allblankvalues),orachromatographicpeakthreetimesthe
Forisoprene,however,theyindicatedsimilartrendsinverti-
standard deviation of the background noise in the base line
calprofiles,butwithasystematicrelativeunderestimationof
of the chromatograph. Variability in the blank usually de-
theabsolutemixingratiosbyafactorof0.55.Sincethesedif-
termined the detection limit, which was typically 30ppt for
ferences cannot be explained, and no systematic difference
isopreneand10pptformonoterpenes. Hence,typicaluncer-
wasfoundcomparingisoprenemixingratiosderivedbythe
taintiesreached10%forisopreneat1ppbandrangedfrom
GC-FID (SLG) and GC-MS (REA) during the tower-based
5to30%at100pptformonoterpenes,dependingonthein-
measurements, it was decided to use the airborne isoprene
dividualmonoterpenepeakresolutionandblankvariability.
data derived by GC-FID to calculate VOC fluxes from the
All REA samples were accumulated on cartridges filled MLGapproach,basedonpurelystatisticalconsiderations.
withthreegraphiticcarbonadsorbentsaccordingtoBranca-
leonietal.(1999).VOCwerecollectedusingglasscartridges 2.3 Ground-basedfluxmeasurements
(160mmlength, 3mmI.D.)packedwith118mgCarbopack
C(12m2g−1,Supelco,Bellefonte,USA),60mgCarbograph Isoprene and monoterpene fluxes from the forest canopy
1 (90m2g−1, Lara s.r.l., Rome, Italy), and 115mg Carbo- were calculated from ambient air measurements using dif-
graph 5 (560m2g−1) in sequential beds. They were ana- ferentmicrometeorologicalapproaches, namely, therelaxed
lyzedbythermaldesorptiongaschromatographywithmass eddy accumulation (REA) and the surface layer gradient
spectrometric analysis (GC-MS). Positive identification and (SLG) techniques. The CO eddy covariance (EC) fluxes,
2
quantitativedeterminationsofallVOCsfromC toC car- and concentration profiles of CO , H O, temperature, and
4 16 2 2
bon atoms were accomplished by running the mass spec- momentum at the K34 site were part of a long-term mon-
trometer in the scan mode and by using reconstructed mass itoring project described in detail by Arau´jo et al. (2002).
chromatographyonspecificionsforselectivedetection. Re- TheVOC-REAsystemwithareversedgeometry(Ciccioliet
Atmos. Chem. Phys.,7,2855–2879,2007 www.atmos-chem-phys.net/7/2855/2007/
U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest 2859
al., 2003), wasinstalledataheightof21mabovethemean andHtheturbulentsensitiveheatflux(Wm−2). Thequanti-
canopy top. Equations used for calculating VOC fluxes are tiesTv,u∗,andH havebeenprovidedbythemeasurements
described in detail in Valentini et al. (1997) and Ciccioli et of a sonic anemometer which is part of the CO and H O
2 2
al.(2003). Onlythosecaseswereaccepted,inwhichhourly eddycovariancesystemattheK34tower. VOCfluxeshave
averaged values of the vertical wind (w) were close to zero beencalculatedaccordingto
(between −0.45 and +0.45ms−1) and the volume accumu-
lated in the updraft and downdraft traps did not differ more FVOC =Vtr ×(cid:2)CVOC(cid:0)zr,2(cid:1)−CVOC(cid:0)zr,1(cid:1)(cid:3) (3)
than 10%. From the experience gathered at this site, we
Thefollowingcriteriahavebeenappliedtoreduceunavoid-
havefoundthatthesamplingvolumeswerebalancedbestif
ablenoiseoffluxestimates
athresholdof±0.65σ (w)wasused. Thismeansthatairwas
sampled through the updraft (or downdraft) cartridge only −1< zr,i +1, and CVOC(cid:0)zr,2(cid:1)−CVOC(cid:0)zr,1(cid:1) ≤1
iftheinstantaneousverticalwindspeedexceededthisthresh- L (C (cid:0)z (cid:1)+C (cid:0)z (cid:1))/2
VOC r,2 VOC r,1
old. In the conditions selected, the volume diverted in the
(4)
two reservoirs ranged between 50 to 60% of the total vol-
ume. MonitoringVOCfluxesbyREAispartofalong-term Applying the SLG approach within the roughness layer
project aimed at assessing the seasonality of VOC fluxes in of the forest may underestimate the flux rates (e.g. Garratt,
relationtoCO2exchange. 1980). Theimportanceofthiseffectisdependentoncanopy
Intheperiodof17–25July2001,theREAfluxmeasure- structures and surface characteristics (Simpson et al., 1998)
ments were complemented by canopy-scale flux measure- andwasnotaccountedforinthepresentstudy. Thiswaythe
mentsusingthesurfacelayergradient(SLG)approach.Both fluxes calculated by SLG are assumed to be a lower bound
methods were based on VOC collection on solid adsorbent estimate. According to the footprint analysis of Araujo et
cartridges,withsimultaneoussampleperiodsof30min,and al.(2002)themeasureddaytimefluxesattheK34towerare
amassflowof150mlmin−1. ForSLGsamples,VOCswere representativeofa2–3km2 areaaroundthetower,although
collected on the tower at four different heights simultane- asmallerproportion(∼10%)ofthefluxesoriginatesfroman
ously(51,42.5,35.5,28maboveground)onanhourlybasis areabeyond10km2 aroundthetower. Theanalysissupports
using automated VOC sampling systems that are described theassumptionthatthemeasuredfluxdatarepresentaneffec-
indetailbyKuhnetal.(2005). Testsamplingswithallfour tive spatial average over most medium-scale topographical
inletsmountedat thesameheightshowed thatthesamplers landscape elements in this area, including plateaus, slopes,
didnotproduceanybiasexceedingtheanalyticaluncertainty. andvalleys.
VOC fluxes have been estimated according to the classical
aerodynamic method (Hicks et al., 1987; Ammann, 1999) 2.4 Airbornemeasurements
as the product of vertical surface layer VOC gradients and
theturbulenttransfervelocity,V . TheverticalVOCgradi- FortheaccurateverticalprofilingofVOCmixingratiosfrom
tr
ent has been determined as the difference between the two 50to3000mabovethecanopy,weemployedaBandeirante
uppermost VOC concentration measurements (z =51m and aircraft (model EMB 110B1) equipped with the same auto-
2
z =42.5m above ground), which had the greatest distance mated VOC sampler as used for the tower-based SLG mea-
1
fromthecanopytop(h =35maboveground). V ,issimply surements. The eight profiles reported here were obtained
c tr
theinverseoftheaerodynamicresistanceR betweenthetwo during the period of 5–17 July 2001, just in advance of the
a
measurementheights,namely intensivetower-basedfluxmeasurements. Fiveofthesepro-
(cid:20) (cid:18)z (cid:19) (cid:16)z (cid:17) (cid:16)z (cid:17)(cid:21) file measurements were made around midday (10:00-12:00
Ra =Vt−r1=(κu∗)−1× ln zr,2 −9H Lr,2 + 9H Lr,1 local time, LT), and three profile measurements were con-
r,1
ductedinthelateafternoon(16:00-18:00LT).Foreachofthe
(1)
respectiveflightsasimilarflightpathandschedulewasused.
where κ is the von Karman constant (=0.4), u∗ is the fric- Amapshowingthefootprintareaandthetypicalflightpath
tion velocity (ms−1), z =z −d is the relative height (m), isgiveninFig.1. AftertakeoffattheairportofManausand
r,i i
d=0.75h is the so-called zero-plane displacement height transfertotheK34towersite,theprofileflightschedulecon-
c
(m), 9 (z /L) is the dimensionless integrated similarity sistedof(i)acontinuousspiralupfrom>50mtoaltitudesof
H r,i
function(orintegratedstabilitycorrectionfunction)forheat >3000mabovecanopy,followedby(ii)astepwisedescend-
(Hicksetal.,1987),andL istheObukhovlength(m)given ingpatternwith6flightlegs, oneontopoftheother, inthe
by vicinity of the K34 tower (Fig. 1). During each flight 2–4
samples were collected within the CBL for MLG flux cal-
T ×u3×c ×ρ
L=− v ∗ p air (2) culations. A Global Positioning System (GPS) tracked the
κ ×g×H
aircraft’spositioninlongitude,latitude,andaltitude.
whereT isthevirtualtemperature,c isthespecificheatof VOC sample collections were accomplished at constant
v p
airatconstantpressure,gtheaccelerationofgravity(ms−2), altitude and aircraft speed along the main wind direction,
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2860 U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest
upwind of the K34 tower site. Each of these flight legs could be estimated as the lowest altitude at which the po-
allowed for a 15min cartridge sampling interval (at flow tential temperature profile showed a persistent change from
rates of 200mlmin−1). Each flight path covered a length well mixed to subadiabatic conditions. Where this was not
of ca. 65km to provide sufficient integration time to spa- clearly indicated, the profiles of H O, CO , and other mea-
2 2
tiallyaverageoveratleastseveraleddies, inordertoobtain sured trace constituents like CO, ozone, and aerosols were
representative mean CBL concentration profiles. The auto- taken into account. The convective velocity scale (w*) was
maticVOCsamplerforsolidsorbentcartridgesallowedthe definedas(Deardorff,1970)
collection of two samples simultaneously, and was used (i)
(cid:18) (cid:19)1
for quality assurance samples, using the same type of car- w∗ = g ×Q×z 3 (5)
i
tridges and analysis, and (ii) to collect samples on different θ
0
typesofadsorbentsthatwereanalysedbyGC-FIDandGC-
wheregistheaccelerationofgravity,θ isthepotentialtem-
MS to cross-check identification and quantification of VOC 0
perature,andQisthesurfacevirtualheatflux. Thepotential
species. Occasionally, canister samples were collected in
temperature and the virtual heat flux were derived from EC
the middle of each flight leg, with sampling times of a few
measurements for the specific time periods of the airborne
seconds. These were used as a third backup system specif-
measurementswithintheCBL.
icallyforisoprenedetermination. CO andH Omixingra-
2 2
Using w*, the characteristic timescale τ for turbulent
tiosweremeasuredbyaninfraredgasanalyzer(Licor6261)
transport(mixing)withintheCBLcanbeapproximated(e.g.
setupaccordingtoLloydetal.(2002). Aftertake-off,ambi-
Kroletal.,2000;BantaandWhite,2003)by:
ent air from outside of the aircraft was continuously drawn
through a ca. 5m long 1/400 Teflon tube. The inlet end was τ = zi (6)
forward of all engines, and was equipped with a Teflon fil- w∗
ter of 2µm pore size that was replaced prior to each flight.
Entrainmentfluxeswereestimatedfromthegrowthratesof
Toavoidpossibleozoneinterference,anozonescrubbercon- the CBL (between 0.01 and 0.05ms−1) and the observed
sistingofmultiplepliesofMnO -coatedcoppermesh(Type
2 concentration differences across the CBL top. The surface
TO341FC003, Ansyco, Karlsruhe, Germany) was mounted
fluxeswereobtainedbyfittingtheMLGequationtothemea-
inthesamplelineaheadoftheVOCsampler.
sured concentrations within the CBL with a least squares
method. ForfurtherdetailsseeSpirigetal.(2004).
2.5 Fluxestimationbythemixedlayergradientapproach
2.6 ModelingofVOCemissionandatmosphericchemistry
TheCBLissubdividedintoasurfacelayer(lowest10%)and
a mixed layer (ML). While transport in the surface layer is For comparison and interpretation purposes, the biogenic
dominatedbymechanicalturbulenceprovidedbythesurface emission and chemical processing of VOCs in the tropi-
friction,transportintheMLismainlyduetoconvectivetur- cal atmosphere have been simulated with a single-column
bulence, whichisthermallydriven(Fischetal., 2004). The chemistryandclimatemodel(SCM;Ganzeveldetal.,2002;
topoftheCBLisdeterminedingeneralbyacappinginver- Ganzeveld and Lelieveld, 2004). The SCM has been
sion in the potential temperature profile. The ML is dom- constrained with the meteorological analysis of the Euro-
inatedbyconvectiveeddieswhosescalesarecomparableto peanCentreforMediumrangeWeatherForecast(ECMWF)
thedepthoftheentireCBL.Itisconsideredtobewell-mixed, model, tosimulatearealisticrepresentationoftheobserved
i.e.,themeanmixingratioofaconservedscalarthatisemit- meteorology(Ganzeveldetal.,2006a).VOCemissionswere
ted from the ground is expected to decrease only gradually calculatedaccordingtoGuentheretal.(1995). Thesimula-
withaltitude. TheobservedverticalprofilesofVOCmixing tionsrepresentatmosphere-biosphereexchangesforaforest
ratioswereusedtoestimatefluxesonalandscapescale. ecosystemwithaLAIof5,acanopyheightof30m,arough-
Themixedlayergradient(MLG)techniquecalculatessur- nesslengthof2m,andVOCandNOemissionfactorsforthe
face fluxes of passive scalars based on mean mixing ratio Olson (1992) ecosystem class “tropical broadleaf seasonal,
differences in the ML. The MLG approach assumes a hori- withdryorcoolseason”.
zontallyhomogenoussystem,implyingspatialhomogeneity The model was nudged towards the observed free tropo-
in emissions and vertical mixing of the CBL. Vertical mix- sphericmixingratiosofaselectionoflong-livedtracegases,
ing ratio gradients within the mixed layer of any conserved e.g.ozone,COandVOCstomimicadvectivetransport. The
scalar dC/dz are then solely determined by surface and en- soil NO emission flux was calculated using a soil mois-
trainment fluxes, the height of the CBL (z ), the convective ture representative for wet season conditions. Because the
i
velocityscale(w*),andthenon-dimensionalbottom-upand simulated nocturnal concentrations of isoprene, MVK, and
top-downgradientfunction(Davisetal.,1994;Pattonetal., MACR in the model were generally higher than were ob-
2003). TheheightoftheCBLisdefinedastheheightwhere servedintropicalareas(e.g.Kesselmeieretal.,2000,2002),
the potential temperature and other scalar profiles have a the model-simulated nocturnal mixing ratios of these com-
maximum of the variance (Stull, 1988). In most cases z pounds have been forced (e.g., Ganzeveld et al., 2006b) to
i
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U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest 2861
decrease steadily during the night, reaching minimum mix-
0
ing ratios below 0.2ppb before sunrise. The reason for the
model’s misrepresentation of the nocturnal depletion, e.g. 315 45
17-25 July 2001 (REA, SLG)
fromnocturnalremovalbyturbulentmixing,drydeposition,
orresiduallayerchemistry,needsfurtherinvestigation.
270 90

3 Resultsanddiscussion 05-17 July 2001 (MLG)
225 135
3.1 MeteorologicalconditionsandVOCmixingratios 180
05-17 July 2001 (MLG)
The LBA-CLAIRE 2001 campaign was carried out in the
beginning of the dry season, when the Inter-Tropical Con- -1s] 3000 30 C]
lvoewrgeesntc5ekZmonoefwthaes alotmcaotespdhaetrecaw.a6s◦dNo,mainndataeidr bflyoweaisntetrhlye -2ol m 2000 20 ure [°
m at
tfrraodmethweinAdtsl,anwtihciocvhertrtahnespfoorretestdsohfutmheidAomcaezaonnicBaaisrinm(aSsislveas AR [µ 1000 10 mper
Diasetal.,2002). Airborneprofilemeasurementswerecar- P Te
0 0
ried out on six days in the period of 05–17 July 2001, and
17-25 July 2001 (REA, SLG)
tower-based flux measurements on seven days in the pe-
3000 temp 30
riod of 17–25 July 2001. Weather conditions were simi- -1s] C]
lar for both periods, mostly dry and sunny, with intermit- -2m 2000 20 e [°
tent cloud patches travelling over the site. The local mean mol atur
dfraoymtimthee(0e9a:s0t0f–o1r8t:h0e0tLoTw)ewr-ibnadseddireflcutixonmweaassuprreemdoenmtsin,awntitlhy R [µ 1000 10 mper
onlyaslightshifttothesouthduringtheairbornemeasure- PA PAR Te
0 0
ments (Fig. 3). Back-trajectories calculated using the HY- 00:00 06:00 12:00 18:00 24:00

bridSingle-ParticleLagrangianIntegratedTrajectory(HYS- Local time [hh:mm]

PLIT) model showed a consistent flow of air from the East

tothemeasurementsitethroughouttheexperimentalperiod. Fig.3.Comparisonofmeteorologicalconditionsduringthetwoin-

Theairparcelback-trajectoriesforthetimeframeofthein- tensivemeasurementperiods,of5–17July2001forairborneMLG,
dividualflightsareshownwiththelandcoverinFig.1. The versus 17–25 July 2001 for tower-based REA and SLG. Upper
trajectoryanalysisshowednoevidenceofairhavingpassed panel: frequency distribution of daytime wind directions (09:00–
18:00LT);lowerpanels: ambienttemperatureandPARmeanval-
overthenearbycityofManaus,overareasoflarge-scalede-
ues(solidlines)±standarddeviation(dottedlines).
forestation,oroverextendedareasofopenwaterinriversor
lakes. With vast expanses of pristine forest situated to the
eastoftheK34tower, thissitecanbeexpectedtoberepre-
sentativeofanundisturbedremoterainforestecosystemdur- tower-based measurements in the surface layer versus the
ing easterly winds. Previous measurements in the Amazon dataobservedwithintheCBL,and(ii)middayversusafter-
Basinhaveshownthatradiativecoolingatduskresultsinthe noon values were compared. Toluene and benzene mixing
formationofashallow,decouplednocturnalboundarylayer ratios were relatively low (≤0.1ppb) and did not show dis-
overtheforest, whilstheatingofthesurfaceinthemorning tinctverticalprofiles,indicativeoftheabsenceofhumanac-
causesawellmixedCBLtodevelopataverticalgrowthrate tivitiesorothersignificantairpollutionsourcesatthisremote
ofca.10mmin−1 (Garstangetal.,1988). TheCBLheights, forestsite.
deducedfromtheairborneprofilemeasurementsofpotential Figure 4 shows a contour plot of a typical diel course of
temperatureandothertraceconstituents,rangedbetween450 the isoprene and α-pinene mixing ratios observed directly
and1115m,andwerewithinthetypicalmixedlayerheights abovethecanopy,whichwasinferredfromsurfacelayerpro-
thatareexpectedaboveAmazoniantropicalforests(Diaset filesmeasuredsimultaneouslyatfourdifferentheights,onan
al.,2002;Fischetal.,2004). hourlybasis. Isopreneandα-pinenemixingratiosfollowed
IsoprenewasthedominantbiogenicVOCobservedinam- a clear diel pattern as a function of light and temperature
bientaircontributingupto90%ofthemeasuredspecies.The (see also Rinne et al., 2002; Kuhn et al., 2002). Mean day-
mixingratiosofα-pinenewereanorderofmagnitudelower, timemixingratiosforisopreneandα-pinenereached3.4ppb
and comprised about half of the detected sum of monoter- and0.34ppbat51maboveground. Maximumisopreneand
penespecies. TheVOCspeciescompositionwasverysimi- α-pinene values reached 6.6ppb and 0.6ppb, respectively.
lar throughout theboundary layer, when (i)values fromthe Due to intermittent cloud patches travelling over the site in
www.atmos-chem-phys.net/7/2855/2007/ Atmos. Chem. Phys.,7,2855–2879,2007
2862 U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest
-2-1PAR [µmol m s]1230000000000 0123000 Temperature [°C] -2-1AR [µmol m s] 1234000000000000 PPAARR stdev tteemmpp stdev 12340000 mperature [°C]
50 isoprene 0 [ppb] P 0 0 Te
1.3 -1h] RREEAA smtdeaenv flux
Height above ground [m] 45340000 α-pinene 000235.....01570[61ppb] -2prene flux [mg C m 0369....0000 SSSLCLGGM msmtedoaednve lflleudx
0.17 o -3.0
s
30 I
0.22
10:00 12:00 14:00 16:00 18:00
Local time [hh:mm] -1h] 0.9 RRSLEEGAA mmstedeaaennv fflluuxx
Fig.4.Above-canopymixingratiosofisoprene(middlepanel)and -2m SLG stdev
α-pinene(lowerpanel)measuredon25July2001, andrespective C 0.6
g
lightintensity(PAR,solidline)andairtemperature(dashedline). m 0.3
ThecontourplotsareinferredfromVOCgradientsmeasuredonan x [
u
hourlybasissimultaneouslyonfourdifferentheightsaboveground e fl 0.0
at51,42.5,35.5,and28m;canopytopwas∼30m. nen -0.3
pi
-α -0.6
03:00 06:00 09:00 12:00 15:00 18:00 21:00
theafternoon,themaximummixingratioswereobservedas
Local time [hh:mm]
earlyasmidday.

Fig. 5. Hourly mean values of isoprene and α-pinene fluxes and
3.2 Surfacefluxesofisopreneandmonoterpenes
environmental conditions for the period 17–25 July 2001 (n=3–
8). The data points represent arithmetic mean values ± standard
3.2.1 FluxesderivedbySLGandREA deviation(squareswithfloatingcolumnsfortheREA;andcircles
withshadedareaforSLG).Themiddlepaneladditionallyshowsthe
VOCfluxeswerecalculatedbytheSLGapproachusingthe meanvaluesfortherespectivetimeframepredictedbythesingle-
concentration gradient between the two uppermost heights columnchemistryandclimatemodel(SCM).
ofthetower-basedprofilemeasurements. AllSLGmeasure-
ments were accompanied by simultaneous (synchronized in
time and sampling period) REA measurements, the latter TheSLGfluxesexhibitedahigherscatter,asexpectedforthe
assumed to have the least degree of uncertainty of all flux complex terrain of heterogeneous tall vegetation canopies,
methodsappliedinthisstudy. Figure5showsacomparison andexceededtheREAfluxesinmostcases(Fig.5).
of the mean diel cycle of isoprene and α-pinene fluxes The tower-based canopy scale fluxes, with an estimated
determined by both methods for the whole measurement footprint area of 2–3km2, are assumed to be representative
period. Mean daytime (10:00–15:00LT) fluxes mea- ofthecombinationofthecharacteristiclocal-scalelandscape
sured by the REA approach were 2.1±1.6mgCm−2h−1 elements in this area, including plateaus, slopes, and val-
for isoprene, 0.20±0.18mgCm−2h−1 for α-pinene, leys. The observed range of VOC flux values are consis-
and 0.39±0.43mgCm−2h−1 for the sum of monoter- tentwiththosereportedpreviouslyforremotetropicalAma-
penes. With the SLG approach, the respective fluxes zonforestecosystems(Table1),andareinaccordancewith
were 3.4±3.6mgCm−2h−1, 0.20±0.33mgCm−2h−1, fluxesusedforthisregioninglobalmodelestimates(Guen-
and 0.38±0.58mgCm−2 h−1. The maximum fluxes that ther et al., 1995). The mean daily integrated emission de-
were measured with REA were 5.4mgCm−2h−1 for iso- terminedbyREAsumsupto21mgCm−2d−1 forisoprene
prene (11.3mgCm−2h−1 with SLG), 0.89mgCm−2h−1 and3mgCm−2d−1forthesumofmonoterpenes.Measured
for α-pinene (1.19mgCm−2h−1 with SLG), and surfacefluxesofbiogenicVOCwereroughlyproportionalto
1.70mgCm−2h−1 for the sum of monoterpenes mixingratioswithintheboundarylayer,asexpectedforcom-
(1.86mgCm−2h−1 with SLG). In general, flux esti- poundswhoseatmosphericlifetimeissubstantiallylessthan
mates by both approaches were in reasonable agreement. 1day.
Atmos. Chem. Phys.,7,2855–2879,2007 www.atmos-chem-phys.net/7/2855/2007/
U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest 2863
Table1. VOCfluxesintheAmazontropicalrainforest: estimatesanddirectmeasurementsreportedinliteratureandmeanvaluesfrom3
methodologiesusedinthisstudy. Allnumbersarebasedoncarbon. NotethatfluxestimatesofJacobandWofsy(1988),Zimmermanet
al.(1988),andDavisetal.(1994)werederivedfromthesameobservationaldatabase.
study isoprene α-pinene sumofmonoterpenes site technicalapproach comments
[mgCm−2h−1] [mgCm−2h−1] [mgCm−2h−1]
JacobandWofsy(1988) 2.44(max.∼6) ABLE2Acampaign, one-dimensionaldynamicalchemistry meandaytimeflux(08:00–18:00LT)
Dukeforestreserve modelonverticalprofileswithinCBL
10kmnorthofManaus,Brazil
Zimmermanetal.(1988) 2.73 0.11 0.24 ABLE2Acampaign, MassBudget meandaytimeflux(08:00–16:00LT)
Dukeforestreserve fromverticalprofileswithinCBL
10kmnorthofManaus,Brazil
Davisetal.(1994) 3.63±1.4 ABLE2Acampaign, MixedLayerGradient meandaytimeflux(08:00–15:00LT)
Dukeforestreserve fromverticalprofileswithinCBL
10kmnorthofManaus,Brazil
Helmigetal.(1998) 2.64–7.24 0.11–0.33 0.19–0.45 remoteAmazonforest,Peru MixedLayerGradientandMassBudget meandaytimeflux(10:00–18:00LT)
fromverticalprofileswithinCBL
Stefanietal.(2000) 1.1 0.20 K34tower60kmnorthofManaus,Brazil RelaxedEddyAccumulation 30◦C,1100µmolm−2s−1PAR
Rinneetal.(2002) 2.12 0.23 FlorestaNacionaldoTapajos,Para,Brazil EddyCovariance(isoprene) 30◦C,1000µmolm−2s−1PAR
DisjunctTrueEddyAccumulation(pinene)
Geronetal.(2002) 2.2 LaSelva,Heredia,CostaRica RelaxedEddyAccumulation 28◦,1100µmolm−2s−1PAR
partlycloudyconditions
Karletal.(2004) 1.19(max.2.56) 0.09(max.0.33) LaSelva,Heredia,CostaRica DisjunctEddyCovariance meandaytimeflux
Greenbergetal.(2004) max.1.94 max.0.08 max.0.16 FLONATapajo’s,Brazil one-dimensionalchemistrymodel maximalflux
max.4.68 max.0.11 max.0.20 Balbina,150kmnorthofManaus,Brazil onverticalprofileswithinCBL
max.8.65 max.0.16 max.0.54 ReservaBiologicadoJaru,Rondonia,Brazil
thisstudy 2.1±1.6(max.5.4) 0.20±0.18(max.0.89) 0.39±0.43(max.1.70) K34tower60kmnorthofManaus,Brazil RelaxedEddyAccumulation meandaytimeflux(10:00–15:00LT)
3.4±3.6(max.11.3) 0.20±0.33(max.1.19) 0.38±0.58(max.1.86) SurfaceLayerGradient meandaytimeflux(10:00–15:00LT)
3.7±5.2(max.13.8) 0.40±042(max.1.05) MixedLayerGradient(corrected) MLGcorrected(meanofallflights)
Likeisoprene,themonoterpeneswerealsoemittedbythe
8
tropical vegetation in a light-dependent manner. In agree- 1] observations (REA)
-h G95 leaf level
mentwithpreviousstudies,thereisnowincreasingevidence -2m 6 G95 canopy level
thatlightdependenceofmonoterpeneemissionscanbegen-
C
eralizedfortropicalforests(Rinneetal., 2002; Kuhnetal., g
2002;2004a;Karletal.,2004),aswellasfordeciduoustree m 4
[
species in temperate ecosystems (Staudt and Seufert, 1995; x
u
Ciccioli et al., 1997; Kesselmeier et al., 1996; Spirig et al., fl 2
2005;Dindorfetal.,2006). ForanthropogenicVOCmainly ne
e
deposition was observed. In particular average daily fluxes r 0
p
of–0.21,–0.02andlessthan–0.01mgCm−2h−1weremea- o
s
sured for benzene, toluene and CCl4 during this campaign. I
-2
Thesevaluesaresimilartothosereportedpreviouslyforthe
17.Jul 19.Jul 21.Jul 23.Jul 25.Jul
samesite(Andreaeetal.,2002).
Date

3.2.2 Comparisonwithmodeledfluxes
Fig.6. ComparisonofisoprenefluxesobservedbyREA(squares),
and modelled with a single-column chemistry and climate model
The isoprene surface flux for the measurement region has
(SCM), with implementation of the Guenther et al. (1995) algo-
been simulated with the single-column chemistry and cli-
rithm(G95),andconstrainedwiththeanalyzedmeteorologyofthe
mate model (SCM), which was constrained with the ana-
European Centre for Medium range Weather Forecast (ECMWF)
lyzedmeteorologyfromtheECMWFdatabaseasdescribed
model. Thedottedlinerepresentsfluxesbasedontheleafsurface,
inSect.2.6. Thedielcourseofmeanmodelledfluxesofall whilethesolidlineshowsthefluxescalculatedforthecanopylevel,
measurementdayswereinarangebetweenthosecalculated including within-canopy interactions of gas phase chemistry, soil
byREAandSLG(Fig.5,middlepanel),andhenceshowed uptake,andturbulentmixing.
goodagreementwithobservations. Theevaluationindicates
that the implementation of the Guenther et al. (1995) algo-
rithmintheSCMreproducedtheobservedfluxesreasonably cloudcoverinmodelgridsdoesnotcaptureanysubgridscale
well. As shown in Fig. 6, on sunny days the model simu- variability of PAR. The influence of the non-linearity in the
latedmaximumfluxescomparabletotheREAfluxes(e.g.,19 light-dependence of isoprene emissions on this issue is cur-
and20July),butdidnotcapturetheobserveddielvariability rentlyunderfurtherevaluation.
during partly cloudy days, with intermittent cloud patches It has to be considered that the surface flux measured by
travelling over the site (e.g. 17 and 19 July). This differ- micrometeorological techniques at the canopy level likely
ence is likely due to the fact that average model grid data differsfromtheprimaryplantemissionusuallyimplemented
of PAR do not ideally reflect the highly variable light con- inthemodels.Thedifferencedependsonthedegreetowhich
ditions at the measurement site, as the treatment of average isoprene, that is emitted from leaves, is oxidized while still
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2864 U.Kuhnetal.: IsopreneandmonoterpenefluxesfromCentralAmazonianrainforest
3.2.3 Implicationsforthelocalcarbonbudget
12 1200
-1h] mean total VOC mean CO2
-2m 8 total VOC stdev CO2 stdev 800 The CO2 sink strength of the Amazon rain forest is of spe-
mg C 4 400 -2-1m h] chioaulsientgearessbtufdogreretgsitoundaielsa.sBwieolgleansicglVobOaClscadroboonffasnetdtghreeenne-t
OC flux (REA) [ -04 -0400 ux (EC) [mg C ecKwcoeeomsrsespyeorslemtneleamentieetdrcoaeftrottbhaotelhn.e,g2lflco0oub0nxa2cel)usc.,raTrraehbnneodtnoChbcOeysne2ccrlvneeee(dtcGoeVunxeOscnthCittahuneetgermeeiatsonsafiloi.t,nhn2teer0gaf0otre2ars-l;
al V -8 -800 O fl2 est,inordertoestimatethecontributionofVOCsinthelocal
ot C ecosystemcarboncycleduringLBA-CLAIRE2001.
T -12 -1200 Figure 7 shows hourly mean fluxes of the sum of mea-
00:00 06:00 12: 00 18:00 24:00
suredVOCsfortheperiodof17–25July2001withthecorre-
Local time [hh:mm]
spondingdirectmeasurementsofCO2fluxesbytower-based
Fig. 7. Mean values of total VOC fluxes (isoprene and monoter- EC. Daytime net CO2 influxes during the period of maxi-
mal net photosynthesis were as high as 20µmolm−2s−1.
penes)measuredbyREA,andCO2 fluxescalculatedbyeddycor-
relation(EC)duringtheperiodof17–25July2001.Thedatapoints Nocturnal net CO2 effluxes (including autotrophic and het-
represent arithmetic mean values ± standard deviation (squares erotrophic respiration) were typically in the range between
with floating columns for VOC-REA; and green line with shaded 5–10µmolm−2s−1, resulting in a mean daily (24h) inte-
areaforCO2-EC). grated net CO2 sink strength of 2017mgCm−2d−1 for the
local net ecosystem exchange (NEE). Relative to these re-
sults of the CO -EC, the daily integrated amount of carbon
2
inthecanopyairspace,priortobeingventilatedtothewell- re-emittedbyVOCsasmeasuredbyREA(24mgCm−2d−1
mixed atmosphere. The simulated difference between iso- forisopreneplusmonoterpenes)was1.2%(0.6%duringthe
prene fluxes modelled at the leaf level and modelled at the photoperiod),whichissomewhatlowerthantherange(2.7–
canopy scale is explicitly shown in Fig. 6. This difference, 3.7%)derivedfromeddycovariancemeasurementsofVOCs
ontheorderof10%,reflectstheinfluenceofwithin-canopy andCO fluxesforalowlandtropicalwetforestsitereported
2
interactions of gas-phase chemistry, soil uptake (Cleveland, byKarletal.(2004).
1997), and turbulent mixing in the SCM model. This is in Yet, as is extensively discussed in the literature (Culf et
agreement with recent results by Stroud et al. (2005), who al.,1999;Baldocchietal.,2000,2003;Martensetal.,2004;
suggesteda5to10%reductionoftheisopreneemissionrate Kruijt et al., 2004; Ometto et al., 2005), the EC-CO mea-
2
due to chemical loss within the canopy. It is also similar surementsduringnighttimehavetobeusedwithcaution,as
to results of Karl et al. (2004), who inferred from the ra- they are subject to potential systematic errors at low wind
tio (MVK+MACR)/isoprene in in-canopy air, that less than speeds,andthedailyintegratednumbersofECmaybeover-
10%ofisopreneisoxidizedwithinatropicalforestcanopy. estimated,especiallyinspatiallycomplexterrain.Theuncer-
Allofthisleadstotheconclusionthatin(anddirectlyabove) tainties stem from difficulties related to (i) night time calm
atropicalforestcanopythetransporttimescaleisconsider- conditions, when respired CO tends to accumulate within
2
ablyfasterthanthetimescaleforchemicallossofisoprene, the forest canopy and intermittent updraft events transfer it
andthatthebiasofin-canopyatmosphericchemistryforthe to the atmosphere in a complex spatial pattern not consis-
overall isoprene (and α-pinene) flux measurements is rela- tently caught by the flux tower sensors (Staebler and Fitz-
tivelysmall,i.e.,withintheuncertaintyofthemicrometeoro- jarrald,2004),and(ii)anirregulartopography,wherelateral
logicalfluxmeasurementapproaches. CO advectiontolowerpositionsonthelandscapecanresult
2
Even though the flux values of isoprene are reasonably in CO2 draining out from the eddy covariance tower foot-
well reproduced by the SCM, the comparison of the simu- print (Arau´jo et al., 2002). Considering the methodological
lated and observed mixing ratios showed a significant over- uncertainties and applying different methods of data treat-
estimation for isoprene, as is commonly the case in atmo- ment, Arau´jo et al. (2002) gave a relatively wide range for
spheric chemistry models (Poisson et al., 2000; Bey et al., the annual NEE of 1–8tCha−1a−1 estimated from a one
2001;vonKuhlmannetal.,2004). Thesimulatedmaximum year record of EC data for the same measurement site. A
daytime surface layer mixing ratios are as large as 12ppb, lineardown-scalingtodailyvaluesresultsinarangeof270–
whereasthemaximumobservedmixingratiosdonotexceed 2200mgCm−2d−1, which suggests the above EC result of
6.6ppb. Apotentialexplanationforthegeneraloverestima- 2017mgCm−2d−1 asbeingclosetotheupperlimit. Rela-
tion of isoprene may be found in an underestimation of the tive to the lower range limit, carbon re-emitted as isoprene
modelled chemical destruction of isoprene, associated with andmonoterpeneswouldaccountforaconsiderablefraction
toolowOHradicalconcentrationssimulatedbycurrentmod- ofupto9%.
els,aswillbediscussedinSect.3.3.
Atmos. Chem. Phys.,7,2855–2879,2007 www.atmos-chem-phys.net/7/2855/2007/