Measurements of total hydroxyl radical reactivity during CABINEX 2009

15 Pages·2014·2.75 MB·English

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doi:10.5194/acp-14-2923-2014 Chemistry A
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Measurements of total hydroxyl radical reactivity during CABINEX
2009 – Part 1: field measurements
R.F.Hansen1,2,*,S.M.Griffith2,3,S.Dusanter3,4,5,P.S.Rickly2,3,P.S.Stevens1,2,3,S.B.Bertman6,M.A.Carroll7,8,
M.H.Erickson9,J.H.Flynn10,N.Grossberg10,B.T.Jobson9,B.L.Lefer10,andH.W.Wallace9
1DepartmentofChemistry,IndianaUniversity,Bloomington,IN,USA
2CenterforResearchinEnvironmentalScience,IndianaUniversity,Bloomington,IN,USA
3SchoolofPublicandEnvironmentalAffairs,IndianaUniversity,Bloomington,IN,USA
4MinesDouai,CE,F59508,Douai,France
5UniversitéLilleNorddeFrance,59000,Lille,France
6DepartmentofChemistry,WesternMichiganUniversity,Kalamazoo,MI,USA
7DepartmentofChemistry,UniversityofMichigan,AnnArbor,MI,USA
8DepartmentofAtmospheric,Oceanic,andSpaceSciences,UniversityofMichigan,AnnArbor,MI,USA
9DepartmentofCivilandEnvironmentalEngineering,WashingtonStateUniversity,Pullman,WA,USA
10DepartmentofEarthandAtmosphericSciences,UniversityofHouston,Houston,TX,USA
*nowat:SchoolofChemistry,UniversityofLeeds,WoodhouseLane,Leeds,LS29JT,UK
Correspondenceto:R.F.Hansen([email protected])
Received:31May2013–PublishedinAtmos.Chem.Phys.Discuss.:28June2013
Revised:8January2014–Accepted:6February2014–Published:21March2014
Abstract. Total hydroxyl radical (OH) reactivity was mea- come from oxidation products of VOCs. These results sug-
sured at the PROPHET (Program for Research on Oxi- gestthatadditionalunmeasuredtracegases,likelyoxidation
dants: PHotochemistry, Emissions, and Transport) forested products, are needed to fully account for the OH reactivity
field site in northern Michigan during the 2009 Commu- measuredduringCABINEX.
nityAtmosphere–BiosphereINteractionEXperiment(CAB-
INEX). OH reactivity measurements were made with a
turbulent-flow reactor instrument at three heights from the
forestfloorabove(21and31m)andbelow(6m)thecanopy 1 Introduction
at three different time periods during the CABINEX cam-
paign.InadditiontototalOHreactivitymeasurements,collo- Thehydroxylradical(OH)isanimportantoxidantinthetro-
catedmeasurementsofvolatileorganiccompounds(VOCs), posphere. OH controls the chemical lifetimes of many im-
inorganicspecies,andambienttemperatureweremadeatthe portant atmospheric trace gases (Ehhalt, 1998) and is also
differentheights.Theseancillarymeasurementswereusedto involved in the formation of tropospheric ozone and sec-
calculate the total OH reactivity, which was then compared ondary organic aerosols. Because of the key role that OH
tothemeasuredvalues.Discrepanciesbetweenthemeasured plays in tropospheric chemistry, an accurate understanding
andcalculatedOHreactivity,ontheorderof1–24s−1,were of OH sources and sinks is important. Due to its high re-
observedduringthedaytimeabovethecanopyatthe21and activity,therearenumeroussinksofOHintheatmosphere.
31m heights, as previously reported for this site. The mea- Among the most important are nitrogen oxides and volatile
sured OH reactivity below the canopy during the daytime organic compounds (VOCs). However, of the hundreds of
was generally lower than that observed above the canopy. possible VOCs within the atmosphere, many still elude de-
Closer analysis of the measurements of OH reactivity and tection by traditional VOC measurement techniques (Gold-
trace gases suggests that the missing OH reactivity could steinandGalbally,2007).Thus,acompletecharacterization
ofalltheindividualOHsinksisadauntingtask.Onewayto
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
2924 R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009
characterizeOHsinkswithouttheneedformeasurementsof al., 2003a; Di Carlo et al., 2004; Mao et al., 2010). How-
theirconcentrationsistodirectlymeasurethetotalOHloss ever, some missing OH reactivity on the order of 5–30s−1
rateduetochemicalreactionsthroughmeasurementsofthe (10–50%ofthetotal)hasbeenseensometimesinNashville
totalOHreactivity. (Kovacsetal.,2003),NewYorkCity(Renetal.,2006a),and
The OH reactivity (k ) is defined for a chemical species Tokyo(Chatanietal.,2009;Sadanagaetal.,2004a;Yoshino
X
X as the product of the second-order rate constant (or, for etal.,2006).
termolecular reactions, the bimolecular rate constant calcu- In contrast, the agreement between calculated and mea-
latedattheworkingpressure)forthereactionofXwithOH sured total OH reactivity has been poorer in forested envi-
(k )andtheconcentrationofX: ronments(Table1).PreviousmeasurementsofOHreactivity
X,OH
madeatthePROPHET(ProgramforResearchonOxidants:
kX=kX,OH[X]. (1) PHotochemistry, Emissions, and Transport) site in northern
MichiganfoundmissingOHreactivityvaluesbetween1and
ThetotalOHreactivitykOH,whichistheinverseoftheOH 4s−1(33–50%ofthetotal)thatexhibitedacleardependence
lifetime, can be derived by summing the OH reactivity for
on temperature (Di Carlo et al., 2004). OH reactivity mea-
eachchemicalspeciesX :
i surementsinaconiferousforestinCaliforniaalsofoundsig-
X nificantmissingOHreactivitythatalsodisplayedatemper-
k = k [X ]. (2)
OH Xi,OH i ature dependence, although the absolute values were higher
i (2.5–10s−1, 30–40% of the total) (Mao et al., 2012). Even
Instruments to measure ambient total OH reactivity have highermissingOHreactivity(36s−1,68%ofthetotal)was
been recently developed and deployed in several field cam- measured in a rainforest in Suriname in 2005 (Sinha et al.,
paigns(Louetal.,2010;Dolgoroukyetal.,2012;Nölscheret 2008).ThemagnitudeofmissingOHreactivityfromaboreal
al.,2012;Sinhaetal.,2012).Theseinstrumentsemployone forestinFinlandwasless,beingonly4.6s−1onaverage,but
ofthreemethods,allofwhichmeasuretheOHreactivityby thisaccountedfor50%ofthetotalOHreactivityinthisen-
mixingambientairwithalargeamountofOHinasampling vironment(Sinhaetal.,2010).Similarresultswereobtained
reactorandmonitoringthechangeintheconcentrationofOH atthesamesitein2010,with8.9s−1ofmissingreactivityon
oratracermolecule.Thefirstmethod,knownasthetotalOH averagecomprising77%ofthetotalOHreactivity(Nölscher
loss rate method (TOHLM) (Kovacs and Brune, 2001; Ing- etal.,2012).
hametal.,2009),usesaflowtubereactorwithamovableOH Of all the studies of OH reactivity conducted in forested
injector coupled to a laser-induced fluorescence detection environments(summarizedinTable1),onlyonestudydoes
cell (fluorescence assay with gas expansion, FAGE). A sec- not report significant missing OH reactivity (Ren et al.,
ondmethod,thedual-laserpump-probemethod(Sadanagaet 2006b). This study was conducted in a deciduous forest in
al.,2004b),employsonelaserforOHgenerationat266nm eastern New York State, which is influenced by urban air
from the photolysis of ambient O in the sampling reactor masses. It is unclear why no significant missing OH reac-
3
and a laser-induced fluorescence detection cell (FAGE) to tivity was observed at this site, although the median con-
probeOH.BothmethodsrelyonthedirectdetectionofOH tribution of isoprene to OH reactivity was only 14% (Ren
to measure the first-order OH loss rate. The third method, et al., 2006b). In other campaigns conducted in forested
known as the comparative reactivity method (CRM) (Sinha areas, the measured OH reactivity has been at least 50%
etal.,2008),isanindirectmethodinwhichatracermolecule higherthantheOHreactivitycalculatedfrommeasuredtrace
isdetectedinsteadofOHtodeterminetheambientOHloss species.Thismissingreactivityhasbeenattributedtounde-
rate. tectedemissionsofhighlyreactivebiogenicVOCs(BVOCs;
DirectmeasurementsoftotalOHreactivityfromanyone Di Carlo et al., 2004; Sinha et al., 2010), unmeasured VOC
ofthesetechniquescanbecomparedtovaluescalculatedvia oxidation products (Edwards et al., 2013; Lou et al., 2010;
Eq.(2)frommeasuredconcentrationsofknownOHsinksto Maoetal.,2012),orcombinationsofthesetwo(Nölscheret
assess whether unmeasured trace gases are important unac- al.,2012).
countedOHsinks.Thedifferencebetweenthemeasuredand Most of the OH reactivity measurements to date within
calculatedtotalOHreactivityisreferredtoasthe“missing” forested environments have been made above the forest
OH reactivity and represents the loss of OH from unmea- canopy(seeTable1).AslocallyemittedVOCsandtheirox-
suredsinks. idation products, trace gases carried by transport, and solar
Missing OH reactivity analyses have been conducted for radiation are all present above the canopy, this environment
several urban and forested areas. The agreement between isexpectedtobethemostchemicallyactivewithintheforest.
measured and calculated total OH reactivity in urban areas However,Holzingeretal.(2005)haveobservedVOCoxida-
hasgenerallybeengood,withmissingOHreactivityofless tionproductsbothaboveandbelowaforestcanopy,indicat-
than 3s−1 (5–10% of the measured total OH reactivity), ing that the below-canopy OH reactivity may also provide
well within the combined uncertainties of the OH reactiv- useful information to investigate potential sources of miss-
itymeasurementsandtheOHreactivitycalculations(Renet ingOHreactivity.One-dimensionalmodelingbyMogensen
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R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009 2925
Table1.SummaryoftotalOHreactivitymeasurementsinforestedenvironments.
Campaign Site Dates Foresttype zmeas/zca MRb kOH(s−1)c Tech.d Reference
PROPHET2000 Michigan,USA Jul–Aug2000 Mixeddeciduous 1.5 ∼1.5 1–12e T DiCarloetal.(2004)
PMTACS WhitefaceMountain,NY,USA Jul–Aug2002 Mixeddeciduous f ∼1 3–12 T Renetal.(2006b)
(N/A) Brownsberg,Suriname Oct2005 Rainforest 0.88 ∼3.5 30–70 C Sinhaetal.(2008)
OP-3 Borneo,Malaysia Apr–May2008 Rainforest g ∼3h 5–80 T Edwardsetal.(2013)
SMEAR/BFORM Hyytiälä,Finland Aug2008 Boreal 0.86 ∼3–4i 3–30 C Sinhaetal.(2010)
BEARPEX2009 California,USA Jun–Jul2009 Coniferous 1–1.7j ∼1.5 8-25e T Maoetal.(2012)
HUMPPA-COPEC2010 Hyytiälä,Finland Jul–Aug2010 Boreal 0.87k l 3–76 C Nölscheretal.(2012)
1.17k 5.22m 3–60 C
CABINEX Michigan,USA Jul–Aug2009 Mixeddeciduous 0.32p 2.26q 1–39 T Thiswork
1.1p 2.07q 1–39 T
1.63p 2.34q 1–42 T
aRatioofmeasurementheighttoaveragecanopyheight.bMissingreactivityfraction:ratiooftotalmeasuredOHreactivitytototalcalculatedOHreactivity.cRangeofindividualOHreactivitymeasurementsforentire
campaign,unlessotherwiseindicated.dMeasurementtechniqueused:T=totalOHlossratemethod;C=comparativereactivitymethod.eRangeoftemperature-binnedaverages.fMeasurementsmadeincanopy,no
measurementheightgiven.gMeasurementmadeinaforestclearingataheightof5m.hCalculatedfromdiurnalaverageat∼15:00LT.iMedianvalue:missingreactivityfractionsrangedfrom1to12.jMeasurementsat
multipleheights(9,12and15m),combinedforanalysis.kMeasurementsfrommultipleheights(18and24m).lCalculatedOHreactivityfromin-canopymeasurementsnotreported.mCalculatedfromwhole-campaign
averagevalues.pMeasurementsfrommultipleheights(6,21,and31m).qMedianvalues.
etal.(2011)showsthatambientOHreactivitycanvarywith airisintroducedthroughtheintakearmperpendiculartothe
height in a forest environment. These observations moti- main axis of the reactor. To determine the loss rate of OH
vatefurthermeasurementsbothwithinandbelowtheforest duetowallreactionsintheflowtube(k ),high-puritynitro-
b
canopy. genfromaliquidnitrogendewar(IndianaOxygen,industrial
TheCommunityAtmosphere–BiosphereINteractionEX- gradeforlaboratorymeasurements;Airgas,99.999%purity
periment(CABINEX)campaignduringthesummerof2009 forfieldmeasurements)isintroducedthroughanintakearm
providedanopportunitytomeasureOHreactivityatmultiple perpendiculartothemainaxisofthereactor(Fig.1).TheOH
heightswithinaforestcanopywherepreviousmeasurements radicalsourceinjectorisinsertedalongthemainaxisofthe
from this site highlighted significant missing OH reactivity reactor(Fig.1).Thegasvelocityismeasuredneartheexitof
abovethecanopyataheightof29m(DiCarloetal.,2004). theflow tubereactor witha Pitot-statictube (DwyerInstru-
This paper presents measurements of total OH reactivity at ments 167-6) connected to a 0–1.8torr differential pressure
threeheights(6,21,and31m)abovethefloorofamixedde- gauge (Omega Instruments PX654-015DV). The end of the
ciduousforest.Themeasurementsmadebelowthecanopyat Pitottubeisplacedwithinthereactorfacingtheoppositedi-
6mandabovethecanopyat21and31mhavebeenanalyzed rectionofthemainflow,approximately1cmawayfromthe
andcomparedtoOHreactivityvaluescalculatedfromcollo- reactor wall at a distance of 4.5cm upstream of the OH de-
catedmeasurementsofVOCsandinorganictracespeciesto tectionsystem.ThisplacesthePitottubewithintheturbulent
confirmwhethermissingOHreactivityisobservedabovethe core region of the reactor. Since the OH detection axis, due
canopyandtogaininsightsintoprocessesthatcouldleadto toitsorientationrelativetotheflowtube,samplespreferen-
somemissingOHreactivityatthissite. tiallyfromtheturbulentcoreregion,theflowvelocityshould
bemeasuredinthisregionforaccuratekineticmeasurements
(Seeleyetal.,1993).Toconfirmtheaccuracyofflowvelocity
2 Experimental measurements,thePitottubewascollocatedwithaportable
hot-wireanemometer(Testov405),andstandardflowsofni-
2.1 Instrumentdescription trogen(40–200Lmin−1)werepassedthroughtheflowtube.
Theflowvelocitymeasurementsmadeusingtheanemometer
The total OH reactivity instrument used for these measure- andthePitottubewerewithin10%ofeachother.Tomeasure
ments is based on the total OH loss rate method (TOHLM) thetemperatureoftheairinsidetheflowtube,athermocou-
(KovacsandBrune,2001),whichitselfisanadaptationofthe ple was placed inside the flow tube within a port closest to
flow-discharge technique used in kinetic studies (Seeley et theexit,approximately12cmfromtheexitoftheflowtube.
al.,1993).TheIndianaUniversityinstrument(IU-TOHLM) TheOHradicalsourceinjectorconsistsofamercurypen
consistsofthreemajorcomponents(Fig.1):amovableOH lamp (UVP Pen-Ray) housed in a 2m-long, 1.25cm-o.d.
radicalsource,aflowtubereactor,andanOHradicaldetec- stainlesssteeltube.Thetopofthemercurypenlampisposi-
tionsystemthatisconnectedtotheflowtubereactorthrough tionedapproximately1cmupstreamoftheexitoftheinjec-
anadapterblock. tor. On the downstream end of the injector, there is an alu-
Theflowtubereactorconsistsofa5cm-i.d.(innerdiame- minum cap with small holes drilled concentrically with an
ter)glasstubethatis75cminlengthwithseveral1.2cm-o.d. approximately 60◦ angle of coverage to promote the mix-
(outer diameter) ports (at 25cm intervals) along the length ing of OH radicals with the main flow. In addition to en-
ofthetube.Attheupstreamendofthereactor,therearetwo hancing mixing of OH radicals within the reactor, this cap
5cm-o.d. intake arms perpendicular to each other. Ambient
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2926 R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009
Fig.1.DiagramoftheIU-TOHLMinstrument.
also minimizes the dispersion of ultraviolet radiation into 70pptvwithanaveragemorningpeakaround150pptv,this
thereactor,wheresomeambienttracespeciescouldbepho- correctionwasnotnecessary.
tolyzed.Aturbulizer,madeofTeflonandconsistingoffour TheOHdetectionsystememploystheFAGEtechniquefor
1cm-widefinsplacedat90◦ anglesalongthecircumference OHmeasurements,andisdescribedindetailbyDusanteret
oftheflowtube,isattachedtotheinjector10cmupstreamof al.(2009).Briefly,airfromthereactorissampledthrougha
the cap to help promote mixing and to help establish turbu- 1mm pinhole and is expanded into a low-pressure cell held
lentflowconditions(Seeleyetal.,1993). between 5 and 10torr. This range of pressures is dependent
TogenerateOHradicals,a5–10Lmin−1 flowofnitrogen ontheorificeusedwiththeFAGEinlet.Withtheorificeand
isbubbledthroughhigh-puritywater(EMDChemicals),pro- pumping setup used during CABINEX, the pressure in the
ducingahumidflowofcarriergas.Thewatervaporisthen FAGEcellwas10.8±0.5torr.Atunabledyelaser(Lambda
photolyzedbythe185nmemissionfromthemercurylamp, Physik ScanMate 1) pumped by a Nd-YAG laser (Spectra
producingapproximatelyequalconcentrationsofOHandH Physics Navigator) operating at a pulse repetition rate of
atoms. 5kHz is used to produce coherent light at a wavelength of
616nm.ThefrequencyofthisoutputisdoubledwithaBBO
H O+hv(185nm)−→Hq+OHq (R1)
2 doubling crystal to produce radiation near 308nm. A small
fractionofthe308nmlightissenttoareferencecell,where
The H atoms produced from reaction R1 can then react
a high concentration of OH is produced by thermal decom-
rapidlywithmolecularoxygenpresentasanimpurityinthe
positionofwatervapor.Theresultingfluorescencesignalis
carriergastoproduceHO radicals.AnyunreactedHatoms
2
thendetectedbyaphotomultipliertubeandisusedtodeter-
leaving the injector would react with ambient O to form
2
minewhenthewavelengthofthelaseristunedinresonance
HO radicals due to the large excess of oxygen in air com-
2 withtheA26+(v0=0)←X25(v00=0)electronictransition
paredtotracegases.
of OH. Approximately 38% of the output from the laser is
Hq+O +M−→HO q+M (R2) transmitted to the detection cell through a 2m optical fiber
2 2
(ThorLabsFG-200-UCR).Thelaserlightthenentersamulti-
Under field measurement conditions, it was determined passWhitecellandisreflected20timesacrosstheairstream.
throughcalibrationoftheOHdetectionaxis(Dusanteretal., Theresultingfluorescenceisdetectedbyagatedmicrochan-
2008)thattheinitialconcentrationofOHradicalsinthereac- nelplatedetector(HamamatsuR5916U-52),andthesignalis
toraftermixingwasontheorderof1to5×109cm−3,which processed by a preamplifier (Advanced Instruments F100T)
is approximately 3 to 4 orders of magnitude above typical before being monitored by a photon counter (Stanford Re-
ambientOHconcentrations. searchSRS400).
KovacsandBrune(2001)demonstratedthatHO2 radicals Aregenerativeblower(SpencerVB001)connectedtothe
producedinthereactorviaReaction(R2)canregenerateOH adapter block pulls ambient air through the instrument at
at ambient NO mixing ratios above 1ppbv. This secondary a flow rate of 175–400Lmin−1 (v≈150–350cms−1), es-
sourceofOHincreasestheOHconcentrationinthereactor, tablishing turbulent flow conditions within the reactor with
causing the OH decay to appear slower than it actually is, Reynoldsnumbersbetween5000and11000.Turbulentflow
thus yielding a lower value of OH reactivity. These authors hasseveraladvantagesoverlaminarflowconditionsformea-
proposedacorrectionprocedureforenvironmentswhereNO surements of reaction kinetics. In fully developed turbulent
is present at mixing ratios greater than 1ppbv (Ren et al., flows,exchangebetweenthelaminarsublayeratthewallsof
2003b), based on concomitant measurements of HO2, OH the reactor and the turbulent core is minimal (Seeley et al.,
and NO in the reactor. However, since the ambient daytime 1993).ThisminimizeslossesofVOCsandOHtothereactor
mixingratiosofNOduringCABINEXweregenerallybelow
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R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009 2927
walls.TurbulentflowalsopromotesthemixingofVOCsand
OH, which reduces the possibility of reactant segregation.
To verify that the flow is turbulent within the reactor, flow
velocity profiles were measured at the exit of the reactor
with the Pitot tube. The flow velocity profile was measured
approximately 2cm upstream of the exit of the flow tube.
Measurements of flow velocity were made at increments of
2mmalongtheinternaldiameteroftheflowtube.Theflow
velocity was uniform throughout the inner diameter of the
flowtubeforcarriergasflowratesabove100Lmin−1,with
a 10% higher flow velocity at the center and sharp drop-
offs near the wall of the flow tube. This profile matches the
characteristics of turbulent flow as described by Seeley et Fig.2.TwosubsequentOHdecaysfromCABINEX,onemeasured
al.(1993). withtheinjectortravelingintheforwarddirection(crosses)andone
For ambient measurements, air is sampled through an measuredwiththeinjectortravelinginthereversedirection(circles)
on10July2009.Thesolidlineshowsalinearfityieldingavalueof
8.1cm-i.d.hoseconstructedofperfluoroalkoxy(PFA)poly-
OHreactivityof16.4s−1.
merfilm(thickness0.13mm)andattachedtotheambientair
intake. Different lengths of inlet tubing were used to sam-
pleatthevariousheights.A14minletwasusedforthe6m
measurements, while a 36m inlet was used for the 21 and where k represents the OH loss rate due to wall reactions
b
31m heights. The 14 and 36m lengths of tubing resulted thatismeasuredbyflowingpureN inthereactor.Notethat
2
in residence times of air samples in the inlets of approxi- dilution of OH in the core flow at the exit of the injector
mately 20 and 30s, respectively, at the sampling flow rates is also accounted for in k . Measurements of k were con-
b b
used with each inlet during field measurements. To ensure ductedbothinthelaboratoryandinthefieldusingnitrogen
thattheinletdidnothaveaneffectonthetotalOHreactiv- gas output from a liquid nitrogen dewar. Tests in the field
ity measurements, various lengths of tubing (6–40m) were were conducted on four separate days (see below). To test
attached to the flow tube reactor during laboratory experi- theeffectsofimpuritiesfromthecarrierN intheOHsource
2
mentstochangetheresidencetimeofairsamplesintheinlet on k , measurements of k were performed alternately with
b b
(10–75s)whilemeasuringambientOHreactivityatthesame nitrogen from an industrial-grade N dewar (Indiana Oxy-
2
location.NostatisticallysignificantdifferenceinOHreactiv- gen) and VOC-free nitrogen (purity of 99.9995%, Mathe-
itywasseeninthesetests.Asimilartestwasalsoperformed son). The average difference between the k values from
b
inthefield.FortheCABINEXcampaign,anadditionalinlet these two sources was 0.24s−1, which corresponds to ap-
section(7mlong)wasattachedtoasamplinginletof6.7m, proximately7%ofthek measuredinthesetests.
b
doublingtheinletlength.Theaddedsectionofinletwasal- Measurements of k conducted in the laboratory exhib-
b
ternatelyattachedandremovedevery30min.Thedifference itedsimilarrelativevariationtothoseconductedinthefield.
inreactivitybetweenthe13.7minletandthe6.7minletwas The day-to-day variability for the laboratory tests was ap-
0.32±0.5s−1(n=9). proximately 10% (from 1σ of 0.6s−1), and the variation
An automated linear motion system controls the move- among the four field tests was approximately 8% (from 1σ
mentoftheOHinjector.Theinjectorismovedovera37.5cm of 0.28s−1). The average value of k for the field and lab-
b
pathwhichstarts5cmupstreamoftheFAGEdetectioninlet. oratory tests was 3.6±0.2s−1 (n=39) for total flow rates
The OH fluorescence signal is measured continuously (at a between130and200Lmin−1,wheretheuncertaintyrepre-
frequencyofapproximately2Hz)overthepathoftheinjec- sentsonestandarddeviationoftheaveragevalue.Thisvalue
torinboththeforwardandreversedirections(Fig.2).Forthe of k measured with the IU-TOHLM instrument is compa-
b
IU-TOHLMinstrument,eachOHdecaymeasurementtakes rable to those reported previously for similar instruments
approximately2.5mintocomplete. (Kovacs and Brune, 2001; Mao et al., 2009; Ingham et al.,
Under pseudo-first-order conditions (i.e., [OH](cid:28)[X]), 2009). The laboratory tests suggest that k may decrease
b
theconcentrationofOHovertimecanbeexpressedasafirst- withincreasingflowratesinthereactor,andmaybe1–2s−1
orderexponentialdecay: lowerforflowrateshigherthan200Lmin−1.Forthebelow-
canopy measurements the average flow rate was approxi-
[OH] =[OH] exp(−(k +k )t). (3) mately 220Lmin−1, while for the above-canopy measure-
t 0 OH b
ments,theaverageflowratewasapproximately350Lmin−1.
SolvingEq.(3)fortheOHreactivityk resultsinEq.(4), Unfortunately,measurementsofkb atflowratesgreaterthan
OH 200Lmin−1 were not made in the field during CABINEX.
−1(ln[OH]) As a result, the average value of kb measured in these tests
kOH= −kb, (4) likelyrepresentsanupperlimittothebackgroundreactivity
1t
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2928 R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009
fortheabove-canopymeasurements,leadingtoalowerlimit
fortheoverallreactivitymeasuredat21and31m.
Since the OH fluorescence signal is proportional to OH
concentration, the net OH signal S (the difference of the
OH
on-resonance and off-resonance OH fluorescence signals)
can be substituted for [OH] in Eq. (4). Thus, the OH reac-
tivitycanbedeterminedfromtheslopeofaplotofthenatu-
rallogarithmofS asafunctionofreactiontime(Fig.2).
OH
Since the OH generated within the injector is still not com-
pletelymixedwiththereactantsinthereactoratshortreac-
tiontimes(<50–60ms),itisdifficulttoseparatetheOHloss
due to dilution from the OH loss due to chemical reaction
(Inghametal.,2009).Forthisreason,analysisoftheOHde- Fig.3.SummaryoflaboratorycharacterizationsofIU-TOHLMin-
cayswasstartedatareactiontimeof60ms,whentheinjector strument.Thedashedlinerepresentsthe1:1line;thesolidlineisa
wasapproximately12cmupstreamoftheFAGEinlet.How- linearfitofthedatasetrepresentedbythesolidmarkers.Errorbars
ever,thisplacesanupperlimitof45s−1ontheOHreactivity represent1σ standarddeviationoftheaveragevalues.
measurementswiththeIU-TOHLMinstrument.
The accuracy and precision of the IU-TOHLM mea-
surements wereevaluated during laboratoryexperiments by 2.2 FieldmeasurementsduringCABINEX
measurements of the OH reactivity of four standard mix-
tures(propane,butane,tetramethylethylene(TME),andiso- TheCABINEXfieldcampaignwasconductedfrom1Julyto
prene). These are representative of compounds that are 8August2009atthePROPHETfieldsiteattheUniversityof
present in ambient air and have well-established rate con- MichiganBiologicalStationinnorthernMichigan(Carrollet
stant values spanning two orders of magnitude (10−12– al.,2001).Thissiteislocatedwithinamixeddeciduousfor-
10−10cm3molecule−1s−1) (Atkinson et al., 1995, 2006; estwithanaveragecanopyheightof20m.Apreviousstudy
Atkinson, 1997, 2003). A correlation plot of the measured of the biomass within the area surrounding the site found
and calculated OH reactivity (Fig. 3) shows that measure- anaverageof156±11gm−2 ofisoprene-emittingbiomass,
mentsareonaverage38%lowerthancalculatedvalueswhen 90%ofwhichwasattributedtoaspen(Westberg,2001).
themeasuredvelocityoftheturbulentcoreisusedtodeter- The CABINEX campaign consisted of two phases; the
mine the reaction time. Similar discrepancies are observed first (1–20 July) focused on measurements below the for-
forallthetestcompoundsandsuggesteitheranincomplete est canopy (6m above ground level), and the second (21
mixing between the reactants or a systematic underestima- July–8 August) focused on measurements above the forest
tion of the reaction time. In addition, subsequent tests with canopy(21and31mabovegroundlevel).Furtherdetailson
trans-2-butene and propane showed similar behavior. Mea- theCABINEXcampaigncanbefoundincompanionpapers
suredambientOHreactivityvaluesarescaledupbyafactor (Bryanetal.,2012;Griffithetal.,2013).Duetothelimited
determined from the fit shown in Fig. 3 to account for the instrumentationandvariationinsamplingschemes,theavail-
averagedifferenceobservedintheselaboratoryexperiments. ability of collocated measurements varies among the differ-
Giventhesimilarbehaviorforallthetestcompounds,whose entheights.Measurementtechniquesandtheirassociatedun-
rateconstantsforreactionwithOHspantwoordersofmag- certaintiesaregiveninTable2.
nitude, and the linearity of the measured OH reactivity as a VOCs were measured with a proton transfer re-
function of the calculated OH reactivity, it appears that this action mass spectrometer (PTR-MS) from Washington
discrepancy does not vary with OH reactivity up to 35s−1. State University modified to incorporate dehumidified
We therefore assume a linear scaling factor to account for sampling for increased sensitivity toward formaldehyde
thediscrepancy. (Jobson and McCoskey, 2010). The PTR-MS measure-
The limit of detection (1σ), determined from measure- ments include isoprene, the sum of methyl vinyl ketone
mentsofk describedabove,is0.7s−1underfieldconditions and methacrolein (MVK+MACR), total monoterpenes,
b
for a 10min average. Measurements performed at multiple formaldehyde(HCHO),acetaldehyde,methanol,methylhy-
OHreactivityvaluesindicatethattheIU-TOHLMinstrument droperoxide,acetone,methylethylketone,toluene,benzene,
canmeasureOHreactivityupto45s−1 witha1σ precision and the sum of C2-alkylbenzenes. The VOC measurements
of (1.2s−1+4% of the measured value) for a 10min aver- were alternated between the three heights (6, 21, and 31m)
age. at10minintervals.
Ambient temperature was measured with temperature
probes (R. M. Young) placed at a height of 6m on a
trailer deployed by Washington State University and on the
PROPHET tower at heights of 20.4 and 31.2m. Ozone was
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R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009 2929
Table2.SummaryofsupportingmeasurementsfromCABINEXusedforOHreactivitycalculations.
Measurement Technique Timeresolution Institution
(min)/uncertainty
(%,1σ)/LOD(ppbv)
12VOCs: Protontransfer WashingtonState
reactionmass University
spectrometry
(PTR-MS)
Isoprene 1/5/0.05
Methylvinylketone
+methacrolein 1/5/0.07
Formaldehyde 1/10/0.14
Acetaldehyde 1/5/0.09
Acetone 1/5/0.12
Methylethylketone 1/5/0.12
Methanol 1/5/0.12
Benzene 1/5/0.04
Toluene 1/5/0.04
∗
Methylperoxide 1/– /0.05
Totalmonoterpenes 1/5/0.07
Ozone UVabsorption 1/10/1 IndianaUniv./Univ.
(TeledyneAPI ofMichigan
400E/Thermo
Electron)
NO2 Chemiluminescence 1/12/0.002 WashingtonState
(AirQualityDesign) University
CO IRabsorption 1/6/40 Universityof
(ThermoElectron48C) Michigan
Meteorologicaldata: 1/–/– WashingtonState
temperature, Univ.(6m)
pressure
1/–/– Universityof
Michigan(21m,31m)
J(NO2) SAFS 1/30/– UniversityofHouston
∗Uncertaintyformethylperoxidenotyetquantified;refertoJobsonandMcCoskey(2010)forfurtherdetails.
measured by UV absorption using a Teledyne Model 400E. 17days(5–21July)werespentmeasuringtotalOHreactiv-
COwasmeasuredbyIRabsorptionusingaThermoElectron ity below the forest canopy, while measurements above the
48CbytheUniversityofMichiganinadditiontoozonemea- canopywereconductedon18days(22July–8August);this
surementsusingaThermoElectron49Cinstrument(Carroll includedfourdaysofmeasurementsatthe21mlevel.Total
etal.,2001).Nitrogenoxides(NO =NO+NO )weremea- OH reactivity was measured continuously with a time reso-
x 2
sured by Washington State University using an instrument lution of approximately 2.5min, corresponding to the time
based on chemiluminescence of NO and equipped with a required for measurement of a single OH decay. Measure-
blue light photolytic converter for NO measurements (Air mentsoftheOHlossrateinN (k inEq.4)wereperformed
2 2 b
Quality Design). The NO measurements were alternated on3July,10July,16July,and7August.
x
between the 6, 21, and 31m heights in a similar manner
to the VOC measurements. Photolysis frequencies for NO
2
werecalculatedfrommeasurementsmadebytheUniversity 3 Results
of Houston with a scanning actinic flux spectroradiometer
(SAFS)(Shetteretal.,2003)locatedat32.6m. All relevant measurements, including total OH reactiv-
Measurements of total OH reactivity were made at three ity and ambient VOC mixing ratios, were averaged over
locations:belowtheforestcanopyataheightof6m,approx- a 10min time step. Total OH reactivity was calculated
imately1mabovethetopofthecanopyataheightof21m, from all measurements listed in Table 2. The tempera-
and above the canopy at a height of 31m. Approximately ture measured within the flow tube was used to calcu-
latetemperature-dependentrateconstants.Furthermore,only
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2930 R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009
Fig. 4. Thirtymin diurnal averages of measurements for the 6m Fig. 6. Time series of 10min averages for the 21m height. The
(left),21m(center),and31m(right)heights. dashed line in panel a indicates values of J(NO2) calculated by
theTUV4.4modelforclear-skyconditions.
The measured OH reactivity at all heights shows a dis-
tinct diurnal trend, with diurnal averages ranging 8–16s−1
at6m,5–33s−1 at21m,and3–20s−1 at31m(Fig.4,bot-
tom panels; Fig. 8). The diurnal trend in the measured OH
reactivityisespeciallypronouncedatthe21mheight.Asex-
pectedinasitedominatedbyisopreneemissions(Westberg,
2001), isoprene was the dominant contributor to OH reac-
tivityduringthedaytimehours,contributing60–70%ofthe
calculatedOHreactivityonaverageatallheightsfrom12:00
Fig. 5. Time series of 10min averages for the 6m height. The to 18:00 (Fig. 8). However, the contribution of isoprene de-
dashed line in panel a indicates values of J(NO2) calculated by creasedto20–40%whencomparedtothemeasuredOHre-
theTUV4.4modelforclear-skyconditions. activity for the 21 and 31m heights. Isoprene mixing ratios
weresimilarateachheight,withsustaineddailymaximaof
around2ppbv,althoughtherewereseveralhigh-temperature
totalOHreactivitymeasurementsthatwerecoincidentwith
events(7July,10July,24July,27July)wheretheisoprene
VOC and NO measurements are included in comparisons
x mixing ratios ranged from 4 to 6ppbv (Figs. 5–7, panel b).
ofmeasuredandcalculatedOHreactivity.Measurementsof
OxygenatedVOCs(OVOCs,whichincludemethylvinylke-
ambient NO, SO , HONO, and glyoxal mixing ratios have
2 tone, methacrolein, methyl ethyl ketone, acetone, formalde-
notbeenusedintheOHreactivityanalysis,astheirtotalcon-
hyde,acetaldehyde,methanol,andmethylperoxide)werethe
tributiontotheOHreactivityislessthan0.15s−1.
nextlargestcontributorduringthedaytime,contributing10–
Figure4showsthe30mindiurnalaveragevaluesofambi-
15% to the calculated OH reactivity from 12:00 to 18:00.
enttemperature,J(NO )andOHreactivity,aswellasambi-
2 Among these, MVK+MACR was the largest contributor,
entmixingratiosofozoneandsomeVOCs,atthe6,21,and
accountingfor30to50%oftheOHreactivityfromOVOCs
31mheights.Individualtimeseriesofthemeasurementsare
during this time period. Mixing ratios of MVK+MACR
shownasFigs.5–7.Figure8showsthe30mindiurnalaver-
weretypically0.25–1.0ppbvonaverageateachheight,with
age values of OH reactivity and the speciated contributions
somediurnalvariation,mostevidentat6m(Fig.4).Typical
oftracegasesforthe6,21,and31mheights.Averageambi-
HCHO mixing ratios were between 0.5 and 1.0ppbv for all
entdaytimetemperaturesfrom12:00to18:00spanthesame
heights(Fig.4),althoughtheHCHOmixingratiowasclose
range of values among the three heights (20–26◦C, Fig. 4),
to2.0ppbvduringthe27–28Julyepisode(Fig.7,panelc).
althoughtheaveragediurnalvariationintemperatureat6m
Monoterpenes,oxygenatedVOCsandCOwerethedom-
(12.4◦C)isgreaterthanthatat21m(9.5◦C)or31m(7.2◦C)
inant contributors to OH reactivity at night, each contribut-
(Fig.4,toppanels).ThemagnitudeofthemeasuredOHre-
ing roughly 20–30% of the calculated OH reactivity on av-
activitywasdependentontemperature,asthedayswiththe
erage from 00:00 to 06:00 and from 20:00 to 00:00 for all
highest ambient temperatures (20–25◦C) exhibited some of
heights.Mixingratiosofmonoterpeneswerehighestatnight,
thehighestmeasuredOHreactivity(10,24,27and28July)
approaching1ppbvat6mand21m,butlessthan0.5ppbvat
(Figs.5–7,panele).
31m(Figs.5–7,panelb).MixingratiosofCOshowedvery
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R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009 2931
issmallerthanduringtheday(Figs.4,8).Theseresultssug-
gestthatthereareadditionalsinksofOHthatwerenotcap-
turedbythetracegasmeasurements.
The OH reactivity measurements and calculations were
also binned by the ambient temperatures observed at each
height(Fig.9)toinvestigatethetemperaturedependenceof
OH reactivity contributions (Di Carlo et al., 2004; Mao et
al., 2012). The width of each bin is 1K, with the bin cen-
teredonthetemperatureofinterest.Atallheights,isoprene
isthedominantcontributortothecalculatedOHreactivityat
temperatureshigherthan293K,whilemonoterpenesandCO
dominatetheOHreactivityattemperatureslowerthan291K.
Thenegativevalueat284Kobservedat31mlikelyreflects
Fig. 7. Time series of 10min averages for the 31m height. The
theoverestimationofthebackgroundreactivityatthisheight
dashed line in panel a indicates values of J(NO2) calculated by
asdiscussedabove.Themeasurementsat31mlikelyreflecta
theTUV4.4modelforclear-skyconditions.
lowerlimittothetotalOHreactivity(lowerby1–2s−1).An
inspectionofFig.9(topandcenterpanels)indicatesthatthe
missing OH reactivity at 21 and 31m exhibit a dependence
onambienttemperature,withthemissingreactivityincreas-
ingwithincreasingtemperature.Thistrendissimilartothat
observed previously at this site during the PROPHET 2000
campaign(DiCarloetal.,2004),althoughthemagnitudeof
the missing OH reactivity is higher in this study. Di Carlo
etal.(2004)observed1–4s−1 ofmissingreactivityoverthe
temperature range of 286–298K, whereas values up to 14
and24s−1 wereobservedforthisstudyoverthesametem-
perature range for the 21 and 31m heights, respectively. In
contrast, the measurements below canopy at the 6m height
do not reveal a significant trend of missing reactivity with
Fig. 8. Thirtymin diurnal averages for OH reactivity at the 6m increasingtemperature(Fig.9,bottompanel).
(left),21m(center),and31m(right)heights.MeasuredOHreactiv-
ityisshownbytheline;calculatedOHreactivityisindicatedbythe
colored bars. OVOCs include methyl vinyl ketone, methacrolein,
4 Discussion
MEK,acetone,formaldehyde,acetaldehyde,methanol,andmethyl
peroxide.Topplotsshowthemissingreactivityateachheight.Er-
Asmentionedabove,DiCarloetal.(2004)foundthattheir
rorbarsonthecalculatedreactivityrepresenttheuncertaintyprop-
agatedfromthe1σ uncertaintiesontherateconstantsandconcen- measurements of total OH reactivity made during summer
trationsofeachspecies;errorbarsonthemeasuredOHreactivity 2000 above the canopy at the PROPHET site were 1–4s−1
representthe1σuncertaintyonasingleOHreactivitymeasurement, greaterthantheOHreactivitycalculatedfrommeasurements
propagatedfromuncertaintiesonmeasurementsofflowvelocity,in- of the mixing ratios of OH sinks. From these observations,
jectordistance,kb,andthecorrectionfactordescribedinSect.2.1. these authors suggested that emissions of an unmeasured
highly reactive terpene could be responsible for the miss-
ing OH reactivity. However, subsequent measurements of
little diurnal variation, with an average mixing ratio on the total monoterpenes and sesquiterpenes at this site suggest
orderof150ppbvthatapproached200ppbvonsomedays. that these compounds would only account for 0.7s−1 of
TheOHreactivitymeasuredbelowthecanopyatthe6m the reported missing OH reactivity, which corresponds to
height was generally lower than that measured above the 30%ofthemissingOHreactivityobservedat296Kduring
canopyduringtheday(10:00–18:00),withanaveragemiss- PROPHET2000(Kimetal.,2009).
ing OH reactivity of 6.2±0.2s−1 (46% of total; Figs. 4, DiCarloetal.(2004)basedtheirconclusionsonthetem-
8). Missing reactivity was also observed at night (00:00– perature dependence of the observed missing OH reactiv-
06:00,22:00–00:00)atthisheight,withanaveragevalueof ity. The missing OH reactivity derived from these measure-
6.5±0.2s−1 (65% of total). Above the canopy the average ments was fitted with an equation often used to describe
missingreactivityduringtheday(10:00–18:00)wasonaver- temperature-dependent emissions of monoterpenes (Guen-
age10.3±1s−1 (48%oftotal)for21mand9.32±0.5s−1 theretal.,1993;Schadeetal.,1999):
(61% of total) for 31m (Figs. 5–7, panel e). At night, the
discrepancybetweenthemeasuredandcalculatedreactivity E(T)=E(293)exp(β(T −293)). (5)
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2932 R.F.Hansenetal.:MeasurementsoftotalhydroxylradicalreactivityduringCABINEX2009
Fig.9.Measured(lines,markers)andcalculated(bars)totalOHre-
activity,binnedbyambienttemperatureforthe6m(bottom),21m
(center), and 31m (top) heights. The color scheme for the bars is
thesameasthatforFig.8.Errorbarsonthecalculatedreactivity
representtheuncertaintypropagatedfromthe1σ uncertaintieson
therateconstantsandconcentrationsofeachspecies;errorbarson
themeasuredOHreactivityrepresentthe1σ uncertaintyonasingle Fig.10.MissingOHreactivity,binnedbyambienttemperaturefor
OHreactivitymeasurement,propagatedfromuncertaintiesonmea- the21m(squares)and31m(circles)heightsfortemperaturesfrom
surementsofflowvelocity,injectordistance,kb,andthecorrection 288to298K.DashedlinesindicatefitstoEq.(5)forthe21mheight
factordescribedinSect.2.1. (green), 31m height (blue), and the fit of the measurements from
PROPHET2000reportedbyDiCarloetal.(2004)(black).Forthe
fitsoftheCABINEXdata,MR(293)inEq.(5)wasdeterminedfrom
E(T) represents the emission rate for a given temperature exponentialfitsofthemeasurementsfrom21and31m.Duetothe
T. To compare the temperature dependence of the missing lownumberofpointsinthe297and298Kbinsforthe21mheight,
OHreactivitytothetemperaturedependenceofmonoterpene these points have not been included in the fit nor on this figure.
emissions described by Eq. (5), the temperature-dependent Errorbarsrepresenttheuncertainty(1σ)propagatedfromtheun-
certaintiesonindividualmeasurementsofOHreactivity(described
missing OH reactivity MR(T) was substituted for E(T)
incaptiontoFig.9)andthecalculatedOHreactivity.
and MR(293) for E(293) in Eq. (5). Typical values for β,
whichisthefactorrepresentingthetemperaturedependence
of the emission, range from 0.057 to 0.144K−1 for a vari-
ety of monoterpenes, including α- and β-pinene, as well as duringPROPHET2000andattributedtomonoterpeneemis-
limonene(Guentheretal.,1993).Thesevalueshavebeenes- sions (Fig. 10). However, comparisons of β factors derived
timatedfrommeasurementsofemissionrates.Thelargevari- formonoterpeneemissionsmustbemadewithcaution,asβ
ability in β is to be expected, as emission of monoterpenes maynotbethebestmetricfordeterminationofthesourceof
canvarywithplantspecies,location,andevenamongleaves missing OH reactivity. Missing OH reactivity can be influ-
on a specific plant. Experimental error can also factor into encedbyprocessesthatdonotaffectBVOCemissions,such
thevariabilityinβ (Guentheretal.,1993). asboundarylayerheightandverticalmixing.
The missing OH reactivity measurements from the 2000 Inaddition,missingOHreactivityatthe31mheightdoes
PROPHETcampaignwerefittoEq.(5),resultinginavalue notappeartocorrelatewiththemeasuredmixingratioofto-
for β of 0.11 (Di Carlo et al., 2004), a value similar to that talmonoterpenes(Fig.11).Theseresultsareconsistentwith
previouslyobservedforthetemperature-dependentemission branch enclosure measurements of total OH reactivity dur-
of 1-3-carene (Schade et al., 1999). A similar treatment of ing CABINEX that found that the measured OH reactivity
themissingOHreactivityseenattheBEARPEX2009cam- was consistent with the calculated reactivity based on the
paign, in which the missing OH reactivity is attributed to measured VOC concentrations, including measurements of
BVOCoxidationproducts,resultedinaβvalueof0.17(Mao isopreneandmonoterpeneconcentrations(Kimetal.,2011).
etal.,2012),slightlyhigherthantheupperlimitof0.144K−1 ThesetwofindingsindicatethatdirectlyemittedBVOCsare
observedformonoterpeneemissions. likelynotthecauseofthemissingOHreactivityobservedat
The missing OH reactivity data from CABINEX for the thissite.
21 and 31m heights was also fit to Eq. (5). In contrast to The high missing OH reactivity measured during CAB-
theresultsdescribedabove,thevaluesofβ determinedfrom INEX at temperatures greater than 296K is larger than the
the fits of the data for the 21 and 31m heights (0.34 and missingOHreactivityobservedatthesetemperaturesbyDi
0.35, respectively) are not consistent with those observed Carlo et al. (2004) during the 2000 PROPHET campaign.
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