ionic-strength and ph dependent reactivities of ascorbic
TRANSCRIPT
Ionic-strengthandpHdependentreactivitiesofascorbicacidtowardozonein
aqueousmicro-dropletsstudiedbyaerosolopticaltweezers
Yuan-PinChang,1,2,3*Shan-JungWu,1Min-SianLin,1Che-YuChiang,1GeninGary
Huang3
1.DepartmentofChemistry,NationalSunYat-senUniversity,Kaohsiung80424,
Taiwan
2.AerosolScienceResearchCenter,NationalSunYat-senUniversity,Sizihwan,
Kaohsiung80424,Taiwan
3.DepartmentofMedicinalandAppliedChemistry,KaohsiungMedical
University,Kaohsiung80708,Taiwan
*E-mail:[email protected]
AbstractThe heterogeneous oxidation reaction of single aqueous ascorbic acid (AH2)
aerosol particleswith gas-phase ozonewas investigated in this study utilizing
aerosoloptical tweezerswithRamanspectroscopy.Themeasured liquid-phase
bimolecular rate coefficients of the AH2 + O3 reaction exhibit a significant pH
dependence,andthecorrespondingvaluesationicstrength0.2Mare(3.1±2.0)
×105M−1s−1and(1.2±0.6)×107M−1s−1 forpH≈2and6, respectively.These
resultsmeasuredinmicron-sizeddropletsagreewiththosefrompreviousbulk
measurements, indicating that the observed aerosol reaction kinetics can be
solelyexplainedby liquidphasediffusionandAH2+O3 reaction.Furthermore,
theresultsindicatethathighionicstrengthscouldenhancetheliquid-phaserate
coefficients of theAH2+O3 reaction.The results also exhibit a negative ozone
pressure dependence that can be rationalized in terms of a Langmuir–
Hinshelwood typemechanism for the heterogeneous oxidation of AH2 aerosol
particles by gas-phase ozone. The results of the present work imply that in
acidifiedairway-lining fluids theantioxidantabilityofAH2againstatmospheric
ozonewillbesignificantlysuppressed.
IntroductionAscorbicacid (AH2) isoneofmainantioxidants inepithelial lining fluids (ELF)
which work as thin fluid layer (typical ~0.1-0.2 μm thick) on the surfaces of
airwaysandalveolitoprotectagainstatmosphericoxidants,suchasozone.The
reaction pathways of the ozonolysis of aqueous AH2 (pKa = 4.1) significantly
dependonpH.ForthecaseofpH>pKa,suchaspH≈7, thedominant formof
AH2 is monoanion, AH−, and the ozonolysis of AH− produces dehydroascorbic
acid(DHA)andsingletO2inhighyields(>90%):1,2
AH! + O! → DHA+ O!(!Δ!)+ OH! (1)
ThesingletO2productassecondaryoxidantmaytransduceoxidationdamages
throughELF,3asitiswellknowntoreactactivelywithbiomoleculesinproteins
andDNA.4,5Ontheotherhand,theozonolysisofAH2inthefreeacidformfirstly
produces an unstable primary 1,2,3-trioxolane ozonide (POZ), which could
subsequently subject to unimolecular decomposition and form a secondary
ascorbateozonide(AOZ)orthreonicacid(THR):1
AH2+O3→POZ→AOZorTHR (2)
AOZmay also be qualified as harmful secondary oxidant, as the finding of its
shortlifetimeinbulkwaterimpliesitspotentialhighreactivity.1
Besides the reaction mechanisms, pH can also affect the reactivities of AH2
toward ozone. The reaction rate coefficients of the AH2 + ozone reaction at
different pHhave beendeterminedby various experimental studies,while the
measurement results seem to highly depend on the experimental designs or
conditions.ThemeasurementsinbulkwatercarriedoutbyGiamalvaetal.6show
thattheratecoefficientsatpH=2.0and4.8are6.9×105and5.6×107M−1s−1,
respectively.ThemeasurementsofKanofskyandSima7utilizingatypeofreactor
where the liquid contactedwith gaseousozoneyielded similar values: atpH=
2.0and7.0themeasuredratecoefficientswere5.6×105and4.8×107M−1s−1,
respectively.Twolaterstudies8,9utilizedthesimilartypeofreactorsbutcarried
out the reactions in ELF model solutions (pH = 7.4) instead. However, the
measuredratecoefficientsweresignificantlysmallerthanabovereportedvalues
ofpH>pKa:5.5×104M−1s−1fromKermanietal.9Recently,Enamietal.1studied
this reaction inmicro-droplets as amodel system of air-water interfaces. The
aqueousAH2dropletsinsizeoffewμmwereexposedtogaseousozoneforvery
shorttime,andthenthecompositionoftheinterfaciallayersofreactingdroplets
wasanalyzedbyonlineelectrospraymassspectrometry.Theyobservedthatthe
reaction rates at the interfacial layer (a few nanometers) were at least two
ordersofmagnitudelargerthanthosefrombulkmeasurementsofGiamalvaetal.
Ontheotherhand,theirobservedratioofreactionratecoefficientsatpH>pKa
and pH < pKa was about one order of magnitude smaller than that in bulk
water.1,6
Inthisstudy,weutilizedanaerosolopticaltweezers(AOT)apparatus,whichcan
trapsingleliquiddropletsviaopticalgradientforce,tostudythereactiveuptake
processes of single micro-droplets of aqueous AH2 to gaseous ozone. The
interactionofmicro-dropletswithgaseousenvironmentscanbeamodelsystem
fortheinteractionbetweengasandfluidfilms1andexhaledbioaerosols,asthe
reactiveuptakebehaviorofmicro-droplets involves thedynamicsprocessesof
both thebulkphaseand thegas-liquid interface.10Also the rate equationsand
kineticsmodels for interpreting the reactive uptakes of aerosol droplets have
beenwellestablished,providingastraightforwardstrategytoretrievingtherate
coefficientsof interest fromexperimentaldataofAOTdirectly.10,11Particularly,
oneofthemainadvantagesofAOTisthatthemicrophysicalpropertiesofeach
singletrappedmicro-droplet,suchasradius,refractiveindex,viscosity,diffusion
coefficient,surface tension, temperature,pH,vaporpressureandcompositions,
canbedeterminedviaspectroscopicmeansinhighaccuracyandinrealtime.12–25
Thus,AOTcanbeutilizedasawall-lessreactor to investigatingheterogeneous
reactionsindetails,asthereactionkineticsandmicrophysicalpropertiesofeach
particlecanbemonitoredsimultaneously.26
Therehavebeenseveralexperimental investigationsof thereactionkineticsof
heterogeneous oxidations in single droplets via AOT combined with Raman
spectroscopy.26–31 King et al.27 studied the ozonolysis of aqueous and organic
dropletscontainingunsaturatedorganiccompoundsandobtainedtheirreactive
uptakecoefficients,andthe liquid-phasebimolecularratecoefficientsretrieved
from the reactive uptake coefficients agreed with literature values.
Dennis-Smither et al.28 studied theheterogeneousoxidationof aqueousmaleic
aciddropletsbyozone,and theirmeasuredreactiveuptakecoefficientsagreed
with literature values. Recently, Hunt et al.26 investigated the heterogeneous
oxidation of nitrite anion in aqueous droplets by ozone, attempting to clarify
whetherthesurfaceexcessordepletionofnitriteanionscouldaffecttheaerosol
reactionkinetics.Theirmeasuredratecoefficientagreedwithliteraturevaluesof
bulk measurements, implying that it is not necessary to include the surface
effects in the reaction kinetics of aqueous nitrite droplets under atmospheric
conditions.
In thisworkweutilizedAOTcombinedwithRamanspectroscopy to study the
kineticsof thereactionofaqueousAH2micro-dropletswithgaseousozone.We
measured the bimolecular reaction rate coefficients at different values of pH,
ozonepressuresandionicstrengths,andtheireffectstotheratecoefficientsof
theAH2+ozonereactionwillbeaddressedinthisreport.Finally,wediscussthe
implications of these results for antioxidant kinetics in ELF and exhaled
bioaerosols.
ExperimentalTheaerosoloptical tweezersapparatusused in thiswork issimilar to thoseof
previous studies of AOT,32 consisting a trapping laser, an invertedmicroscope
andaRamanspectrometer.WeutilizedaCWNd:YVO4laseroperatingat532nm
(Coherent, Verdi V2) as the trapping laser. The typical laser power before
entering the microscope was about 30 to 130 mW. For achieving an optimal
trapping force, the laser beamwas expandedby twoplano-convex lenses. The
expanded laserbeamwasguided into theepi-illuminationportof the inverted
microscope (Nikon,EclipseTi2), and then itwas reflected to anoil-immersion
objective(Nikon,CFIPlanApochromatlambda,60X,NAof1.4,WDof0.13mm)
by a dichroicmirror (Chroma, ZT532dcrb). The focused light passed from the
objective through an index matching fluid and through a coverslip into an
aerosol trapping chamber that was mounted on the sample stage of the
microscope. TheRaman scattering light emitted from the trapped dropletwas
collectedbytheobjective,passingthroughaholographicnotchfilter(Semrock,
NF03-532E-25), and imaged onto the entrance slit of a 0.5 m spectrograph
(Andor, SR-500i, using 1200 l/mm grating in this work) coupled with a
TE-cooled spectroscopic CCD (Andor, DU401-BVF). The integrated time for
acquiring Raman spectra was 1 second. The brightfield image of the trapped
dropletwasacquiredbyutilizinga455nmLED(Thorlabs,M455L3)asthelight
source, a CCD camera (Basler, ICDA-ACA640-90UM), and a 451 nm bandpass
filter(Semrock,FF01-451/106-25).
AdenseflowofaqueousL-ascorbicacid(Sigma-Aldrich)aerosolwasgenerated
by a medical nebulizer (Sumo, V-15), and the sizes of aerosol droplets were
about3to8μmindiameter.Theaerosolflowwasintroducedintothetrapping
chamber,andafterwaitingaboutafewminutes,asingledropletwillbecaught
by the focused laser. The relative humidity (RH) in the trapping chamberwas
maintained around85%via injecting awet nitrogen gas flowwith a flow rate
about300sccm,whichwascontrolledbyamassflowcontroller(MKS,1179A),
and itwasmonitoredbyahumiditysensor (Honeywell,HIH-4602-C,accuracy:
3.5%)whichwascalibratedbyamoreaccurateone(Rotronic,HC2A-S,accuracy:
0.5%). To maintain this high RH inside the trapping chamber, all of its
componentswereimmersedinwaterpriorexperiments.Afterthechamberwas
assembled, it was further flashedwith the wet nitrogen flow at least over 30
minutes,beforestartingthetrappingexperiments.Duringatypicalexperimental
time, such as a couple of hours, ~85% RH in the chamber may slowly drift
around 5%, while supplying the constant wet nitrogen flow.We chose not to
change the flow rate to regulate RH during trapping experiments, as a stable
trapping is also affected by the gas flow. The temperature inside the chamber
wasmaintainedat297K.Anozonegenerator(Airphysics,C-L010-DSI)wasused
to produce gaseous O3 from a pure O2 gas flow, and the generated O2/O3 gas
mixturewasfurtherdilutedwithdryN2inastainlesssteelchamber.Duringthe
reactionkineticsmeasurements, thepremixedO2/O3/N2gas flowof fewsccms
wasmixedwiththewetN2gasflowpriorflowingintothetrappingchamber.The
concentrationofozonewason-linemonitoredviaitsUVabsorptionpeakat250
nmbyusinganabsorptioncell(lengthof75cm),adeuteriumlamp(Hamamatsu,
S2D2module),andaCCDspectrometer(OceanOptics,Flame-S-UV-VIS-ES).For
themeasurementsatpH≈2,thesolutionforgeneratingaerosolcontained0.3M
AH2and0.1MNaCl.ForthecaseofpH≈6,the0.3MAH2solutioncontained0.5
M sodium phosphate buffer and 0.1M NaCl, and pH of the final solutionwas
adjustedto6.0usingsolutionsofHClandNaOH.
Results Ramanspectraoftrappeddroplets
Figure 1 shows the representative Raman spectra of a trapped aqueous AH2
dropletinpH≈2,beforeandafterthereactionwithozone.ThemeasuredRaman
bandsofaqueousAH2,suchasthecoupledC=CandC=Ostretchingmodeat1690
cm−1, indicated that thedominate formofaqueousAH2 is freeacid.33After the
trapped AH2 droplets were exposed to ozone, the Raman peak at 1690 cm−1
gradually decreased, due to the cycloaddition of ozone toward theC=Cdouble
bondofAH2.1,34Therelativelybroadbandsofproductspeakedataround1790
cm-1 and 3000 cm−1 were also formed, as shown in Figure 1, and they were
tentatively assigned to C=O and C-H stretching modes of threonic acid,
respectively.Enamiatal.observedthattheotherreactionproductAOZdepicted
inreaction(2)onlysurvivedontheair-waterinterfaceduetoitsshortlifetimein
water.1TheseinterfacialAOZwereexpectedtohaverelativelylowconcentration
andthusnotobservedinthisstudybecauseofthelimitedsensitivityofRaman
spectroscopy. For the trappedAH2 dropletswith pH≈ 6, the coupledC=C and
C=OstretchingmodeofAH− iscenteredat1590cm−1,asshowninFigureS1.33
AftertheAH-dropletswereexposedtoozone,severalRamanbandsofproducts
were formed, such as those centered at 1400 cm−1, 1720 cm−1 and 3000 cm−1
(seeFigureS1andS2), and theywere tentativelyassigned todehydroascorbic
acid,whichisthemainproductofO3+AH−reactiondepictedinreaction(1).
Figure1.RepresentativeRamanspectraofanopticallytrappedaqueousAH2dropletinpH=1.8
before(topdata)andafter(bottomdata)thereactionwithozoneat85%RH.Twospectraare
offsetforclarity.NotethatrelativelysharppeaksinthespectraareCERSsignals(labelledwith
asterisk).Theinsetshowsthebrightfieldimageofthedropletbeforethereactionwithozone.See
TableS1(ESI)(expt.01)fordetailsoftheexperimentalconditions.
Determiningmicrophysicalpropertiesoftrappeddroplets
In theRamanspectraofaqueousAH2droplets therearealsoseveral relatively
sharp peaks superposed on the relatively broad molecular Raman band (see
Figures1andS1).Theyaresocalledcavity-enhancedRamanscattering(CERS),
which are whisper-gallery modes amplified by stimulated Raman scattering.
32,35,36 These CERS signals can be used to retrieve the values of radius and
refractiveindex(RI)ofthedropletwithahighaccuracy.WeutilizedaMietheory
simulation programmrfit developed by Preston et al.13 to analyse the spectral
positionsofCERSsignalsandobtainedfittedRIandradius.ThefittedRIvalues
were further used to retrieve the absolute concentration of AH2 before the
reaction,[AH2]0,andionicstrengthinsidethedroplet.Wealsoutilizedthetime
evolutionof fittedradius to trackthechangeofdroplet ionicstrengthduringa
reactionprogress.WhennoobviousCERSsignalsexisted,theradiusand[AH2]0
of the trapped droplet were determined by brightfield imaging and Raman
intensityratiosofAH2andwater,respectively.Asallsolutesindroplets,suchas
AH2,Na+,Cl−andphosphates,areinvolatile,thedropletionicstrengthcanalsobe
determinedviacomparingtheconcentrationsofAH2indropletsandinthebulk
solution. The values of [AH2]0 determined by CERS and spontaneous Raman
1600 2000 2400 2800 3200 3600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Ram
an In
tens
ity (a
rb. u
.)
Raman shift (cm-1)
Before reaction with ozone After reaction with ozone
*
*
*
*
*
*
CH stretch
OH stretch
C=C and C=O stretch
*
spectroscopyarealsoingoodagreement,justifyingbothmethodsutilizedinthis
study.
Tables S1 and S2 list the values of radius, [AH2]0 and droplet ionic strength
determined by above methods for each trapped droplet. The typical droplet
radius in the present work is about 3 μm. Both [AH2]0 and ionic strength of
trappeddropletswere found to be several times larger than those of the bulk
solutiononaverage,probablycausedbyavaporizingenrichmentbeforeflowing
intothemoisturizedtrappingchamber.
Figure2.Representativetimeprofilesofradius(crosses)andintegratedRamanintensitiesofa
trapped AH2(aq) droplet which was exposed to gaseous ozone after 0th second. The Raman
intensitiesofAH2(squares)andproducts(triangles)wereobtainedfromintegratingtheareasof
theRamanpeakscenteredat1690cm-1and1790cm-1,respectively.Mostintensityjumpsinthe
RamanintensitytimeprofilesareascribedtodriftingCERSpeaks.Hollowsymbolsrepresentthe
intensity interferences caused by frequency-drifting CERS peaks. For clarity, only the typical
errorbarsareprovidedat theendofdata.Theozonepressure is1.13ppm.SeeTableS1(ESI)
(expt.17)fordetailsoftheexperimentalconditions.
DeterminingpHoftrappeddroplets
ForthekineticsmeasurementsatpH≈6,weestimatedthevaluesofpHinside
thetrappeddropletsviacomparingmolecularRamansignalsof inorganicacids
andtheirconjugatebases, followingthemethodsdevelopedbypreviousworks
utilizing sodium bisulfate buffer. We utilized the Raman peak area ratios of
H!PO!! (874cm-1)and HPO!!! (987cm-1)(seeFigureS3)todeterminepHover
0 1000 2000 3000 4000 5000 6000 7000
0
0.2
0.4
0.6
0.8
1
1.2
Ram
an in
tens
ity /
arb.
u.
Time / s
2100
2200
2300
2400
2500
2600
2700
Rad
ius
/ nm
therangeof5.5to7.3,withthecalibrationcurveshowninFigureS4.Notethat
the Raman intensities of H!PO!! and HPO!!! at pH ≈ 6 are very similar,
facilitating the determination of pH around this range. The pH measurement
resultsshowthatthevaluesofpHinsidedropletsbeforeexposedtoozonewere
equal to about 6.2 on average. After the reactionswith ozone finished, the pH
values of droplets slightly increased to about 6.7 on average (see Table S2),
agreeingwith thepredictedmechanismofreaction(1).For thecaseofpH≈2,
the values of pH before reactionwere estimated from themeasured values of
[AH2]andknownpKa(seeTableS1).
It shouldbenoted that the above estimationsof pHperformedby thepresent
work did not consider the effect of high ionic strengths insidemicro-droplets.
Two recent studies utilizing AOT to determine aerosol pH have demonstrated
that thenon-ideal thermodynamicsdue tohigh ionicstrengthshas tobe taken
into account via thermodynamic model calculations, such as specific ion
interactiontheoryorE-AIMaerosolthermodynamicsmodel,inordertopredict
theproton activity correctly.23,25Otherwise, the estimated values of pH canbe
lessaccurate.For thecaseofaqueoussulfate/bisulfatesystems,suchdeviation
couldbeuptoabout2pHunits.23,25Weassumethatthismayberegardedasthe
maximum deviation of pH reported in this work. The main limitation of the
presentwork topredict theprotonactivitymoreappropriately isa lackof the
thermodynamic coefficients of AH2 required by the non-ideal thermodynamic
modelsdescribedabove,andlotsofexperimentalandtheoreticaleffortsarestill
neededforsuchpurpose.
KineticsoftheAH2+ozonereaction
Figures2andS5showthetimeevolutionsofradiusandRamanintensitiesofthe
aqueousAH2dropletsexposedtoozoneatpH≈2andpH≈6,respectively.After
exposed to ozone at 0 second, the Raman intensities of AH2 decreased
immediately, and the product Raman bands emerged at the same time. Their
Raman intensity timeprofiles furthershowthat the formationofproductswas
concomitantlyaccompaniedwiththedecayofAH2,indicatingthattheobserved
productwassolelycreatedfromtheAH2+ozonereaction.Thisobservationalso
indicatesthattheproductsareinerttowardozone,agreeingwiththefindingsof
Enamietal.1TheRaman intensity timeprofiles inFigure2alsoexhibitseveral
intensity fluctuations for short periods of time, and they are caused by the
progressofCERSpeaksthroughthecorrespondingmolecularRamanbands.28In
dataanalysis, suchCERS interferencewas identifiedby tracing theprogressof
CERSpeaks inRamanspectra timeseriesand labelling the timeswhenaCERS
peak superposeswith themolecular Raman band of interest, such as the data
pointslabelledwithhollowsymbolsinFigure2.Suchassignmentofprogressing
CERS peaks can be assisted and justified by Mie theory simulation, when
assuming continuous changes of droplet size and RI and nomode hopping of
CERSpeaks.Figure2alsoshowsthat theradiusonlydecreasedslightlyduring
the reaction progress, implying involatile reaction products and slow
evaporationofwater.Forsimplifyingthefollowingkineticsanalysis,theradiusis
assumed tobea fixedvalue in thedataanalysis,andweassume thechangeof
radiusduringthereactiontobetheerrorofradius.
Figure3.SquarerootofthenormalizedRamanintensitiesoftheAH2C=Cbandversusreaction
time(symbols)atpH=1.8andozonepressureP=1.4ppm(expt.19)andthefit(straightline)
basedonequation(3).Eachdatapointrepresentstheaverageover20consecutivespectra.Two
rugged lines represent an error band which corresponds to 1σ uncertainties of data. The
significantburstsofintensities(symbolsnotingreyareas)wereattributedtofrequency-drifting
CERSpeaks,andthustheywerenotincludedinthefit.
AccordingtoSmithetal.10andKingetal.,27thereactiveuptakeofgaseousozone
ontheaqueousAH2dropletstudiedinthepresentworkcanbeapproximatedas
thediffusion-limitedspecialcase,wherethedissolvedaqueousozonereactswith
the aqueous AH2 near the surface and thus the reaction rate depends on the
bimolecularratecoefficientoftheAH2+ozone reactioninaqueousphase(k2)
andthediffusioncoefficientofaqueousozone(DO).Theintegratedrateequation
canbeexpressedasfollowing:
[AH2](t)[AH2]0
=1− 3HP2r
DOk2[AH2]0
t (3)
where[AH2](t) is [AH2]as functionoft, [AH2]0represents[AH2]at0secondof
reactiontime,HistheHenry'sLawconstantofozoneintheaqueoussolution,P
is the pressure of ozone and r is the radius of the droplet. The value ofDO is
about1.8×10−9m2s−1.35ThevaluesofHatvarioussoluteconcentrationsused
here can be estimated from a Sechenov relation (see ESI).38 According to
equation(3),aplotof([AH2]/[AH2]0)1/2versust shouldyieldastraight line,as
shown in Figure 3, where [AH2] is represented by the C=C stretching mode
RamanintensityofAH2,andthefittedgradientcanbeusedtoderivethevalueof
k2. The values ofk2 determined from all data of this study are summarized in
TablesS1andS2,forpH≈2and≈6,respectively.Finally,notethatweonlyfit
the datawithin the reaction timewhere the values of ([AH2](t)/[AH2]0)1/2 are
between1and~0.3(seeFigure3),becauseofsignificantstandarddeviationsof
data at the longer reaction times. Note that the CERS interferences described
previously can cause significant deviations of theRaman intensity timeprofile
fromEquation(3),asshowninFigure3.Thus,thedatapointsusedtoderivek2
did not include those associatedwith CERS interferences, such as the symbols
notingreyareasinFigure3.Finally,forthecaseofpH≈2measurements,when
assuming that pH of the trapped droplet was solely regulated by [AH2], the
estimatedchangeofaerosolpH in this fittingrange isabout0.5,whichdefines
theuncertaintyofpHforthefittedvaluesofk2.
Figure 4. Bimolecular reaction rate coefficientk2 (symbols) of pH≈2plotted as a functionof
dropletionicstrengthandthefit(line)usingequation(4).Thepressureofozoneappliedtoeach
dataisbelow3.5ppm.Eachplottederrorbarincludesallexperimentalerrors(seeESI).
DiscussionEffectofionicstrengthonbimolecularratecoefficients
Toclarifytheeffectsofionicstrength,wemeasuredthereactionratecoefficient
k2atdifferentionicstrengths.Figure4plotsallthemeasuredratecoefficientsof
pH≈2asafunctionofionicstrength,indicatingapositivecorrelationbetween
ionicstrengthsandreactionratecoefficients.Whentheionicstrengthsincrease
from0.6Mto1.6M,whicharecontributedbyenrichedconcentrationsofNaClin
thetrappeddroplets,thereactionratecoefficientscanincreaseabouttentimes.
AccordingtoLaidler, theratecoefficientkofareactionbetweenan ionandan
neutralmolecule,orbetweentwoneutralmolecules,ationicstrengthImayhave
anapproximaterelationwiththeratecoefficientatzeroionicstrengthk0:39
log!"𝑘 = 𝑙𝑜𝑔!"𝑘! + 𝑏!𝐼 (4)
where b' is an empirical constant. Figure 4 also shows the fitting based on
equation(4)todataofpH≈2atdropletionicstrength0.5Mto1.7M,whichwas
dominatedbyenrichedNaCl,andthefittedvaluesofk0andb'are(1.7±1.1)×
105M−1s−1 and1.2±0.2M−1, respectively.Figure5plots all themeasured rate
coefficientsofpH≈6atdropletionicstrength1.7Mto5.3M,whichweremostly
contributed by enriched AH−, Na+ and phosphates in the trapped droplets.
Similar todataofpH≈2, thereactionratecoefficientsofpH≈6alsoexhibita
weakpositivedependencewithionicstrengths.Wealsoappliedequation(4)to
fitthesedataofpH≈6,asshowninFigure5.Thefittedvaluesofk0andb'are
(1.1±0.6)×107M−1s−1and0.23±0.04M−1,respectively.
Figure 5. Bimolecular reaction rate coefficientk2 (symbols) of pH≈6plotted as a functionof
dropletionicstrengthandthefit(line)usingequation(4).Thepressureofozoneappliedtoeach
data is below 1.5 ppm. Each plotted error bar includes all experimental errors (see the
SupportingInformation).
The activity coefficients of solutes in liquid phase can be affected by the ionic
strength,andsucheffectforreactionscontainingneutralmoleculesisconsidered
in Equation (4) by means of Debye-McAulay approach.39 Base on such
framework, the positive ionic-strength dependence could be ascribed to
increases in activity coefficients of reactants or/and the stabilization of the
activatedcomplexwhichareapproximatedby theb'I term inEquation (4).40,41
ThereactionofozoneandAH2involvesanattackofozonetothecarbon-carbon
doublebondofAH2,thecorrespondingpre-reactivecomplexcouldhavealarger
dipolemoment than reactants. Thus, under conditions of high ionic strengths,
theactivatedcomplexcouldbemorestabilized thanreactantsdue to its larger
ion-dipoleinteractions.41Sucheffectcanfurtherresultinalowerenergybarrier
andthusafasterreactionrate.ForbothcasesofpH≈2and6,weassumethat
thiseffectmayplayaroleintheirpositiveionic-strengthdependences.However,
forthecaseofpH≈2,theincreaseintheionicstrengthcouldalsoincreasethe
activitycoefficientofAH2inthefreeacidformandthusdecreaseitssolubility,so
called salting-out effect.42 When increasing the ionic strength, the water
0 1 2 3 4 5107
108
k 2 /
M−1
s-1
Ionic strength / M
moleculesinthesolvationshellofAH2willbedisplacedbysaltions,reducingthe
availablevolumeoftheaqueoussolutiontodissolveAH2.Inaerosolphase,such
effectcouldfurthercauseAH2moleculestoberepelledtothewater-airinterface,
and the concentration of AH2 near the surface would be higher than in bulk
phase,enhancingthereactionsofAH2withozonenearthesurface.Suchsurface
enrichmentduetoincreasingtheionicstrengthhasbeeninvestigatedbyseveral
previous studies.40,43–45 For the case of pH ≈ 6, this salting-out effect probably
doesnotplayarole,astheincreaseoftheionicstrengthcandecreasetheactivity
coefficient and thus increase the solubility of AH− as charged species instead.
Thismaypartiallyexplainwhythefittedvalueofb'forpH≈2islargerthanthat
forpH≈6.
Figure6.Bimolecularreactionratecoefficientsk2(symbols)ofpH≈2anddropletionicstrength
≈1Masafunctionofgas-phaseozoneconcentrationandthefit(line)utilizingEquation(5).The
plottederrorbarofeachratecoefficientcorrespondsto1σstatisticerrors.
Effectofgaseousozonepressureonbimolecularratecoefficients
Wealsomeasuredthereactionratecoefficientsk2atdifferentgas-phaseozone
pressures.Figure6plotstheratecoefficientsofthesameionicstrengthatpH≈2
andfromP≈2ppmto100ppm.Thehigherozonepressuresyieldthesmaller
valuesofk2.Suchozonepressuredependencemaybeattributedtothesurface
saturation effects for reactants, so called Langmuir-Hinshelwood mechanism,
which could cause the decrease of reactive uptake coefficients and the
underestimationofthemeasuredk2.46,47Therelationshipbetweenthemeasured
rate coefficient k2 and the gas-phase ozone pressure P for the
Langmuir-Hinshelwood-typemechanismcanbemodelledusing:39
WhereKozone is the ozone gas-to-surface equilibrium constant and k2,max is the
maximumbimolecularratecoefficientmeasuredatthelowozonepressurelimit.
Figure6showsthatthefitbasedonEquation(5)hasagoodagreementwiththe
data.Thefittedvaluesfork2,maxandKozoneare(3.1±0.3)×106M−1s−1and(3.2±
0.3) × 10−15 cm3molecule−1, respectively. The fitted value ofKozone for aqueous
AH2dropletsissimilartothosepreviouslyreportedonaqueousaerosolparticles
ofmaleicacid((9±4)×10−15cm3molecule−1)andfumaricacid((5±2)×10−15
cm3molecule−1).38 The results of fitting also verify that the ozone pressures
appliedforthedatainFigures4and5aresmallenough,sothatthissaturation
effectshouldnotappearinourkineticsmeasurements.
Experimentaluncertainty
The error reported for each bimolecular reaction rate coefficient k2 obtained
fromEquation (3) includes theerrorsofmeasuring theozonepressure, [AH2]0
andtheradiusandfittingthegradientfromtheRamanintensitytimeprofile.The
percentageerrorofmeasuringtheozonepressureisfixedto2.6%,whichisthe
assumedmaximumerrorofthemeasuredUVabsorptioncrosssectionofozone
at250nmestimatedbyliterature.48Thepercentageerrorofmeasuring[AH2]0is
about4%,whichisattributedtofittingerrorofcalibrationcurvesandstatistics
ofRamanintensities.Theerrorofthedropletradiusisassumedtobe0.15μm,
whichisanaveragedchangeofmeasuredradiiduringthereactionprogress,as
thefittingerrorfromthesimulation(fewnanometers)andstatisticsofdata(~
20nm,seeFigure2)aresignificantlysmaller.ForthecaseofpH≈2,thetypical
percentageerrorof fittedgradients is about1%.Suchsmall fittingerror could
probably be ascribed to the removal of CERS interferences and small Raman
intensity fluctuations gained from concentrated AH2 (~3.3 M on average) in
aerosols.ForthecaseofpH≈6,thetypicalpercentageerroroffittedgradientsis
about24%.Thisrelatively large fittingerrormaybeattributedto largeRaman
intensityfluctuationsduetotherelativelylowconcentrationsofAH2(~0.6Mon
k2 =k2,max
1+KozoneP (5)
average)inaerosolsandapotentiallyinsufficientremovalofCERinterferences,
whichwillbediscussedlater.
Overall, theseerrorswerepropagated intothedeterminationof theerrorofk2
byEquationsS4andS5showninESI.Asaresult,thetypicalpercentageerrorof
thederivedk2atpH≈2isabout12%,whichisdominatedbytheuncertaintiesof
reactantconcentrationsandradius.ThisvalueatpH≈6isabout36%,whichis
dominatedbythefittingerrorofgradient.Itshouldbenotedthattheerrorsofk2
reportedheredonot include the systematic errors potentially associatedwith
sizechangesofdroplets.Thetime-varyingdropletvolumeduetosizechangecan
alterthespeciesconcentrationsandalsotheionicstrengthduringtheprogress
of reaction, causing additional deviations to the value of k2 determined by
Equation(3),whichassumesafixeddropletvolume.Forthecaseof3μmdroplet
radius,thecorrespondingdecreaseofdropletvolumeduetoradiusdropof0.15
μmis14%.AccordingtoEquation(4),anincreaseof14%inionicstrengthcan
lead to an increase of about 32% or 7% in k2 for pH ≈ 2 or 6, respectively,
causinganoverestimationofk2.Ontheotherhand,theenrichmentofreactants,
suchasAH2,duringthereactioncouldprolongthereactiontimeandleadtoan
underestimation of k2. Based on Equation (3), we could roughly estimate that
suchunderestimationofk2issimilartothechangeofvolume,suchas14%.Asa
result,themaximumsystemicdeviationofk2duetoshrinkingdropletvolumeis
estimatedtobeabout18%overestimationor7%underestimationforpH≈2or
6,respectively.
Finally, we would like to discuss the potential contributions of CERS
interferencestothefittingerrorofgradientinEquation(3)andhowtodealwith
them.Asdescribedbefore,wedealtwiththeCERSinterferencesviatracingthe
CERSpeakssuperposedonthemolecularRamanbandofinterestandexcluding
thedatapointsassociatedwithsuchinterference.Withoutsuchtreatment,thefit
inFigure3cansignificantlydeviatefromthetime-dependenttrendinintensity
definedbyEquation(3).However,wewerenotabletoassignweakCERSsignals
which resemble the noise of spontaneous Raman spectra, and they could just
manifest themselves as an intensity fluctuation in the Raman intensity time
profile.Thisisprobablyoneofmainsourcesoftherelativelylargeuncertaintyof
k2 at pH ≈ 6. To further reduce such interference noise, we propose that it is
actuallydesiredtoenhanceCERSsignals,sothattheycanbeeasilyassignedand
deconvolutedindataanalysis.AstheintensityoftheCERSpeakisproportional
to the spontaneous Raman intensity at the same wavelength,35 one feasible
optionistoenhancethespontaneousRamansignalsviaincreasingthepoweror
lowering the wavelength of the Raman excitation laser. In future works, we
prefer to upgrade the single-beam AOT utilized in the present work to a
dual-beam AOT with a counter-propagating geometry which has been
demonstratedbyseveralresearchgroups,49–51assuchdual-beamgeometrycan
facilitateastabletrappingwitharelativelylargelaserpower.
Table 1. Reaction rate coefficient k2 determined from the present work and
previousmeasurements.
Study I/M freeacida/M−1s−1 monoanionb/M−1s−1
Thiswork 0.0c (1.7±1.1)×105 (1.1±0.6)×107
0.2d (3.1±2.0)×105 (1.2±0.6)×107
Giamalvaetal.6 (6.9±2.3)×105 (5.6±2.6)×107
KanofskyandSima7 ~0.2 (5.6±0.4)×105 (4.8±0.4)×107
Kermanietal.9 ~0.2 (5.5±0.4)×104
a.pH=1.9forthiswork,and2.0forGiamalvaetal.andKanofskyandSima.
b.pH=6forthiswork,4.8forGiamalvaetal.,7.0forKanofskyandSimaand7.4
forKermanietal.
c.k0fromthefittingresultsofFigures4and5.
d.estimatedviaequation(4)andthefittedk0andb'.
Comparingtheresultswithliterature
Table1summarizesthebimolecularratecoefficientsk2determinedbythisstudy
andpreviousmeasurements.Tocomparewiththeliteraturevaluesmeasuredat
ionicstrength0.2M,weestimatedthevaluesofk2atthisionicstrengthfromour
resultsfittedwithequation(4).Forthecaseoffreeacid(pH<pKa),ourresult,
(3.1±2.0)×105M−1s−1,hasthesimilarmagnitudesasthoseofpreviousworks,
such as Giamalva et al.6 and Kanofsky and Sima.7 Such agreement with the
literature valuesmeasured in bulk solution (Giamalva et al.6) implies that our
observedreactionkinetics isdominatedbydiffusion-limitedkinetics inside the
droplets.Forthecaseofmonoanion(pH>pKa),ourestimatedvalueofk2ationic
strength0.2M,(1.8±0.8)×107M−1s−1,hasthesameordersofmagnitudewith
thosemeasured by Kanofsky and Sima7 or Giamalva et al.,6while our value is
about three times less their values. Finally, our results exhibit the significant
dependenceofthereactivitywithpH,agreeingwiththeresultsoftheseprevious
works. Such pH dependence can be attributed to the different reaction
mechanismsoftheAH2freeacid(pH<pKa)andmonoanion(pH>pKa)forms.
Ontheotherhand,theagreementwithliteraturevaluesalsojustifiestheaerosol
pHmeasurementsofthisstudy.
Table2.Estimateddiffuso-reactivelengths,l,forozoneundervariousconditions
ofdifferentpHandconcentrationsofAH2.
Condition k2/M−1s−1 [AH2]/M l/nm
Thiswork(pH≈6) 1.2×107 0.4-1.1 12-19
Thiswork(pH≈2) 3.1×105 1.6-5.6 32-60
ELF(pH>pKa) 1.2×107 4×10−3 ~190
ELF(pH<pKa) 3.1×105 4×10−3 ~1200
Comparisonwithsurfacereactionkinetics
The kinetics of the AH2 + O3 reaction at the air-water interface has been
investigated, and the correspondingsurface reaction rateswere found tobeat
least twoordersofmagnitude larger thanthose fromthebulkmeasurements.1
To observe the surface kinetics in micro-droplets by means of AOT, this may
ideallyrequirethediffuso-reactivelengthstobewithinfewnanometers.26Table
2 lists thepredicteddiffuso-reactive lengthswith theexperimentalparameters
used in thisstudy.For theconditionof thisworkatpH≈2, thecorresponding
diffuso-reactive lengths are about few tens of nm, implying that any interface
effect should not be observable in this study. Thus, the observations of the
enhanced reactivities at pH ≈ 2 in this study could bemainly ascribed to the
effectofionicstrengthinstead.
Fortheconditionof thisworkatpH≈6, theestimateddiffuso-reactive lengths
are reduced to about 12-19 nm, because of the significantly large value of
bimolecular reaction rate coefficient k2. However, even such small lengthmay
still be too large to observe the desired interface enhancement. Indeed, the
maximumvalueofk2atpH≈6observedinthisstudyis(2.4±0.7)×108M−1s−1,
which is only about three times larger than the literature value, but not the
expectedtwoordersofmagnitudeenhancementduetointerfacereaction.Ifwe
wanttofurtherreducethediffuso-reactivelengthtofewnanometers,wehaveto
increasetheconcentrationofAH2tooverfewmolarities.Ontheotherhand,the
relativelyhighconcentrationsofsodiumphosphaterequiredformaintainingpH
≈6alsoresultinlargeionicstrengthsintheaqueousAH2droplets.Theeffectof
ionic strength to the reactive uptake of liquid phase aerosols have been
investigated by several aerosol studies, such as the reactions of SO2 with
peroxidesandthereactionsofmethoxyphenolswithozone.52,53Asthedataofk2
canbereasonablymodelledbyequation(4),asshowninFigure5,theobserved
enhancementofk2atpH≈6 in thisstudy isalsoascribedto theeffectof ionic
strength.
The potential role of interfacial chemistry in aerosol reaction kinetics
investigatedherecouldalsobeexaminedbyvaryingthedropletsizetochange
thesurfacearea-to-volumeratio.Thisapproachhasbeenoftenusedinprevious
studiesofaerosolkineticstounderstandwhethertheheterogeneousreactionis
dominated by volume- or surface-limited case, such as the studies on
heterogeneous reactions of N2O5 on organic or inorganic aerosol particles.54,55
These studies demonstrated that the volume-limited case can manifest the
dependence of reactive uptake coefficients on particle size. In contrast, the
surface-limitedcasedoesnotexhibitsuchdependence.54,55Forthepresentwork,
we plotted the bimolecular rate coefficients corrected to zero ionic strength
(ke−b'I,accordingtoEquation(4))atpH≈2and6asafunctionofdropletradius,
asshowninFiguresS6andS7,respectively.Theseresultsexhibitnoobservable
dependence of size, verifying that the observed kinetics of this work is
dominatedbythesurface-limitedcase(diffuso-reactivelength<<radius).54,55
ImplicationsforELFandbioaerosol
Several previous studies have suggested that the diffusions of potentially
unreacted ozone or secondary oxidants generated from ozonolysis of
antioxidants across the ELF could induce the oxidation damages and even the
cellinjuryofbiomembranes,whilethetypeofsecondaryspecies,suchassinglet
O2orascorbateozonide,coulddependontheacidityofELF.1,3Thefindingsofthe
present study indicate that the acidity can also affect the kinetics of the AH2
oxidation reaction and the diffusion lengths of ozone. Table 2 summarizes the
estimateddiffuso-reactivelengthsofozoneinELFattwodifferentpH,assuming
thattheionicstrengthofELFis0.2M.56Normally,thepHofELFisabout6.9,57
andtheconcentrationofAH2iswithinfewmM.58Thus,accordingtoTable2,the
large reaction rage coefficient of the AH2 + O3 reaction at this pH can yield a
relatively small diffuso-reactive length for ozone which could avoid the full
penetration of ozone through the ELF and the direct contacts of ozone to the
biomembranes. On the other hand, at higher acidities the significantly smaller
reaction rate coefficients of the AH2 + O3 reaction can yield larger
diffuso-reactivelengths,whichcanbeevenmuchlargerthanthedepthofELF.As
a result, the higher acidity of ELF provided by various pathologies or inhaled
particularmatterscannotonlygeneratemoreAOZ,1butalsoresultinthedeeper
penetrationsofAOZandozonethroughtheELFandthusthehigherchancesof
theirdirectcontactstotheaerialbiosurfaces.
TheaqueousAH2aerosolsstudied inthepresentworkcanalsoberegardedas
prototypebioaerosols.Humanscanexhalesmalldropletsofairway-liningfluids,
such as ELF, during normal breathing, coughing or sneezing. These exhaled
bioaerosolsmaycarryairbornepathogens,andadetailedunderstandingabout
the influences of aerosol chemistry and microphysics to these pathogens in
aerosolsbecomescrucialrecently.59–61Theairway-liningfluidstypicallycontain
trace amountsof antioxidants, suchasAH2, uric acid and reducedGlutathione,
besides salts and proteins. The findings of this study suggest that the
concentrations of above species and the ionic strength inside the bioaerosol
dropletscouldincreaseatleastaboutafewtimesduetoevaporation,whilethe
bioaerosolsmaystillremainthesimilarpHat6.9.Asaresult,theconcentrated
AH2inbioaerosolremainsthehighreactivitiesandyieldssmalldiffuso-reactive
lengths forozone (~120nm,assumingenriched [AH2]=10mM), compared to
the typical sizeofbioaerosol (<1-10μm).Thus, theantioxidants, suchasAH2,
actuallybecometoprotect thepathogens insidebioaerosolagainstozone from
environments for a period of time. The reaction time required to scavenge
enrichedAH2(~10mM)inbioaerosols(~5μminsize)withindoorozone(~10
ppb)couldtakeaboutafewhours.Forthemore"toxic"atmosphericozone(~50
ppb),thescavengetimeisthenreducedtoaboutahalfhour.
Finally,wewouldliketojustifytheusageoftherelativelyhighpressureofozone
inthiswork(~2ppmonaverage),whencomparingtoambientozone(~50ppb).
Firstly,theexperimentalresultsofpressuredependenceinthisworkjustifythat
the typical ozone pressure used here is sufficiently low, so that the
correspondingvalueofk2,3.0×106M−1s−1(atpH≈2,ozonepressure=1.4ppm
and ionicstrength=1M,seeFigure6), isalreadyveryclose to thatat the low
pressurelimit,3.1×106M−1s−1.ThismeansthattheozonolysisofAH2dropletsat
both~2ppmand~50ppbozonepressures canbe approximated to the same
special case of heterogeneous reaction kinetics, i.e., diffusion-limited case
characterized byEquation (3) anddiffuso-reactive length,without the need to
include any ozone-pressure dependent kinetics, such as
Langmuir-Hinshelwood-type mechanism. As a result, the main features of
heterogeneousreactionkineticsinvestigatedbythisworkshouldbethesameas
thoseforambientozone.Ontheotherhand,theprimarypurposeofhigherozone
pressuresusedinthis laboratory investigation is toshortentheentirereaction
time, allowing for building statistics via repeating more measurements on
reasonableexperimentaltimescales.
ConclusionsIn this work, we investigated the kinetics of aqueous AH2 reaction in
micron-sizeddropletswithgaseousozonebymeansofaerosolopticaltweezers,
at different ozone pressures, ionic strengths and pH. Particularly, this study
demonstrates that the kinetics and pH of single aerosol droplets can be
determinedsimultaneously.Themeasuredbimolecularreactionratecoefficients
atlowozonepressuresandlowionicstrengthsagreewiththosefromprevious
bulkmeasurements,indicatingthattheobservedaerosolreactionkineticscanbe
solelyexplainedintermsofliquidphasediffusionandAH2+ozonereaction,and
no necessary to include any surface effect. We found that the measured
bimolecular reaction rate coefficients exhibit a Langmuir-Hinshelwood
dependenceonozonepressures, ascribed to theheterogeneousnatureof such
aerosol reaction. We also found that the measured bimolecular reaction rate
coefficients have positive correlations with ion strengths. This study also
confirmsthatthereactionratecoefficientsoftheAH2+ozonereactionatpH<
pKa,suchaspH≈2,canbeabouttwoordersofmagnitudesmallerthanthatat
pH > pKa, such as pH ≈ 6, agreeing with previous bulkmeasurements. These
resultsimplythatthehigheracidityinairway-liningfluidscouldcausethelower
reactivityofAH2toozone.Ontheotherhand,AH2inexhaledbioaerosolscould
actually protect pathogens within against the oxidative damages caused by
atmosphericozoneforaperiodoftime,suchasafewhours.
ConflictsofinterestTherearenoconflictsofinteresttodeclare.
AcknowledgementsThis work was supported by the Ministry of Science and Technology, Taiwan
(MOST107-2113-M-110-004-MY3 and MOST109-2113-M-110-010-) and
NSYSU-KMUjointresearchproject(NSYSUKMU108-I002).Wealsothankforthe
financialsupportsfromAerosolScienceResearchCenter,NSYSU,Taiwan.Finally,
wethankProf.AdamJ.Trevittforthetutorialofmanipulatingaerosols,andwe
also thank Prof. Chia C. Wang and referees of this manuscript for their very
helpfulcomments.
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