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Atmos.Chem.Phys.Discuss.,14,28571–28608,2014 D
www.atmos-chem-phys-discuss.net/14/28571/2014/ isc
doi:10.5194/acpd-14-28571-2014 us
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©Author(s)2014.CCAttribution3.0License. io
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Thisdiscussionpaperis/hasbeenunderreviewforthejournalAtmosphericChemistry pe
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andPhysics(ACP).PleaserefertothecorrespondingfinalpaperinACPifavailable.
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Complex chemical composition of D
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colored surface films formed from ss
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reactions of propanal in sulfuric acid at P
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upper troposphere/lower stratosphere r
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aerosol acidities D
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A.L.VanWyngarden1,S.Pérez-Montaño1,J.V.H.Bui1,E.S.W.Li1, sio
T.E.Nelson1,K.T.Ha1,L.Leong1,andL.T.Iraci2 n
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1DepartmentofChemistry,SanJoséStateUniversity,SanJosé,CA95192,USA pe
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2AtmosphericScienceBranch,NASAAmesResearchCenter,MoffettField,CA94035,USA
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Received:27October2014–Accepted:28October2014–Published:19November2014
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Correspondenceto:A.L.VanWyngarden([email protected]) c
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. sio
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28571 |
Abstract D
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Particles in the upper troposphere and lower stratosphere (UT/LS) consist mostly s
s
of concentrated sulfuric acid (40–80wt%) in water. However, airborne measure- io
n
ments have shown that these particles also contain a significant fraction of organic P
a
5 compounds of unknown chemical composition. Acid-catalyzed reactions of carbonyl pe
r
species are believed to be responsible for significant transfer of gas phase organic
speciesintotroposphericaerosolsandarepotentiallymoreimportantatthehighacidi- |
tiescharacteristicofUT/LSparticles.Inthisstudy,experimentscombiningsulfuricacid D
is
(H SO ) with propanal and with mixtures of propanal with glyoxal and/or methylgly- c
2 4 u
10 oxal at acidities typical of UT/LS aerosols produced highly colored surface films (and ssio
solutions) that may have implications for aerosol properties. In order to identify the n
P
chemicalprocessesresponsiblefortheformationofthesurfacefilms,AttenuatedTotal a
Reflectance–FourierTransformInfraredand1HNuclearMagneticResonancespectro- pe
r
scopieswereusedtoanalyzethechemicalcompositionofthefilms.Filmsformedfrom
|
propanalwereacomplexmixtureofaldolcondensationproducts,acetalsandpropanal
15
D
itself. The major aldol condensation products were the dimer (2-methyl-2-pentenal) is
and 1,3,5-trimethylbenzene, which was formed by cyclization of the linear aldol con- cu
s
densation trimer. Additionally, the strong visible absorption of the films indicates that s
io
higherorderaldolcondensationproductsmustalsobepresentasminorspecies.The n
P
majoracetalspecieswere2,4,6-triethyl-1,3,5-trioxaneandlonger-chainlinearpolyac- a
20 p
e
etalswhicharelikelytoseparatefromtheaqueousphase.Filmsformedonmixturesof r
propanalwithglyoxaland/ormethylglyoxalalsoshowedevidenceforproductsofcross-
|
reactions.Sincecross-reactionswouldbemorelikelythanself-reactionsunderatmo-
D
spheric conditions, similar reactions of aldehydes like propanal with common aerosol is
c
organicspecieslikeglyoxalandmethylglyoxalhavethepotentialtoproducesignificant u
25 s
s
organic aerosol mass and therefore could potentially impact chemical, optical and/or io
n
cloud-formingpropertiesofaerosols,especiallyiftheproductspartitiontotheaerosol P
a
surface. p
e
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28572 |
1 Introduction D
is
c
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Aerosolsintheuppertroposphereandlowerstratosphere(UT/LS)arecomposedpri- s
s
marilyofsulfuricacid(40–80wt%)(Cleggetal.,1998;Finlayson-PittsandPitts,2000; io
n
Tabazadeh et al., 1997) and water but they also contain significant fractions of or- P
a
5 ganiccompounds(Froydetal.,2009;Murphyetal.,1998,2007,2014).Inthecaseof pe
r
UT aerosols, the amount of organic material can even exceed the amount of sulfate
present(Murphyetal.,1998).Thepotentialimpactsofthisorganicmaterialonchemi- |
cal,opticalandcloud-formingpropertiesofUT/LSaerosolsarehighlyuncertainsince D
is
relativelylittleisknownaboutthechemicalcompositionoftheorganicfractionbecause c
u
10 availablesamplingtechniquesandfrequenciesarelimitedbythehighaltitudeairborne ssio
missionsrequired. n
P
IncontrasttoUT/LSaerosols,troposphericaerosolsarebettersampledsoitiswell a
p
establishedthattheycontainmajorfractionsoforganics(upto90%)(e.g.,Calvoetal., e
r
2013; Hallquist et al., 2009; Jacobson et al., 2000; Jimenez et al., 2009; Kanakidou
|
etal.,2005;Murphyetal.,2006;Zhangetal.,2007),andtherehavebeenmanystudies
15
D
aimedatchemicalcharacterizationoftroposphericorganicaerosol(OA)particlesand is
at determining the physical/chemical pathways for the formation of OA. In particular, cu
s
reactions of carbonyl-containing organic species including aldol condensation, hemi- s
io
acetal/acetal formation, organosulfate formation and various polymerization reactions n
P
have all been identified as potential sources of low-volatility organic products in tro- a
20 p
e
posphericorganicaerosols(BarsantiandPankow,2004;ErvensandVolkamer,2010; r
Gao et al., 2004; Garland et al., 2006; Holmes and Petrucci, 2007; Jang et al., 2002,
|
2004; Kalberer et al., 2004; Liggio and Li, 2006, 2008; Liggio et al., 2007; Lim et al.,
D
2010; Michelsen et al., 2004; Nozière and Esteve, 2007; Nozière and Riemer, 2003; is
c
Sareen et al., 2010; Shapiro et al., 2009; Surratt et al., 2007, 2006; Tan et al., 2010; u
25 s
s
Tolockaetal.,2004;Zhaoetal.,2005;ZiemannandAtkinson,2012).Sincethesere- io
n
actionsarealleitheracid-catalyzedorrequiresulfate,theyarelikelytobeevenmore P
a
favorableatthehighsulfuricacidconcentrationstypicalofUT/LSaerosols. p
e
r
28573 |
Preliminary experiments for the current work, in which various carbonyl species D
is
(propanal, glyoxal and/or methylglyoxal) were combined with highly concentrated sul- c
u
furic acid to simulate UT/LS aerosol acidities, produced highly colored solutions; and ss
io
solutions containing propanal produced reaction products that partitioned to the liq- n
P
5 uidsurfaceasmacroscopicsemi-solidsurfacefilmsthatwerealsohighlycolored.The a
p
possibility that similar organic products could partition to thin layers or films on the e
r
surface of UT/LS aerosols is of particular interest because organic compounds that
coat aerosol particles would have the most dramatic effects on aerosol chemical, op- |
D
tical and/or cloud-forming properties (see Donaldson and Vaida (2006) and McNeill is
10 etal.(2013)forreviewsofaerosolsurfacecoatingsandtheirimpactsonaerosolprop- cus
erties).Forexample,organiccoatingsonaqueousdropletsandsulfuricacidaerosols s
io
have been observed to impede water uptake and/or evaporation in laboratory exper- n
P
iments (e.g., Davies et al., 2013; Otani and Wang, 1984; Rubel and Gentry, 1984; a
p
Seaveretal.,1992;Xiongetal.,1998),soorganiccoatingsonUT/LSaerosolsand/or er
droplets could potentially inhibit water condensation and therefore cloud formation
15 |
and/orgrowth.Organiccoatingsmayalsoimpactheterogeneousreactionsataerosol
D
surfaces;forexample,reactiveuptakeofN2O5hasbeenshowntobeimpededbyvar- isc
ious organic coatings which could reduce the rate of hydrolysis of N O to HNO on u
2 5 3 s
sulfuricacidaerosols,affectingNOxandOHbudgets(Anttilaetal.,2006;Badgeretal., sion
20 2006;CosmanandBertram,2008;Cosmanetal.,2008;Escorciaetal.,2010;Evans P
a
andJacob,2005;Folkersetal.,2003;Gastonetal.,2014;Knopfetal.,2007;McNeill p
e
et al., 2006; Park et al., 2007; Riemer et al., 2009; Thornton and Abbatt, 2005). Sim- r
ilarly, organic coatings on sulfate aerosols would alter optical properties, especially if |
theorganicsarehighlyabsorbingintheUV-visible.Inordertoassesswhetherspecies D
thatformsurfacefilmsonpropanal/H SO mixturesinthelaboratorycouldbeimpor- is
25 2 4 c
u
tantinUT/LSaerosols,thereactionsresponsibleforfilmformationmustbeidentified, s
s
whichis,therefore,thefocusofthepresentwork. io
n
Recent work with various other aldehydes (Li et al., 2011; Sareen et al., 2010; P
a
Schwier et al., 2010) demonstrated that products of reactions of formaldehyde, ac- p
e
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28574 |
etaldehyde,glyoxal,methylglyoxalandtheirmixturesaresurfaceactiveeveninwater D
is
andammoniumsulfate/watersolutionscharacteristicoflessacidiclowertropospheric c
u
aerosols. Their chemical characterization of the reaction products identified hemiac- ss
io
etal oligomers and aldol condensation products, but the surface active species were n
P
5 notspecificallyidentified. a
p
In order to identify the chemical species present in films formed by propanal and e
r
sulfuric acid, we consider the products of the following potential reactions (identified
|
byletterinFig.1):(A)aldolcondensation,(B)trimethylbenzeneformationviacycliza-
D
tionofthelineartrimerproducedbyaldolcondensation,(C)hemiacetal,acetal,and/or is
10 polyacetal formation, (D) trioxane formation via cyclotrimerization and (E) organosul- cus
fate formation. Each of these processes result in higher molecular weight products, s
io
whichcouldresultinpartitioningtothesolidphaseasasurfacefilm. n
P
AldolcondensationproductsareexpectedsinceitcanbeseenfromFig.1thatthey a
p
aretheonlypotentialproductscontainingsufficientconjugationtoabsorbvisiblelight, er
buttheyarenotnecessarilythemajorcomponentofthefilmssinceonlytinyamountsof
15 |
suchchromophoresarenecessaryforcolor(McLaren,1983).Productsofpropanalal-
D
dolcondensationreactionshavebeenobservedinaqueousmediacontainingvarious is
c
catalysts including anion exchange resin (Pyo et al., 2011), ammonium and carbon- u
s
s
ate salts (Nozière et al., 2010), mixed metal oxides (Tichit et al., 2002), and zeolites io
n
20 (Hoangetal.,2010).Inthecaseofzeolitecatalysts,1,3,5-trimethylbenzenewasalso P
a
observedandproposedtoformfromthelineartrimerproducedbyaldolcondensation p
e
reactions (Fig. 1b). Aldol condensation reactions of propanal have also been studied r
inconcentratedsulfuricacid(60–96wt%)solutionsbyNoziereandEsteve(2007)and |
Casaleetal.(2007).NoziereandEstevereportedtheUV-visspectraofaldolconden- D
sationproductsof6carbonylcompoundsincludingpropanalandconcludedthattheir is
25 c
u
absorptionindexcouldbecomesignificantovertheapproximatelytwoyearresidence s
s
time of stratospheric aerosols. Casale et al. (2007) measured bulk reaction rates for io
n
a series of aliphatic aldehydes (C – C ), showing that butanal and propanal had the P
2 8 a
highestreactionrates,butconcludingthattherateswerenotfastenoughtoberespon- p
e
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28575 |
sible for transfer of significant organic mass into tropospheric aerosols. Both studies D
is
focused on aldol condensation reactions due to their potential to form light absorbing c
u
compoundsandthereforeusedUV-visiblespectroscopyforproductdetection,whichis ss
io
notsensitivetoproductsoftheotherpotentialreactions(C-E)consideredhere. n
P
5 Propanalmayalsoundergoacid-catalyzedreactionswithitshydratedform(diol)to a
p
formhemiacetals,acetalsandorlinearpolyacetalsasshowninFig.1c.Inadditionto e
r
these linear species, propanal may also undergo acid-catalyzed cyclotrimerization to
|
formacyclicpolyacetal(atrioxane)(Fig.1d).Thesereactionshavenotbeenreported
D
specificallyforpropanalinsulfuricacid,butGarlandetal.(2006)haveshownthatsulfu- is
10 ricacidaerosolsexposedtohexanalvaporcontainedhemiacetalswhileLietal.(2008) cus
identified a trioxane in bulk reactions of octanal with sulfuric acid (but not in sulfuric s
io
acid aerosols exposed to octanal vapor). (In both studies, aldol condensation prod- n
P
uctswerealsoobserved.)Furthermore,propanalhasbeenshowntoformatrioxanein a
p
aqueoussolution(Corrochanoetal.,2010)andtoformamixtureofaldolcondensation er
products,hemiacetalsandacetalsinthepresenceofananion-exchangeresincatalyst
15 |
(Pyoetal.,2011).
D
Lastly, alcohols may react with sulfuric acid to form sulfate esters (Deno and New- is
c
man, 1950; Iraci et al., 2002; Michelsen et al., 2006; Minerath et al., 2008; Van Loon u
s
s
and Allen, 2004, 2008; Vinnik et al., 1986), so alcohol species including the diol io
n
20 formedbyhydrationofpropanaland/or(hemi)acetals(Surrattetal.,2008)formedfrom P
a
propanal(Fig.1c)couldreactdirectlywithsulfuricacidtoformorganosulfatessimilar p
e
tothoseformedbyreactionofglyoxalonsulfuricacidaerosols(Liggioetal.,2005).An r
exampleisshownforreactionofthepropanalhydrateinFig.1e. |
InthepresentstudywefirstemployacombinationofAttenuatedTotalReflectance– D
FourierTransformInfrared(ATR-FTIR)and1HNuclearMagneticResonance(1HNMR) is
25 c
u
spectroscopiestoidentifythemajorspeciesinthefilmsformedbypropanalonsulfuric s
s
acidsolutions.Inordertoapproachmoreatmospherically-realisticmixturesoforganics io
n
andtoaddressthepossibilityofcross-reactionsbetweendifferentcarbonylspecies,we P
a
alsoexaminedfilmsformedonmixturesofpropanalwithglyoxaland/ormethylglyoxal. p
e
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28576 |
Finally, we also used UV-visible spectroscopy of the liquid solutions to gain chemical D
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insightintotheidentityofthechromophoresandtoillustratetheirpotentialimportance c
u
forUT/LSaerosolopticalproperties. ss
io
n
P
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2 Experimentalmethods p
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5 Aftersurfacefilmswerefirstdetectedonsolutionsofpropanal,glyoxal,and/ormethyl- |
glyoxal in sulfuric acid during a preliminary study (see Fig. S1 in the Supplement for
D
photos of typical surface films), controlled survey studies were performed to examine is
c
u
the conditions required for formation of surface films. In these experiments, samples s
s
of propanal, glyoxal, and/or methylglyoxal in all possible combinations of 1, 2 or all 3 io
n
10 species(0.030Mineachorganicpresent)werepreparedinstocksolutionsof19,37, Pa
48and76wt%sulfuricacid(H SO ).AlthoughUT/LSaerosolconcentrationsofthese p
2 4 e
organiccompoundsareunknown,0.03MislikelymuchlargerthanUT/LSconcentra- r
tionsofanyonecarbonylspecies,butismorereasonableifconsideredasrepresenta- |
tiveofthetotalaldehydeorcarbonylconcentration.Sulfuricacidstocksolutionswere D
15 prepared by dilution of concentrated sulfuric acid (96–98wt%, Sigma-Aldrich, ACS iscu
grade)withMilli-Qwater,andconcentrationswereconfirmedbytitrationwithstandard- s
s
ized sodium hydroxide (0.5 N, Sigma-Aldrich). The following Sigma-Aldrich organics ion
were used: 97wt% reagent grade propanal, 40wt% glyoxal and 40wt% methylgly- P
a
p
oxal in water. 4.0mL aliquots of each mixture were transferred to multiple 8mL glass e
r
vialsandstoredundereachofthefollowingtemperatureandlightingconditions:room
20
temperature (21–24◦C)/constant fluorescent light, room temperature/dark, 0◦C/dark, |
−19◦C/dark.Sampleswerevisuallymonitoreddailyforcolorchangesandformationof D
is
surfacefilmsinordertosurveywhichmixturesformedfilmsandtoassesstheimpact c
u
s
ofacidity,organicmixture,temperatureandfluorescentlightonfilmformationrates. s
Chemical analysis of the films required production of films in sufficient quantity to ion
25
P
allow physical removal of a portion without disturbing the underlying sulfuric acid so- a
p
lutionsandtherebyavoidingspectroscopicinterferencesfromwaterandsulfuricacid. e
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28577 |
Therefore,samplesusedforchemicalanalysiswerepreparedasabove,exceptatthe D
is
higher concentration of 0.30M in each organic and were stored in volumetric flasks c
u
(roomtemperature/fluorescentlight)whichcausedthefilmtoconcentrateonthesmall ss
io
liquidsurfaceareaintheneckoftheflask.Filmsampleswereremovedandtransferred n
P
5 with a glass rod to the surface of an ATR crystal for analysis by FTIR spectroscopy. a
p
ATR-FTIR spectra of the films and standards were collected on a Nicolet 6700 spec- e
r
trophotometer from 4000–700cm−1 at 1cm−1 resolution using an MCT detector, and
a 10-bounce AMTIR ATR crystal with 45◦ mirrors from PIKE. ATR-FTIR was chosen |
D
forchemicalanalysissincethesemi-solidfilmscouldbedirectlyanalyzedonacrystal is
10 compatiblewithconcentratedsulfuricacidandwithoutanyneedtoalterthechemical cus
environment by dissolving the sample in a solvent. In order to provide more chemi- s
io
cal specificity, films were also analyzed by 1H NMR spectroscopy using a Varian IN- n
P
OVA400MHzspectrometer.NMRsampleswerepreparedbydissolvingfilmsamples a
p
in deuterated chloroform (CDCl3) in quartz NMR tubes (5mm outer diameter). ATR- er
FTIRand/orNMRspectrawerealsorecordedforthefollowingcommerciallyavailable
15 |
standards: 2-methyl-2-pentenal (97wt% Sigma-Aldrich), 1,3,5-trimethylbenzene and
D
2,4-diethyl-6-methyl-1,3,5-trioxane(TCI). is
c
Finally,theUV-visibleabsorptionspectraofsolutions(0.030Mineachorganic)were u
s
s
obtainedusingaVarianCary50BioUV/visiblespectrometerwithadiodearraydetec- io
20 torandquartzcuvettesofvariouspathlengthsfrom0.01–10mmfordifferentregionsof nP
a
the spectrum. Prior to analysis, solutions were filtered through 2.5µm Teflon filters to p
e
removeanysuspendedsolidparticulates. r
|
D
3 Results is
c
u
s
3.1 Formationoforganicsurfacefilms s
io
n
25 Carbonyl-containingorganics(propanal,glyoxaland/ormethylglyoxal)mixedwithsul- Pa
p
furicacid(19–76wt%)tosimulateUT/LSaerosolaciditiesproducedcoloredsolutions, e
r
28578 |
precipitates and surface films. At the highest acidities, all individual organics and or- D
is
ganicmixturesexamined(0.030Mineachorganic)producedvisiblycoloredsolutions c
u
that darkened with time. Mixtures containing propanal produced the most deeply col- ss
io
oredsolutions,progressingfromyellowtoorangetoredtobrownovertimescalesrang- n
P
5 ingfromminutestomonths.Thiscolordarkeningprogressedfasterathigheracidities, a
p
consistent with an acid-catalyzed reaction. Many propanal-containing mixtures also e
r
eventuallyproducedcoloredprecipitatesand/orsurfacefilms.(Mixturescontainingonly
|
glyoxaland/ormethylglyoxaldidnotproducesurfacefilms.)Thesesolidsorsemi-solids
D
were observed either as particles suspended in the liquid (usually collecting near the is
10 surface) and/or as semi-rigid macroscopic films on the surface. In principle, the films cus
couldpotentiallybeformedeitherbyheterogeneousreactionsattheair/liquidinterface s
io
orbyliquid-phasereactionsresultinginproductsthatpartitiontothesurface.Thelatter n
P
process,however,issupportedbytheobservationthatwhensolutionswerestoredin a
p
volumetricflaskssolid,darkcoloredmaterialsometimescollectedontheupperslanted er
wallsinthebodyoftheflaskbeforemigratingtothesurface;presumablythematerial
15 |
rose due to its low density relative to the solution, but was temporarily impeded from
D
reaching the surface by the flask walls. Furthermore, the quantity of film material ob- is
c
servedcannotbeeasilyexplainedbyheterogeneoussurfacereactionsalone. u
s
s
Therewasvariabilityinfilmformationratesforreplicatesofthesurveyexperiments, io
n
20 however,thefollowinggeneraltrendsemerged.First,theprecisedependenceoffilm- P
a
formationrateonaciditywascomplex,but,ingeneral,thefilmsformedfasterathigher p
e
acidity, consistent with acid-catalyzed processes. In fact, the most acidic (76wt% r
H2SO4) propanal/glyoxal mixture produced a surface film immediately upon combin- |
ingthereactants,althoughotherorganicmixturesformedfilmsmoreslowlyat76wt% D
than at 48wt% H SO . Second, film-formation rates also varied as a function of or- is
25 2 4 c
u
ganic mixture. In general, mixtures containing glyoxal formed films more rapidly than s
s
thosewithout,whilemixturescontainingmethylglyoxalformedfilmsmoreslowly.Third, io
n
filmsformedbothinthedarkandunderfluorescentlightwithnoconsistenttrendinfor- P
a
mationrate.Finally,filmsformedmoreslowlyatcoldertemperatures,but,importantly p
e
r
28579 |
for application to the cold UT/LS, were eventually observed (after approximately 100 D
days)evenatthelowesttemperature(−19◦C)examined. isc
u
s
s
3.2 Chemicalcompositionofsurfacefilms io
n
P
a
The highly-colored nature of the surface films (only formed on solutions containing p
e
propanal)isstrongevidenceforaldolcondensationproducts,asaldolcondensationis r
5
the only potential reaction (Fig. 1) of propanal in sulfuric acid that can result in prod- |
uctswiththeconjugationrequiredtocauseabsorptionofvisiblelight.Infact,multiple D
aldolcondensationstepsarerequiredtoproducesufficientconjugation,sincethefirst isc
u
aldolcondensationproductofpropanal(2-methyl-2-pentenal,seeFig.1a)iscolorless s
s
10 with λmax for the π→π∗ transition of ∼266 and ∼233nm in 75wt% H2SO4 (Casale ion
etal.,2007)andwater(ourstandard)respectively.Furtherconjugationfromadditional P
a
p
aldol condensation reactions of 2-methyl-2-pentenal with propanal or with itself is re- e
r
quiredtoshiftabsorptionintothevisible.Althoughproductsfrommultiplealdolconden-
sation steps are almost certainly responsible for the film color, these chromophores |
are not necessarily the major chemical components of the films, so ATR-FTIR and D
15 1H NMR spectroscopies were used to analyze the chemical composition of the sur- isc
u
s
face films. The combined results of these two techniques provide evidence that the s
io
films are a mixture of aldol condensation products (mainly 2-methyl-2-pentenal and n
P
1,3,5-trimethylbenzene) and acetals (mainly 2,4,6-triethyl-1,3,5-trioxane and longer- a
p
chainlinearpolyacetals)asdetailedinSects.3.2.1through3.2.3below.Thedetailed e
20 r
chemical analysis in these sections is presented for surface films formed on 0.30M
|
propanal/48wt% H SO as a starting point, since surface films were only formed
2 4
D
on solutions containing propanal and since propanal formed films fastest at 48wt% is
c
H SO . Sections 3.3–3.4 subsequently address the impact of varying the acidity and u
2 4 s
25 organicmixturefromthisbasecase. sio
n
P
a
p
e
r
28580 |
3.2.1 Aldolcondensationproducts D
is
c
u
Figure2presentsatypicalATR-FTIRspectrumofasurfacefilmformedona7dayold s
s
0.30Mpropanal/48wt%H SO mixture(ingreen)alongwithspectraoffourstandards io
2 4 n
forcomparison.Thestrongabsorptionbandinthefilmspectrumat1689cm−1andthe P
a
5 bandat1643cm−1 areconsistentwiththecharacteristicC=OandC=Cstretchingvi- per
brations,respectively,ofanα,β-unsaturatedaldehyde,whichisproducedbyaldolcon-
|
densation (Fig. 1a). The spectrum of neat 2-methyl-2-pentenal shown in Fig. 2 (blue)
displays these bands at 1687 and 1643cm−1 and is scaled to illustrate the maximum D
is
amount of the film spectrum that could be explained by its presence (limited by the cu
size of the C=C band at 1643cm−1). An additional C=O peak at 1722cm−1 occurs ss
10 io
intheunsaturatedaldehydestretchingregionandisassignedtounreactedpropanal. n
P
In Fig. 2, the spectrum of neat propanal (red) is also scaled to illustrate its potential a
p
contributiontothespectrumofthefilm. e
r
The 1H NMR spectrum for this film presented in Fig. 3 indicates that 2-methyl-2-
|
pentenal is the dominant species since it contains strong peaks (assigned in Fig. 3)
15
D
correspondingtoallfivetypesofhydrogensin2-methyl-2-pentenalinthecorrectmulti- is
c
plicityandwithin0.03ppmofourstandard.Althoughsomeofthepeaksaretoosmallor u
s
tooclosetointerferingpeakstointegratereliably,therelativepeakintensitiesarealso sio
roughly consistent with the standard. Residual propanal is similarly positively identi- n
P
20 fied by comparison to the standard as shown by peak assignments in Fig. 3. There ap
e
are no additional detectable NMR peaks consistent with linear compounds with ad- r
ditional units of conjugation due to multiple aldol condensation steps, indicating that |
they must be significantly less abundant than 2-methyl-2-pentenal and therefore will
D
notcontributesubstantiallytotheFTIRspectrum,either.(Forexample,theprotonsla- is
c
25 belled A and B in 2,4-dimethyl-2,4-heptadienal (Fig. 1) would be expected to appear us
s
as singlets with chemical shifts near those for 2,4-hexadienal (Spectral Database for io
n
OrganicCompounds,2014,SDBS)at9.5and7.1ppm,respectively.) P
a
p
e
r
28581 |
Although there is no NMR evidence for linear aldol condensation products beyond D
is
2-methyl-2-pentenal,NMRpeaksat2.27and6.79ppmconfirmthepresenceof1,3,5- c
u
trimethylbenzene (mesitylene) (SDBS), which has previously been observed to form ss
io
in reactions of propanal over acidic zeolite catalysts (Hoang et al., 2010). Hoang n
P
5 et al. proposed that 1,3,5-trimethylbenzene was formed by acid-catalyzed cyclization a
p
andsubsequentdehydrationofthetrimerformedbyaldolcondensation(2,4-dimethyl- e
r
2,4-heptadienal)ofpropanalasshowninFig.1b,whichisareasonablemechanismfor
|
sulfuricacidsolutionsaswell.Furthermore,thetrimerformedbyaldolcondensationof
D
acetonehasalsobeenshowntocyclizetoform1,3,5-trimethylbenzeneinsulfuricacid is
10 (Duncanetal.,1998;Kaneetal.,1999;Klassenetal.,1999).InFig.2,theATR-FTIR cus
spectrumofneat1,3,5-trimethylbenzene(black)isscaledtothefilmspectrumtoindi- s
io
cateitsmaximumpotentialcontribution.Thecomparisonshowsthatthefilmspectrum n
P
isconsistentwiththepresenceofsome1,3,5-trimethylbenzeneinthefilmsinceithas a
p
bands corresponding to the 2 most intense 1,3,5-trimethylbenzene bands at 834 and er
1609cm−1, the latter of which lies in the region for aromatic skeletal vibrations and,
15 |
therefore,cannotbeexplainedbyanyotherpotentialproducts.
D
Although2-methyl-2-pentenaland1,3,5-trimethylbenzeneareshownheretobethe is
c
major products resulting from aldol reactions, both are colorless, so the more highly u
s
s
conjugated compounds formed by further aldol condensation steps that are presum- io
n
20 ably responsible for the film color must be minor constituents. Therefore, there must P
a
also be additional compounds present in the film to explain the strength of the peaks p
thatappearintheFTIRspectrumbetween1500–800and3000–2800cm−1. er
|
3.2.2 Ethers:acetals/hemiacetalsandlinear/cyclicpolyacetals
D
is
c
Inadditiontoaldolcondensationproducts,theFTIRandNMRspectrabothalsodisplay u
s
25 evidenceforethergroups(C-O-C)duetostrongpeaksinthe1200–1000cm−1 region sio
oftheFTIRspectrum(Fig.2)andpeaksinthe4.5–5.1ppmregionoftheNMRspec- n
P
trum.Speciesthatcouldberesponsiblefortheseethersignaturesincludehemiacetals, a
p
e
acetals and/or higher order polyacetal polymers which can form from the reaction of r
28582 |
propanal with one or more of its hydrates (diols) (Fig. 1c) or from cyclo-trimerization D
is
of propanal to form the cyclic acetal, 2,4,6-triethyl-1,3,5-trioxane (Fig. 1d). Of these c
u
potentialproducts,thecyclotrimerismosteasilyconfirmedsinceitisreadilyidentified ss
io
by comparison of the FTIR and NMR spectra of the film to reference spectra (SDBS) n
P
5 of2,4,6-triethyl-1,3,5-trioxaneasindicatedbythepeaksassignedtothetrioxane(T)in a
Figs. 2 and 3. Specifically, the 1H NMR spectrum of the film contains all three of the pe
r
peaksinthereferencespectrum:atripletat4.78ppm,acomplexmultipletat1.67ppm
|
andatripletat0.94ppm(althoughthebroadpeakgroupat0.94ppmcanonlybepar-
D
tiallyduetothetrioxaneduetoitsstrongintensityrelativetotheothertrioxanepeaks). is
10 Sofimthielarfillym, acsorsrheoswponnbdywaitshsiingn2mcmen−t1stionpFeiga.ks2,inatthleearsetfe1r3enpceeaskpseinctrthuemFoTfIaRnsepaetclitqruumid cuss
io
filmof2,4,6-triethyl-1,3,5-trioxane(includingall6ofthestrongestreferencepeaksbe- n
P
tween1500–900cm−1).(Thistrioxaneisnotcommerciallyavailable,soinordertoalso a
p
provideageneralideaofrelativepeakintensitiesexpectedintheATR-FTIRspectrum er
ofthetrioxane,thespectrumofatrioxanethatdiffersonlybyreplacingoneethylgroup
15 |
withamethylgroup(2,4-diethyl-6-methyl-1,3,5-trioxane)isalsoshowninFig.2.)Fur-
D
thermore,previousstudiesof2,4,6-triethyl-1,3,5-trioxanereportthatitphaseseparates is
c
uponformationfrompropanal/catalystsolutions(Satoetal.,1993),consistentwithour u
s
s
surfacefilmformation. io
n
20 Upon assignment of the cyclotrimer peaks, only one major peak in the FTIR spec- P
a
trum of the film remains unexplained by species identified thus far (2,4,6-triethyl- p
e
1,3,5-trioxane, 2-methyl-2-pentenal, 1,3,5-trimethylbenzene and propanal). This peak r
at945cm−1 is,however,thestrongestpeakinthespectrumofthefilmandtherefore, |
must be a major peak in the spectrum of the absorbing species. The hemiacetal and D
singleacetalformedbypropanal(Fig.1c)areunlikelytoberesponsibleforthepeakat is
25 c
945cm−1sincetheywouldbeexpectedtoproducetheirstrongestbandsathigherfre- us
s
quencies. Specifically, the hemiacetal would produce a strong FTIR absorption band io
n
in the 1150–1085cm−1 region from the asymmetric stretch of its single ether group, P
a
whiletheacetalcontainstheC-O-C-O-Cmoietywhichwouldproduce5characteristic p
e
r
28583 |
bands between 1200–1020cm−1 (Bergmann and Pinchas, 1952). Furthermore, both Dis
the hemiacetal and acetal are also unlikely to be major ether constituents since only cu
a weak peak exists in the OH stretching region (3500–3400cm−1) where a stronger ss
io
peak(withrespecttothepeaksintheetherregion)wouldbeexpectedduetotheOH n
P
groups. a
5 p
Instead,thestrongpeakat945cm−1 mostlikelyresultsfromlonger-chainpolymers er
ofpropanal(polyacetalinFig.1c)dueto-C-O-C-O-C-stretchingbandsthatareshifted
|
tolowerfrequencieswiththeadditionofadditionalethergroups.Spectraofpolymers
D
ofvarioussmallaldehydes(formaldehyde,acetaldehydeandpropanal)whichcontain is
c
thesamepolymethoxy(-C-O-) backbonedisplayonlyveryweakOHstretchingbands u
10 n s
butmultipleverystrong,broad,overlappingabsorptionbandsbetween925–975cm−1 sio
(NovakandWhalley,1962,1959a,b;Vogl,1964a,b).Althoughthepeakat945cm−1 nP
does not exactly match any of the three strongest peaks (975, 960 and 925cm−1) ap
e
in this region in the Novak and Whalley (1959a) spectrum of the polymethoxy poly- r
merformedbypressurizationofpropanal,thereissuchbroadabsorptionintheentire |
15
980–920cm−1 regionofthepolymerspectrumthatapeaknear945cm−1 maynotbe D
distinguishable.Furthermore,thepolymerpresentinoursurfacefilmislikelytodisplay isc
different relative intensities of the peaks in this region due to differences in degree of us
s
polymerizationand/ordifferencesinrelativequantitiesofrotationalisomers(Novakand io
n
Whalley,1959a).Additionally,bandsmayalsobeshiftedinfrequencyduetodifferent P
20 a
interactionsbetweenpolymerchains(NovakandWhalley,1959a)inthecomplexsur- pe
r
facefilmmatrix.Finally,theNMRspectrumofthefilmisalsoconsistentwiththepres-
ence of propanal polymer since the 4.5–5.1ppm region contains multiple unassigned |
peaksconsistentwithethersandsimilartothebroadgroupofunresolvedpeaksfrom D
is
4.5–5.0ppmthatcharacterizestheNMRspectrumofthepolymethoxypolymerformed c
25 u
byacetaldehyde(Vogl,1964a),whileCH2 andCH3 protonsfromtheethylchainsare ssio
likelyresponsibleforpeaksinthe1.0–1.7ppmregionandforaportionofthetripletat n
P
0.94ppm,respectively. a
p
e
r
28584 |
Afterthisidentificationofpolymersofpropanal,wenotethatallofthemajorbands D
is
in the infrared spectrum that could not be explained by aldol condensation products c
u
either correspond to 2,4,6-triethyl-1,3,5-trioxane or could reasonably be assigned to ss
io
longer-chainlinearpropanalpolymers. n
P
a
5 3.2.3 Otherpotentialfilmcomponents:organosulfates,etc. pe
r
Inordertotestforthepresenceoforganosulfates,reactionmixtureswerepreparedwith |
hydrochloricacidatthesamepHasthesulfuricacidmixtures.Theformationofsurface D
films on these mixtures demonstrates that organosulfates are not necessary for film isc
u
formation,andthesimilarityoftheATR-FTIRspectraforfilmsformedonsulfuricacid s
s
10 andhydrochloricacidsolutions(exampleshowninFig.4for0.30Mpropanal/48wt% ion
H SO )demonstratesthatorganosulfatesarenotpresentinsignificantquantities.We P
2 4 a
p
note,however,thatorganosulfatescouldstillbeproducedinthesulfuricacidsolutions, e
r
wheretheywouldbeexpectedtoremainduetotheirpolarnature.
Although all of the major peaks (and many of the minor peaks) in both the FTIR |
andNMRspectraofthefilmcanbeassignedtothechemicalspeciesdiscussedthus D
15 is
far, some small unassigned peaks (e.g. NMR peaks at 3.2 and 3.9ppm) indicate the c
u
s
presence of other minor species. These could include products of multiple aldol con- s
io
densationsteps,aldolsthathavenotlostwaterthroughthecondensationprocess(see n
P
Fig.1),thehemiacetalandacetalformedbypropanal,otheracetalsthatcouldalsopo- a
p
tentially be formed by reactions of aldol condensation products with propanal, and/or e
20 r
productsfromoxidationoffilmsbylight/air.
|
3.3 Effectofacidity D
is
c
u
As discussed in Sect. 3.1, acidity has a complex effect on the formation rates of the ss
surfacefilmsthatvariesdependingontheorganicmixture.Ingeneral,filmstendedto ion
25 formfasterastheacidityincreasedfrom19to37to48wt%H2SO4,butfilmsformed Pa
p
more slowly or not at all at the highest acidity (76wt% H SO ) in all mixtures except e
2 4 r
28585 |
propanal/glyoxal.The FTIRspectraoffilms formedonmixtures of0.30Mpropanalin D
is
48 and 37wt% H SO solutions in Fig. 5 (shown in triplicate) demonstrate that there c
2 4 u
are also chemical differences in films formed at different acidities. The spectra are ss
scaled to the C=O peak at 1690cm−1 from aldol condensation products (predomi- ion
5 nantly2-methyl-2-pentenal)inordertoillustratedifferencesinrelativepeakintensities. Pa
p
Althoughthereisconsiderablevariabilityinrelativepeakintensitiesamongthespectra e
r
ofreplicates(mostlikelyduetofilminhomogeneity),thepeaksinthe1200–900cm−1
region are generally larger relative to the C=O peak at the higher acidity (red), in- |
D
dicating a larger relative contribution to the film from the 2,4,6-triethyl-1,3,5-trioxane is
10 afonrdmelodngaetrt-hcehahiinghpeorlyaaccideittaylaplosloymhaevrsetshmatalalebrsoprebakinstahtis16re0g8iocnm.−I1n,aindddiictiaotnin,gthsemfialmllesr cuss
io
concentrationsof1,3,5-trimethylbenzenerelativetoaldolcondensationproducts.Both n
P
ofthesetrendsareconfirmedbyNMRspectroscopy(datanotshown).Thepresence a
p
ofmore2,4,6-triethyl-1,3,5-trioxaneandpolymersathigheraciditiesisconsistentwith er
faster film formation at higher acidities in the 19–48wt% H SO range since these
15 2 4 |
speciesaremostlikelyresponsibleforthephaseseparationintoasurfacefilm.Addi-
D
tionally, slower film formation at the highest acidity (76wt%) is potentially due to low is
c
watercontentthatreducestheformationofthediolsrequiredtobeginthepolymeriza- u
s
s
tionprocess(seeFig.1c). io
n
P
3.4 Cross-reactionswithglyoxalandmethylglyoxal a
20 p
e
r
To examine the potential effect of additional organic species with carbonyl groups on
|
theformationoffilms,mixturesofpropanalwithglyoxaland/ormethylglyoxalwerealso
D
examined. Although glyoxal and methylglyoxal did not form films in the absence of is
c
propanal,themixturesofpropanalandglyoxalformedfilmsfasterthanpropanalalone, u
s
25 whilemixtureswithpropanalandmethylglyoxalformedfilmsmoreslowly.Acomparison sio
ofFTIRspectraoffilmsformedonvariousorganicmixturesin48wt%H SO allpre- n
2 4 P
paredonthesamedayareshowninFig.6.(Becausesignificantvariabilityinrelative a
p
e
peakintensitiesexistsinreplicateFTIRspectraofthefilmsmostlikelyduetoinhomo- r
28586 |
geneity in the solid mixtures of multiple chemical species, a complete set of replicate D
spectra are provided in the Supplement (Fig. S2) to demonstrate that the differences isc
u
between organic mixtures discussed here are in fact due to differing chemical path- ss
waysandarenotsimplysamplingartifacts.)ThespectraareagainscaledtotheC=O ion
5 peakat1690cm−1 frompropanalaldolcondensationproductsinordertoillustratedif- Pa
p
ferencesbetweenrelativepeakintensities.Thespectraofthefilmsfrompropanaland e
r
propanal/glyoxalarenearlyidenticalinthe1800–1600cm−1regionindicatingthatboth
|
filmsinclude2-methyl-2-pentenalandunreactedpropanalinsimilarratios.Conversely,
thespectralpatterninthe1200–900cm−1regionforthepropanal/glyoxalfilmdoesnot Dis
10 correspondtothespectrumof2,4,6-triethyl-1,3,5-trioxaneasitdoesforthepropanal- cus
onlyfilm.Sinceglyoxaldidnotformfilmsbyitself,thisinfraredsignatureismostlikely s
io
duetoproductsofcrossreactionsbetweenpropanalandglyoxal. n
P
TheFTIRspectrumofthefilmfrompropanalandmethlyglyoxaldeviatesevenfarther a
p
from that of propanal-only. Not only does it lack the signature of 2,4,6-triethyl-1,3,5- er
trioxane, indicating products of cross-reactions, but, additionally, absorbance in the
15 |
entire1500–900cm−1 regionismuchstrongerrelativetothealdolcondensationpeak
D
at1690cm−1,indicatingastrongerrelativecontributionfromacetalspecies.Finally,itis is
c
intriguingthat(1)thespectrumofthefilmformedonthepropanal/glyoxal/methylglyoxal u
s
mixture is quite similar to that for the propanal/methylglyoxal mixture, differing only in sio
n
20 relativepeakratiosand(2)thattherateoffilmformationwasdecreasedfromtherate P
a
forthepropanal/glyoxalmixture.Thiscouldindicatethatglyoxalissomehowinhibited p
e
fromparticipatinginfilm-formingreactionsbythepresenceofmethylglyoxal. r
|
3.5 UV-visspectraofsolutions
D
is
c
Althoughthefocusofthisworkisoncharacterizationofthesurfacefilms,theUV-visible u
s
25 absorption spectra of aged organic/sulfuric acid solutions were also examined in or- sio
der to give some insight into the potential formation of highly absorbing species over n
P
the long residence time of lower stratospheric aerosols (∼2years). Figure 7a (blue) a
p
e
shows that a solution of 0.030M propanal in 48wt% sulfuric acid allowed to age for r
28587 |
274dayshadtwostrongabsorptionpeaksaround200and245nmmostlikelycorre- D
is
sponding to species also observed in the films: 1,3,5-trimethylbenzene and 2-methyl- c
u
2-pentenal,whichabsorbinwaterat∼200and234nm,respectively.Moreimportantly, ss
io
theabsorbanceextendssignificantlyintothevisible.Therearenootherdistinguishable n
P
5 peaks,buttheabsorbanceismostlikelyduetooverlappingpeaksfromvariouslonger a
p
oligomers formed by additional aldol condensation reactions of 2-methyl-2-pentenal e
r
with propanal and/or with itself. Each sequential aldol condensation step would add
|
another unit(s) of conjugation and thereby shift the absorption peak to longer wave-
D
lengths.Thisinterpretationissupportedbytheobservationthatwhentheaciditywas is
10 increased to 76wt% sulfuric acid (Fig. 1b), the intensity of the peak corresponding cu
s
to 2-methyl-2-pentenal was reduced (or even absent) and additional peaks became s
io
distinguishableatlongerwavelengths(270,365,388(shoulder)and458nm).Nozière n
P
and Esteve (2007) observed a similar spectrum for reaction products of propanal in a
p
96wt%sulfuricacid,andalsoascribetheselongwavelengthpeakstooligomersfrom er
aldol condensation reactions. Although they suggest that the peak in their spectrum
15 |
near270nmmaybepropanalitself,thiscannotbethecaseforoursamplessincethe
D
molarabsorptivityofpropanalistoosmallat∼9cm−1M−1 (Xuetal.,1993). is
c
The absorption spectra of mixtures of propanal with glyoxal and/or methylglyoxal u
s
arealsopresentedinFig.7.“Effective”molarabsorptivitiesarecalculatedbasedonly sio
n
20 on the concentration of the propanal reactant (0.030M) so that any changes in ab- P
a
sorbance (compared to the propanal-only spectrum) must be due to the presence of p
e
the additional organic species. At both acidities absorbance in most of the spectrum r
is increased, with methylglyoxal having a larger effect than glyoxal, suggesting that |
theaddedorganicspeciesareundergoingaldolcondensationeitherviareactionswith
D
propanalor self-reactions.Althoughsome oftheadditional absorptionmay bedueto is
25 c
u
glyoxal and methylglyoxal themselves, molar absorptivities of these species are too s
s
small (Horowitz et al., 2001; Malik and Joens, 2000; Plum et al., 1983) to contribute io
n
significantlyatleastbelow350nm. P
a
p
e
r
28588 |
4 Discussionandatmosphericimplications D
is
c
u
Themajorspeciespresentinsurfacefilmsformedonbulksolutionsofpropanalinsul- s
s
furic acid were identified as aldol condensation products (mainly 2-methyl-2-pentenal io
n
and 1,3,5-trimethylbenzene) and polyacetals (mainly 2,4,6-triethyl-1,3,5-trioxane and P
a
5 longer-chainlinearpolyacetals).Oftheseproducts,thepolyacetalspecies(bothcyclic pe
r
andlinear)aremostlikelytobeprimarilyresponsiblefortheseparationoftheorganic
speciesfromthesolutionintoaseparatesolidorganicphaseonthesurfaceoftheliq- |
uidduetotheirhighmolecularweightandhigherhydrophobicitycomparedtothetwo D
is
observedaldolcondensationproducts.Sincethesolidmaterialinthelaboratorysam- c
u
10 plesrisestothesurfaceofthesolutiondue,atleastinpart,toitslowdensityrelative ssio
tosulfuricacid,ifsimilarinsolubleacetalswereformedfromreactionsofaldehydesin n
P
liquid UT/LS aerosols, it is unclear whether they would exist as solid inclusions or as a
p
surfacecoatings(fullorpartial),thelatterofwhichwouldbemorelikelytoalteraerosol e
r
optical,chemicaland/orcloud-nucleatingproperties.
|
Neither the solubility nor the reactive uptake coefficient of propanal in sulfuric acid
15
D
has been measured, but, based on the low concentration of propanal vapor in the is
UT/LS(∼15pptat11kmandpresumablymuchlowerinthestratosphere,Singhetal., cu
s
2004)uptakeandreactionofpropanalalonetoformpolyacetalsisnotexpectedtobe s
io
asignificantsourceoforganicmaterialinUT/LSaerosols.However,polyacetalforma- n
P
tionfromaldehydesingeneralcouldbeimportantforthreereasons.First,polyacetals a
20 p
e
may be formed from a variety of organic species since they have been observed to r
formfrommanyaliphaticaldehydes(Vogl,2000)andhavespecificallybeenobserved
|
in sulfuric acid for formaldehyde uptake (Iraci and Tolbert, 1997) and inferred for ac-
D
etaldehydeuptake(Williamsetal.,2010).Second,therateoffilmformationwasgreatly is
c
enhancedbythepresenceofglyoxal,suggestingthatcarbonylspeciesalreadypresent u
25 s
s
inaerosolscouldenhancethereactiveuptakeandpolyacetalformationofsmallalde- io
n
hydes,consistentwithpreviousexperimentsthatdemonstratedenhancedreactiveup- P
a
takeofacetaldehydeonsulfuricacidsolutionscontainingformaldehyde(Williamsetal., p
e
r
28589 |
2010)andenhancedreactiveuptakeofnonanalonmixedorganic/sulfuricacidaerosols D
is
(Chan and Chan, 2011). Third, although aerosol concentrations of any one aldehyde c
u
are unlikely to result in significant self-polymerization, cross reactions between alde- ss
io
hydesand/orbetweenaldehydesandalcoholsmaybesignificantandarespecifically n
P
5 shownheretooccurbetweenpropanalandtwocommonaerosolorganicspecies(gly- a
p
oxalandmethylglyoxal). e
r
Althoughuptakeanddissolutionofaldehydesontosulfuricacidaerosolsisthemost
|
likely method of polyacetal formation directly from small volatile mono-aldehydes like
D
propanal, there may be more favorable methods for polyacetal formation in UT/LS is
10 aerosols.Sincepolyacetalformationrequiresmultiplepolymerizationsteps,thekinet- cus
icsarelikelytobegreatlyenhancedathigherconcentrationsoftheorganicreactants. s
io
One possibility for enhanced concentration of organic reactants is the potential pref- n
P
erence of the reactants for the aerosol surface. If polyacetals partition to the aerosol a
p
surfaceastheydoinourbulkexperiments,theirfurtherpolymerizationwitheachother er
and with condensing organics would be enhanced; polymerization could be similarly
15 |
enhancedifcarbonyland/oralcoholreactantspartitiontotheaerosolsurfaceortoor-
D
ganic inclusions. This possibility is supported by the recent work of Li et al. (2011), is
c
Schwieretal.(2010)andSareenetal.(2010)demonstratingsurfacetensiondepres- u
s
s
sionbysurface-activespeciesformedinsolutionsofformaldehyde,acetaldehyde,gly- io
n
20 oxal and/or methylglyoxal in pure water and/or aqueous ammonium sulfate. Addition- P
a
ally,productsofcross-reactionsbetweenmethylglyoxalandformaldehydeoracetalde- p
e
hyde had a larger effect on surface tension than could be explained by self-reactions r
alone. |
Anadditionalpossibilityforenhancedconcentrationsoforganicreactantsfavorable D
forpolyacetalformationistransportoforganic-richaerosolsfromthelowertroposphere is
25 c
u
totheUT/LS.Polyacetalformationcouldbeinitiatedonsuchaerosolsuponcondensa- s
s
tionofH SO and/orcoagulationwithH SO particlesformednearthetropopause.In io
2 4 2 4 n
order to evaluate the likelihood of this process, carbonyl species more typical of pho- P
a
tochemically aged tropospheric aerosols (less volatile and likely more oxidized than p
e
r
28590 |