Secondary organic material formed by methylglyoxal in aqueous

20 Pages·2010·2.81 MB·English

Atmos. Chem. Phys.,10,997–1016,2010
Atmospheric
www.atmos-chem-phys.net/10/997/2010/
Chemistry
©Author(s)2010. Thisworkisdistributedunder
theCreativeCommonsAttribution3.0License. and Physics
Secondary organic material formed by methylglyoxal in aqueous
aerosol mimics
N.Sareen,A.N.Schwier,E.L.Shapiro,D.Mitroo,andV.F.McNeill
DepartmentofChemicalEngineering,ColumbiaUniversity,NewYork,NY,USA
Received: 15July2009–PublishedinAtmos. Chem. Phys.Discuss.: 24July2009
Revised: 13January2010–Accepted: 14January2010–Published: 1February2010
Abstract. We show that methylglyoxal forms light- loway et al., 2009; Shapiro et al., 2009; Fu et al., 2009; El
absorbingsecondaryorganicmaterialinaqueousammonium Haddad et al., 2009; De Haan et al., 2009a). There is evi-
sulfate and ammonium nitrate solutions mimicking tropo- dencethatSOAformationmayaffectpropertiesoftheseed
sphericaerosolparticles.Thekineticswerecharacterizedus- aerosolsuchasCCNactivity(CruzandPandis,1997;Hartz
ing UV-Vis spectrophotometry. The results suggest that the etal., 2005; Kingetal., 2007, 2009; Engelhartetal., 2008;
bimolecular reaction of methylglyoxal with an ammonium Duplissy et al., 2008; Michaud et al., 2009), optical prop-
or hydronium ion is the rate-limiting step for the formation erties (Saathoff et al., 2003; Nozie`re et al., 2007, 2009b;
oflight-absorbingspecies,withkII =5×10−6M−1min−1 Nozie`re and Esteve, 2007; Casale et al., 2007; Shapiro et
NH4+
and kII ≤10−3M−1min−1. Evidence of aldol con- al., 2009; De Haan et al., 2009a) and heterogeneous re-
H3O+
densation products and oligomeric species up to 759amu activity towards gases such as N2O5 (Folkers et al., 2003;
was found using chemical ionization mass spectrometry Anttila et al., 2006). A variety of potentially surface-active
with a volatilization flow tube inlet (Aerosol-CIMS). Ten- SOAproductshavebeenproposed, includingorganicacids,
tativeidentificationsofcarbon-nitrogenspeciesandasulfur- organosulfates, nitrogen-containing organics, aldol conden-
containingcompoundwerealsomadeusingAerosol-CIMS. sationproducts,andhighlyoxygenatedoligomericmaterial.
Aqueous solutions of methylglyoxal, with and without in- In an aqueous aerosol particle, surface-active products may
organic salts, exhibit significant surface tension depression. partition to the gas-particle interface, lowering the surface
Theseobservationsaddtothegrowingbodyofevidencethat tension (and thus the critical supersaturation required for
dicarbonyl compounds may form secondary organic mate- clouddropletactivation)andactingasabarriertomasstrans-
rialintheaerosolaqueousphase,andthatsecondaryorganic port between the gas and aqueous phases. Light-absorbing
aerosol formation via heterogeneous processes may affect SOAproductswhichcouldincreasetheabsorptionindexof
seedaerosolproperties. theseedaerosolhavealsobeenidentifiedinlaboratorystud-
ies. Aldehydes have been reported to undergo aldol con-
densation in aqueous aerosol mimics to form π-conjugated
species (Nozie`re et al., 2007; Nozie`re and Esteve, 2007;
1 Introduction
Casale et al., 2007). We recently reported the formation
oflight-absorbing,oligomericmoleculesinaqueousaerosol
Laboratoryandfieldstudiessuggestthatcarbonyl-containing
mimicscontainingglyoxalandammoniumsalts(Shapiroet
volatile organic compounds, when absorbed by aqueous
al.,2009).DeHaanetal.(2009a,b)observedbrowningupon
aerosol particles or cloud droplets, participate in aqueous-
thereactionofglyoxalwithaminoacidsinaerosolandcloud
phase chemistry to form low-volatility secondary organic
dropletmimics.
material (SOA) (Jang et al., 2002; Kroll et al., 2005; Lig-
Methylglyoxal (C H O ) is an atmospheric oxidation
gioetal.,2005;Volkameretal.,2006,2007,2009;Loeffler 3 4 2
product of many anthropogenic and biogenic volatile or-
et al., 2006; Zhao et al., 2006; Gao et al., 2006; Altieri et
ganiccompounds(Tuazonetal.,1986;Grosjeanetal.,1993;
al.,2008;Carltonetal.,2008;Nozie`reetal.,2009a,b;Gal-
Smith et al., 1999). There is mixed evidence in the liter-
ature regarding the potential of methylglyoxal to be a di-
Correspondenceto: V.F.McNeill rect precursor for heterogeneous SOA formation in aque-
([email protected]) ous aerosols. Methylglyoxal becomes hydrated and forms
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
998 N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics

2 Methods
Aldol
Pathway 1
2.1 Bulksolutionpreparation
Salt concentrations in atmospheric aerosols at typical rela-
tive humidities exceed bulk saturation concentrations (Tang
Aldol and Munkelwitz, 1994; Tang et al., 1997). In an effort
to mimic atmospheric aerosol compositions to the extent
Pathway 2
possible in a bulk solution, solutions were prepared using
Milliporewaterandhigh(near-saturation)concentrationsof

the salt of interest (3.1M (NH ) SO , 5.1M NaCl, 1.18M
Scheme1.Proposedreactionpathwaysformethylglyoxal. 4 2 4
Scheme 1. Proposed reaction pathways for methylglyoxal Na2SO4, 8.7M NH4NO3). Methylglyoxal concentrations
ranged from 0–2.0M, corresponding to ∼0–25wt% of the
solute. Methylglyoxalwasintroducedfroma40wt%aque-
acetalandhemiacetaloligomersinaqueoussolution(Nemet
ous solution (Sigma Aldrich). Mixing time was counted as
etal.,2004;Paulsenetal.,2005;Loeffleretal.,2006;Zhao
timeafterthe40wt%methylglyoxalsolutionwasintroduced
et al., 2006; Krizner et al., 2009). Kalberer et al. (2004)
to the aqueous salt solution. The aqueous methylglyoxal
suggestedthatmethylglyoxalacetaloligomerscouldexplain
stock solution was pH=2.0 (±0.1) when tested with an Ac-
their observation of polymeric material in secondary or-
cumet model 20 pH/conductivity meter (Fisher Scientific),
ganic aerosols formed in a reaction chamber by the pho-
andthereactionmixturesthatcontained≥16.2mMmethyl-
tooxidationof1,3,5-trimethylbenzene. BarsantiandPankow
glyoxalwerepH=2.0(±0.1),withoutbufferingorfurtherad-
(2005) and Krizner et al. (2009) predicted that aldol con-
ditionofacid. ThisiswithintherangeofpHrelevanttotro-
densationshouldbefavorableformethylglyoxalinaerosols.
pospheric aerosols (Keene et al., 2004; Zhang et al., 2007).
Singly hydrated methylglyoxal has been reported to be the
InexperimentsperformedtotesttheeffectofvaryingpH,di-
dominantmonomericspeciesinaqueousmethylglyoxalsys-
luteHNO wasaddedtothereactionmixturesdropwiseuntil
tems (Nemet et al., 2004). Singly hydrated methylglyoxal 3
thedesiredpHwasreached.
may participate in self-aldol condensation via two possible
pathways initiating with enol formation with the C=C dou- Methylglyoxalstocksolutionisacidicduetothepresence
ble bond forming from either terminal carbon, as shown of a small amount of pyruvic acid impurity. Pyruvic acid
in Scheme 1. Note that we refer to the overall process of is a relatively strong organic acid, with pKa=2.49. There-
aldol addition followed by dehydration as aldol condensa- fore,thefactthatourstocksolutionispH=2correspondsto
tion (Muller, 1994). Zhao et al. (2006) measured non-zero averysmall(0.07%bymole)impurityofpyruvicacidinthe
methylglyoxaluptakeontoaqueoussulfuricacidsolutionsin methylglyoxalstocksolution.
a coated-wall flow tube reactor. However, in aerosol cham- Solutions were prepared in 100mL Pyrex volumetric
ber studies Kroll et al. (2005) observed that methylglyoxal fl38asks. Pyrex is opaque to light with wavelengths <280nm
uptaketoacidicammoniumsulfateseedaerosolsdidnotlead (Corning, Inc.), but the samples were not further protected
tosignificantparticlegrowth. from ambient light except for control experiments as speci-
Westudiedtheformationoflight-absorbingsecondaryor- fiedinthetext. Allexperimentswereperformedatambient
ganic products in aqueous solutions containing methylgly- temperatureandpressure.
oxal and ammonium salts. The kinetics of formation were
characterized using UV-Vis spectrophotometry. We also 2.2 UV-Visspectrophotometry
characterizedthereactionproductsviaatomizationofdiluted
reaction mixtures followed by detection with chemical ion-
TheUV-Visabsorptionspectraofthereactionmixtureswere
izationmassspectrometrywithavolatilizationflowtubein-
measured using an HP 8453 UV-Visible Spectrophotometer
let (Aerosol-CIMS). We found evidence of aldol condensa-
witha10mmopen-topquartzcuvette.
tionproductsandhigh-molecular-weightoligomericspecies,
aswellaspossiblesulfur-containingcompoundsandcarbon-
nitrogen species. Pendant drop tensiometry measurements 2.3 Surfacetensionmeasurements
show that aqueous solutions of methylglyoxal exhibit sur-
face tension depression, and the effect is enhanced when Surfacetensionwasmeasuredusingpendantdroptensiome-
NaCl or (NH ) SO is present. These observations add to tryasdescribedinShapiroetal.(2009). Briefly,dropletsof
4 2 4
the growing body of evidence that dicarbonyl compounds samplesolutionweresuspendedfromthetipofaglasscap-
form secondary organic material in the aqueous phase, and illarytubeusinga100µLsyringemountedinsideachamber
thatSOAformationviaheterogeneousprocessesmayaffect withquartzwindows. Imageswerecapturedasdescribedby
seedaerosolproperties. Anastasiadisetal.(1987). ThemethodofCanny(1986)was
Atmos. Chem. Phys.,10,997–1016,2010 www.atmos-chem-phys.net/10/997/2010/
N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics 999
Quadrupole Mass CI
Spectrometer Region
VFT
n
N /CH I o
2 3
i
or N /H O ut
2 2
l
i
d
dry N
2
210Po
Atomizer
TSI 3076
SMPS
To pumps
Turbo Turbo
pump pump

Fig.1. SchematicofAerosol-CIMSsetupforthedetectionofproductsformedduringthereactionsofmethylglyoxalinaqueoussolution
with (NH4)2SO4 or NaCl. SMPS: scanning mobility particle sizer, VFT: volatilization flow tube, CI: chemical ionization. See text for
details. Figure 1. Schematic of Aerosol-CIMS setup for the detection of products formed during the
reactions of methylglyoxal in aqueous solution with (NH ) SO or NaCl. SMPS: scanning
4 2 4
implemented in MATLAB 7.0 (The MathWorks, Inc.) for 2005,2006a,2007;Hearnetal.,2005,2007;McNeilletal.,
mobility particle sizer, VFT: volatilization flow tube, CI: chemical ionization. See text for
edgedetection. Surfacetensionwascalculatedaccordingto: 2007, 2008) andto characterize aerosolsof unknown, com-
plexchemicalcomposition(HearnandSmith,2006b).
details.
1ρgd2
σ= e (1) Experiments were conducted using a custom-built
H Aerosol-CIMSapparatus. Analytemoleculesweredetected
astheproductsoftheirinteractionswithI−orH O+.(H O)
where σ is surface tension, 1ρ is the difference in density 3 2 n
using a quadrupole mass spectrometer with high mass
betweenthesolutionandthegasphase,gisaccelerationdue
(≤1000amu) capabilities (Extrel CMS). The two reagent
togravity,d istheequatorialdiameterofthedroplet,andH
e ionsused,I−andH O+.(H O) ,arecomplementaryintheir
istheshapefactor(AdamsonandGast, 1997). Themethod 3 2 n
versatility(H O+.(H O) )andselectivity(I−). Aschematic
ofseveralselectedplaneswasusedfordeterminingH based 3 2 n
oftheexperimentalsystemisshowninFig.1.
on the diameter of the drop at five intervals along the drop
Mixtures initially containing 1.62M methylglyoxal and
axis (Juza, 1997). Solution density was measured using an
analyticalbalancereadabletowithin±10µg(DenverInstru- 3.1M (NH4)2SO4 or 5.1M NaCl were prepared using Mil-
lipore water as described in the previous section. After the
ments).
desired reaction time had passed, the mixtures were diluted
2.4 Aerosol-CIMS with Millipore water until the salt concentration was 0.2M.
Reaction time was generally >24h, which was sufficient
Aerosol-CIMS enables measurements of aerosol composi- time for significant light absorption and surface tension de-
tion simultaneously with gas-phase composition, with the pression to develop in the methylglyoxal/(NH4)2SO4 solu-
highsensitivity, selectivity, andfasttimeresponseofCIMS tions. Reaction kinetics at short times were investigated as
(Hearn and Smith, 2004a, 2006b; McNeill et al., 2007, follows:asmallamountofreactionmixtureinitiallycontain-
2008). This technique allows speciated measurements of ing1.62Mmethylglyoxaland3.1M(NH4)2SO4wasdiluted
aerosol organics which are selective based on the choice of 2minaftermixing.Anothersampleofthesamebulkreaction
parent ion. Chemical ionization is a relatively soft ioniza- mixture was diluted 38min after mixing. The mass spectra
tion technique that results in low fragmentation of organ- ofthesesamplesweremeasuredusingAerosol-CIMSimme-
ics, thus simplifying their identification and quantification. diatelyafterdilution. 39
Aerosol-CIMShasbeenusedforlaboratorystudiesoftheox- Two control experiments were performed, the first in
idative aging of organic aerosols (Hearn and Smith, 2004b, whicha0.2M(NH ) SO solutionatpH=2andthesecond
4 2 4
www.atmos-chem-phys.net/10/997/2010/ Atmos. Chem. Phys.,10,997–1016,2010
1000 N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics
inwhicha0.05Mmethylglyoxalsolutionwasatomizedand H O+.(H O) +R→RH++(H O) (2)
3 2 n 2 n+1
analyzed using Aerosol-CIMS. Additional control experi-
ments to test the performance of Aerosol-CIMS in the high orligandswitching(Blakeetal.,2009):
mass detection mode were performed using a solution of
H O+.(H O) +R→H O+.R+(H O) (3)
0.2MNaCland3.9mMpoly(ethyleneglycol)(PEG)(Sigma 3 2 n 3 2 n
Aldrich, 570–630amu) in Millipore water. The instrument
and the species are then detected as the protonated analyte
wascalibratedusingaerosol-phasesuccinicacid(C4H6O4). moleculeoritsclusterwithH O.IntheI−detectionscheme
2
Aerosolsweregeneratedbyatomizingasolutionof0.001M the analyte molecules form clusters with I− via a ligand-
succinicacidinMilliporewater. Sincetheliquidwatercon-
switchingreaction:
tentoftheaerosolparticleswasnotknown, weassumethat
the aerosol mass measured by the SMPS was comprised I−.H O+R→I−.R+H O (4)
2 2
of 100% succinic acid and report the calculated sensitiv-
ity and detection limit values as lower and upper limits, re- ortheyareionizedviaprotonabstraction:
spectively. The instrument sensitivity to aerosol-phase suc-
cinicacidwasmeasuredusingtheI−detectionschemetobe I−+R−H→R−+HI (5)
≥100Hzppt−1 withadetectionlimitof≤0.01µgm−3. Us- IonspassedfromtheCIregionthrougha0.05cm-IDcharged
ingtheH3O+.(H2O)nscheme,thesensitivitytosuccinicacid orificeintoacollisionaldissociationchamber(CDC)which
was≥66Hzppt−1andthedetectionlimitwas≤0.02µgm−3. was maintained at 5Torr by a mechanical pump (Var-
ThedilutesolutionswereaerosolizedwithN2usingacon- ian DS402). Ions may be accelerated through this re-
stant output atomizer (TSI 3076), forming submicron parti- gion using a series of biased cylindrical lenses in order to
cles. The aerosol stream was combined with a dry N2 di- control clustering. The CDC is separated from the MS
lution flow, resulting in a relative humidity of 50–60% as prechamber (∼10−4Torr) by a second charged orifice plate
measured with a hygrometer (Vaisala). The particle popu- (ID=0.05cm). The prechamber contains an ion optics as-
lation was characterized using a scanning mobility particle sembly (Extrel CMS) and is separated by a 0.2cm-ID ori-
sizer (SMPS) (Grimm Technologies, TSI). The aerosol had fice from the final chamber (∼10−7Torr) which houses the
a lognormal size distribution with a typical geometric stan- 19mmquadrupoleanddetector(ExtrelCMS).Thefinaltwo
dard deviation of 1.8 and a mean volume-weighted particle chambersaredifferentiallypumpedbyidenticalturbomolec-
radius of 119±1nm. Typical number concentrations were ular pumps (Varian TV-301 Navigator) backed by a single
7×104cm−3. The aerosol stream passed through a 23cm- mechanical pump (Varian DS302). For regular operation
long,1.25cmIDPTFEtubewrappedinheatingtapeinorder the RF operating frequency for the mass spectrometer was
to volatilize the organics before entering the chemical ion- 1.2MHz; for high mass mode a 0.88MHz RF supply was
ization region of the mass spectrometer. The external tem- used(ExtrelCMS).
perature of the inlet was maintained at 135◦C using a ther-
mocouple and temperature controller (Staco Energy). No 2.5 DFTcalculations
increase in signal was observed when the inlet temperature
wasincreasedto160◦C.Someexperimentswereperformed Geometry optimizations and energy calculations were per-
withnoinletheatinginordertotestforspecieswhichwere formedusingJaguar6.0(Schrodinger,Inc.) withtheChem-
volatileatroomtemperature. Bio3D interface (CambridgeSoft) in order to evaluate the
Flow through the aerosol inlet into the chemical ioniza- UV-Vis absorption of potential products and the energetics
tionregionwasmaintainedat3SLPMusingacriticalorifice. ofreactionpathways,andtoevaluatetheinteractionsofpro-
Thechemicalionization(CI)regionconsistsofa3.5cmID posedproductmoleculeswithI− forCIMSdetection. Den-
stainlesssteelmanifoldwhichis3.8cmlong. Pressureinthe sityfunctionaltheory(DFT)withtheB3LYPfunctionaland
CIregionismaintainedat45–55Torrbyamechanicalpump the cc-pVTZ(-f) basis set (Kendall et al., 1992) was used
(Varian DS302). For the negative ion detection scheme, I− to predict the HOMO-LUMO energy difference (and thus
reagent ions were generated by flowing dilute CH I (Alfa UV-Vis absorption wavelengths) of proposed products. For
3
Aesar,99.5%)in3SLPMN (TechAir,99.999%)througha purposes of comparison with Krizner et al. (2009) some
2
210Poionizer(NRD).Theionizerwasmountedperpendicu- additional calculations were performed with the 6-311G**
lartotheCIregion. FordetectionwithH O+.(H O) , ions basis set and Poisson-Boltzmann solvation (water solvent,
3 2 n
weregeneratedbyflowingacombinedstreamof2SLPMN ε=80.37, probe radius=1.40A˚). The Gibbs free energy of
2
bubbledthroughMilliporewaterand3SLPMdryN through solvatedspecieswascalculatedusinghalfthegasphaseen-
2
the ionizer. Ion-neutral reaction times were 20–30ms. For tropyfollowingKrizneretal.(2009).
theH O+.(H O) detectionscheme, thepredominantpeaks For the CIMS ion-molecule reaction calculations, DFT
3 2 n
inourspectraareH O+.(H O) at55amuandH O+.(H O) wasusedwiththeB3LYPfunctionalandtheERMLER2ba-
3 2 2 3 2 3
at 73amu. The reagent ions react with the neutral species sisset,whichallowsthetreatmentofiodineviatheuseofef-
throughprotontransfer(HearnandSmith,2004a): fectivecorepotentials(Lajohnetal.,1987). Thefreeenergy
Atmos. Chem. Phys.,10,997–1016,2010 www.atmos-chem-phys.net/10/997/2010/
N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics 1001
a b
4
4
)
AU 3 U) 3
( 7 min A 3 min
ce 1 h e ( 1 h
rban 2 312 24h hh banc 2 3919 hh h
so 96 h or 27 h
Ab 1 Abs 1 4794 hh
98 h
0
0
200 300 400 500 600
200 300 400 500 600 700
Wavelength (nm) Wavelength (nm)
U) U) 4
A 4 A
( (
m m
2 n 3 0 n 3
8 5
2 5
at 2 at 2
e e
c c
n n
a 1 a 1
b b
r r
o o
s s
b 0 b 0
A A
0 20 40 60 80 100 0 20 40 60 80 100
Time (h) Time (h)

Fig.2.UV- Visspectraofaqueoussolutionscontaining3.1M(NH4)2SO4and(a)16.2mMmethylglyoxaland(b)1.62Mmethylglyoxalasa
functionoftimeaftermixing.Absorbanceisshownasafunctionofwavelengthintheupperpanels,andabsorbanceatselectedwavelengths
isshowninthelowerpanels.Errorbarsreflectuncertaintyinthemeasuredabsorbancesbasedonvariationobservedinthebaselinesignal.

Figure 2. UV-Vis spectra of aqueous solutions containing 3.1M (NH ) SO and a) 16.2 mM
change for the ligand-switching reaction or proton abstrac- 3.1 UV-Visabsorptio4n2 4
tionwascmalceuthlaytelgdl.yFooxratlh aenldig ban) d1s.6w2it cMhi nmgertehaycltigolnysosxeavl- as a function of time after mixing. Absorbance is
eral geometries for the cluster of the analyte molecule with The products formed by methylglyoxal in aqueous solu-
I− were tesshtoedwfno raesa cha spfuecnicetsi,oann doifn swoamveelceansegsths evienr althe utipopnesr copnatanienlisn,g a(nNdH 4a)2bSsOor4baonrcNe Ha4tN Ose3leacbtseodrb light at
stableclustergeometries(localminima)werefound. Ineach UVandvisiblewavelengths(ref.Figs.2–3andSupplemen-
wavelengths is shown in the lower panels. Error bars reflect uncertainty in the measured
case, 1G for the lowest-energy cluster geometry (global tary Information: http://www.atmos-chem-phys.net/10/997/
minimum)aibssroerpboarntecde.sT bhaesferde eoenn veragryiavtaiolune sobrespeorrvteedd hiner tehe ba2s0e1li0n/aec spi-g1n0a-9l.9 7-2010-supplement.pdf).
arefromtheoutputofthe298.15Kvibrationalfrequencycal-
culationandnofurthercorrectionswereapplied. 3.1.1 Experimentalresults
Aqueous methylglyoxal solutions with no salt have a broad
3 Resultsanddiscussion absorbancepeakat290nmatambienttemperatures(Nemet
et al., 2004). A kinetics study of 16.2mM methylglyoxal
Solutions containing ≥0.16M methylglyoxal and
in 3.1M (NH ) SO (aq) (Fig. 2a) shows that after a delay
4 2 4
(NH4)2SO4 became visibly colored immediately after of ∼1h, peaks grow in at 213nm and 282nm with roughly
mixing and became progressively darker in color with
exponential time dependence. The measured absorbance
time. Thecolorvariednoticeablywithinitialmethylglyoxal
of a solution of 1.62M aqueous methylglyoxal and 3.1M
concentration;solutionswithhigherinitialconcentrationsof
(NH ) SO initiallyincreasesuponmixingacrossallwave-
4 2 4
methylglyoxalweredarkerincolor.
lengths(Fig.2b). Thisinitialincreaseinbaselineabsorption
couldindicateeitherformationofatleastonelight-absorbing
reaction intermediate that is consumed in later steps of the
mechanism,oratransientchangeinthebulkproperties(e.g.
40
refractive index, density) of the solutions. After 1h, the
www.atmos-chem-phys.net/10/997/2010/ Atmos. Chem. Phys.,10,997–1016,2010
1002 N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics
Table 1. Proposed reaction products. Predictions for the energy A
oTfatbheleg 1a.s PprhoapsoeseHdO reMacOti-oLnU pMroOductrtasn. sPirteiodnictainodnst hfoer wthaev eenleenrggyth oof fthe gas phase HOMO-4
ULVU-MViOs tarbasnosritpitoino nanfdro tmhe DwaFvTelBen3gLtYh Pof/c Uc-Vp-vVtzi(s- fa)bssoimrputiloanti ofrnosma DreFT B3LYP/cc-pvtU)z(-f)
A
sshiomwunla.tRioenfse areren csehsoawrne.i Rndeifcearetendcebsy a:r1e) iNndeimcaetteedt bayl.: (12)0 N04e)m,e2t) eZt haal.o, 2004 2) Zhao et am (l., 23006
etal.(2006),3)Krizneretal.(2009).
n
3) Krizner et al., 2009.   2
8
2
Molecule Ref. energy λ at 2
(eV) (nm) e
c
n
a
This orb 1
a) 6.757 183.5 s
work b
A

0
0 1 2 3 4 5x10-2
Initial Methylglyoxal Concentration [M]
b) 1,2 5.728 216.5
B
U) 4
This A
c) 4.574 271.1 m (
work n 3
2
8
2
d) This 3.536 350.6 at 2
work e
c
n
a
b 1
This or
e) 3.354 369.6 bs
work A
0

0 1 2 3 4 5
This pH
f) 6.247 198.5
work C

4
This U)
A
g) work 5.751 215.6 m ( 3
n
2
8
2
h) 3 3.872 320.2 at 2
e
nc
a
b 1
i) 3 3.661 338.7 sor
b
A
0
0.5 1.0 1.5 2.0 2.5 3.0
Initial (NH4)2SO4 concentration [M]

absorption spectrum is saturated for λ≤360nm, and the
baseline at high wavelengths returns to <0.5AU. With in- Fig. 3. Absorbance at 282nm of aqueous solutions containing
ct orelaosninggertwimaev,eltehnegsthatsuarantdedthreegtaioilnshoofwthseinspcreecatrsuinmgeaxbtseonrdps- (galy)o3x.1alM,pF(HNig=H2u4.0r)e2(±S3 O330..41 )Aa,n3bd.s0voahrrybaifantengrcimnei itxiaaitnl gc2.o8n(2bce )nn3tm.r1at Mioonf(s NaoHqf4um)e2eSothOuys4l- ,solutions containing a) 3.1 M (NH4)2SO4 and
1.62mM methylglyoxal, and varying pH 24h after mixing, and
tionathighwavelengths(λ>500). Significantabsorptionat varying initial concentrations of methylglyoxal, pH = 2.0(±0.1), 3.0 hr after mixing. b) 3.1 M
(c) 16.2mM methylglyoxal and varying initial concentrations of
550nmisexhibitedat<1handafter12h,withabsorptionat
(NH4)2SO(N4,H24)hSafOterm1i.x6in2g .mEMrro rmbaersthreyfllgecltyuonxcaerlt,a ianntydi nvtahreying pH 24 hr after mixing, and c) 16.2 mM
upto700nmdevelopingwithin2–3days. measuredabso4rb2ances4, basedonvariationobservedinthebaseline
The effect of initial methylglyoxal concentration on the signal. methylglyoxal and varying initial concentrations of (NH ) SO , 24 hr after mixing. Error bars
4 2 4
UV-Vis spectra of solutions containing 3.1M (NH ) SO
4 2 4
reflect uncertainty in the measured absorbances based on variation observed in the baseline
24h after mixing is shown in Fig. 3a. The absorbance at
282nmafter3.0hislinearlydependentontheinitialmethyl- Fig.3b.sTihgenaabl.s o rbanceat282nmofsolutionsinitiallycon-
glyoxal concentration. The effect of pH on the production taining16.2mMmethylglyoxal24haftermixingislinearly
of light-absorbing products in solutions initially containing dependen tontheinitial(NH4)2SO4concentration,asshown 41
1.62mM methylglyoxal and 3.1M (NH ) SO is shown in inFig.3c.
4 2 4
Atmos. Chem. Phys.,10,997–1016,2010 www.atmos-chem-phys.net/10/997/2010/
N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics 1003
Control samples containing 1.62M methylglyoxal and closetoKrizneretal.’svalueof11.9kcalmol−1 forthefor-
5.1M NaCl or 1.18M Na SO exhibited UV-Vis spectra mation of the pathway (2) enol, suggesting that both enol
2 4
similar to aqueous methylglyoxal in the absence of salt af- speciesshouldbepresentinsmallquantitiesatequilibrium.
ter 24h. A sample initially containing 1.62M methylgly- Aldol addition via pathway (1) is likely to terminate after
oxal and 3.1M (NH ) SO was protected from light until dimer or trimer formation due to the formation of organic
4 2 4
analysisbycoveringthereactionvesselwithaluminumfoil, acidorketoneendgroups(e.g.species(c–g),Table1). Itis
and the resulting spectrum at 24h was identical to that of not energetically favorable for aldol addition at ketone end
an unprotected solution with the same composition. This groups to continue via pathway (1) due to steric hindrance
indicates that the reactions leading to light-absorbing com- from the methyl group. Instead, these ketones may form
pounds in this study are not photochemical. The results an enol and follow pathway (2) for continuing aldol addi-
of these control experiments can be found in the Supple- tion. Additionally,becauseofthemethylgroup,manyofthe
mentaryMaterial(http://www.atmos-chem-phys.net/10/997/ products of aldol addition via pathway (1) cannot proceed
2010/acp-10-997-2010-supplement.pdf). withdehydration(e.g.species(g),Table1). Pathway(2)re-
sultsincarbonyltermination(e.g.species(h)and(i),Table1)
3.1.2 DFTcalculations andthereforealdolcondensationcouldpropagatebeyondthe
trimer.
The results of our B3LYP/cc-pVTZ(-f) calculations of the ReferringtoFig.2a,basedonourB3LYP/cc-pvtz(-f)pre-
HOMO-LUMOenergydifference(andthusUV-Visabsorp- dictions, the species absorbing at 213nm could correspond
tion wavelengths) of several proposed products are listed in toanaldoladditionproductsuchasspecies(f)or(g)inTa-
Table 1. When molecules absorb light, their electrons may ble1. Acetalssuchasspecies(b)inTable1mayalsoabsorb
be promoted from the HOMO (highest occupied molecular atthiswavelength.Asdescribedabove,weestimatetheerror
orbital)totheLUMO(lowestunoccupiedmolecularorbital). rangeofourtheoreticallypredictedabsorbancescomparedto
The energy difference between these levels corresponds to theobservedaqueous-phasespectratoberoughly(−12nm,
thewavelengthofabsorptionaccordingtoE=hc/λwhereE +42nm). Therefore the absorbance band at 286nm could
is the HOMO-LUMO energy difference, h is Planck’s con- correspondtoaspeciespredictedtoabsorbwithintherange
stant, c is the speed of light in vacuum, and λ is the wave- 274nm<λ<328nm. Species which lie within this range
length. include the pathway (2) aldol addition product species (h),
Our calculations were made for gas-phase molecules. which is predicted to absorb at 320nm. Given the approxi-
Meller et al. (1991) reported that gas-phase methylglyoxal matenatureoftheselowerandupperbounds,anothercandi-
has an absorption peak at 280nm. Our B3LYP/cc-pVTZ(- datespeciescouldbespecies(c)whichispredictedtoabsorb
f)calculationspredictthatgas-phase,unhydratedmethylgly- at 271.1nm. Species (c) is the aldol condensation product
oxalhasanabsorptionpeakat291.1nm. Thereforeweesti- correspondingtothealdoladditionproduct(f). C=Nbonds
matethatourpredictionsforthesemoleculesintheabsence couldalsocontributetotheobservedabsorbance.
ofsolventeffectsareaccuratetowithin∼12nm. Furtherde-
viation between the theoretical results and experiment may 3.2 Surfacetension
resultfromsolventeffects.Thentoπ*excitationbandchar-
acteristic of carbonyl compounds appears at ∼290nm and Solutions containing 3.1M (NH4)2SO4 and varying initial
is known to shift toward lower wavelengths (blue shift) for concentrations of methylglyoxal exhibit significant surface
a molecule in aqueous solvent compared to the gas phase tensiondepressioncomparedto3.1M(NH4)2SO4 solutions
(Skoogetal.,1997). Foracetonethisshiftisapproximately withoutorganics(ref.Fig.4).Thesurfacetensiondepression
12nm, and for crotonaldehyde the shift is ∼30nm (Bayliss followsaLangmuir-likedependenceoninitialmethylglyoxal
and McRae, 1954). Therefore, we can estimate an error concentration,withaminimum(saturation)surfacetension,
range of (−12nm, +42nm) for the predicted absorbances. σmin,of41(±2)dynescm−1 basedonafittothedatausing
Note that aqueous methylglyoxal solutions will contain a thefollowingequation:
mixtureofmono-anddi-hydratedmethylglyoxal,aldolcon-
bM
densationproducts,andhemiacetaloligomers(Krizneretal., σ=σ0−S1+bM0 (6)
2009),soitislessstraightforwardtomaptheobservedspec- 0
trumofaqueousmethylglyoxaltothegas-phaseabsorbance where σ is the surface tension, σ is the surface tension of
0
ofasinglemoleculeforpurposesofthisdiscussion. thesolutionwithnomethylglyoxal,M istheinitialmethyl-
0
ReferringtoScheme1, Krizneretal.(2009)showedthat glyoxalconcentration,andSandbarefitparameters. Values
aldolpathway(2)isthermodynamicallyfavorableforaque- of σ for (NH ) SO (aq) and NaCl (aq) were taken from
0 4 2 4
ous methylglyoxal (they did not study pathway (1)). Our the International Critical Tables (2003). The physical inter-
B3LYP/6-311G**calculationswithPoisson-Boltzmannsol- pretationofSisthesurfacetensiondepressionwhenthesur-
vation show that 1G=10.5kcalmol−1 for the formation of faceissaturated,suchthatσ =σ −S,andbisanequilib-
min 0
the pathway (1) enol from singly hydrated methylglyoxal, rium coefficient that describes surface-bulk partitioning. A
www.atmos-chem-phys.net/10/997/2010/ Atmos. Chem. Phys.,10,997–1016,2010
1004 N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics
1.1
HO
2
(NH ) SO
42 4
1.0 NaCl
w 0.9
σ
σ/
0.8
0.7

0.6
Fig. 5. Negative-ion mass spectrum of aerosolized aqueous solu-
Ftiigounres 5i. nNietgiaatlilvye-icono nmtaassi nspinecgtrumm eotf hayerlogsollyizoexd aaqluaeolousn seolu(tgiornesy in)itoiarllyw ciotnhtaiNninagC l
0.0 0.5 1.0 1.5 2.0 m(ebthluylegl)yooxarl (aNlonHe 4(g)r2eyS) Oor4 w(itrhe Nda)C(l s(ebleue)t eoxr t(NfHo4r)2SdOe4t a(rields) )(.seeS teexlte cfotr dpertoaidls)u. ct
Methylglyoxal initial concentration (M) Ssepleectc pireosduwct espreecieds ewteerce tdeedtecutesdi unsgingI I−- asa tshe trheaegernet aiogn.e Pnetakiso anss.ociPateeda kwsitha Is- saondc i-
itas tceludstwer witihth IH−2Oa ans dweiltl sasc thlue smtaesrs-two-icthhargHe 2raOtiosa osf pwroedlulcta pseatkhse arme laabsesle-dt.o -charge

ratiosofproductpeaksarelabeled.
FFigigu.re4 .4.R Reessuullttss ooff ppeenndadnat ndtrodpr otepnstieomnseitroym meetarsyurmemeeanstsu oref maqeuneotusso mfiaxqtuurees- as a
ous mixtures as a function of initial methylglyoxal concentration
function of initial methylglyoxal concentration for aqueous solution, 3.1 M (NH4)2SO4 (aq),
afnodr 5a.1q Mue NoauCsl s(aoql)u. tTihoen r,a3tio.1 oMf me(aNsuHre4d) s2uSrfOace4 t(eansqio),n aton dthe5 m.1eaMsurNeda sCurlfa(caeq t)e.nsion 1500
Theratioofmeasuredsurfacetensiontothemeasuredsurfaceten-
of Millipore water is shown. The measurements were made ≥ 24 h after mixing. Each point
sion of Millipore water is shown. The measurements were made
r≥efl2ec4tsh thaef wteerigmhteixd ianvger.agEea ocfh fipveo tion teirgehflt meceatssutrhemeewntes,i ganhdt ethde aervreorr abgares roefprfiesveent the
sttoandeairgdh dtemviaetiaosnu inre tmhe eranwts d,aatan. dThteh beeestr friot rcubrvaer stor eeapchre dsaetan stett hbeassedta onnd eaqrudatidoen -(6) is ) 1000
avlsioa sthioownni.n therawdata. Thebestfitcurvetoeachdatasetbasedon b.
r
Eq.(6)isalsoshown. (a
al
  n
g 500
Si
time series was performed on a solution initially containing
1.62Mmethylglyoxaland3.1M(NH ) SO . Themeasured
4 2 4
surface tension fluctuated for 2.5h before stabilizing at 45
(±1)dynescm−1,thenslowlydecreasedoverthenext21.5h 0 43
to the minimum value (41 (±2) dynescm−1). Control ex- 222 224 226 228 230
m/z (amu)
periments were performed in order to evaluate the role of 42  
(NH ) SO . For aqueous methylglyoxal solutions with no
4 2 4
saltspresentσ =52(±3)dynescm−1.Therefore,whilehy- Fig. 6.  Detail of a 0.5amu-resolution negative ion mass spec-
min
dratedmethylglyoxaland/ortheoligomersitformsinaque- trum of aerosolized methylglyoxal/(NH4)2SO4 solution. A peak
at225.2Faimguuraen d6a. saDteeltliatiel peoafk aat 202.57. 2aammuu-areresoslhuotwionn. negative ion mass spectrum of aerosolized
ous solution are surface-active, the overall surface-tension
lowering effect is less than when (NH4)2SO4 is present methylglyoxal/(NH4)2SO4 solution. A peak at 225.2 amu and a satellite peak at 227.2 amu are
in solution. Solutions containing 5.1M NaCl and varying
observesdhotwone.n h ance the surface tension lowering effects of
amounts of methylglyoxal follow a trend similar to that of
HULISandorganicdiacids(Shulmanetal.,1996;Kissetal.,
the (NH ) SO solutions, with σ =43 (±2) dynescm−1
4 2 4 min 2005;Asa-Awukuetal.,2008).
(Fig.4).
Glyoxalwaspreviouslyobservednottobesurface-active
Surface tension depression for methylglyoxal solutions
inhydratedformortoformsurface-activeproductsinaque-
containing 5.1M NaCl or 3.1M (NH4)2SO4 is greater than ous (NH ) SO solutions (Shapiro et al., 2009). Com-
4 2 4
that observed for aqueous methylglyoxal in the absence of
pared with glyoxal, the methyl group adds hydrophobicity
salts. Theobservedenhancementinsurfacetensiondepres-
tomethylglyoxalanditsoligomerproducts,increasingtheir
sion is likely to be a physical effect of the salts rather than
surfaceactivity.
an effect of especially surface-active products formed by a
chemicalreactionofmethylglyoxalwiththesalts. Highsalt 3.3 Aerosol-CIMS
concentrationscanresultinadecreasedcriticalmicellecon-
centrationduetochargescreening,andthuscauseenhanced Representative Aerosol-CIMS mass spectra for the
filmformation(MatijevicandPethica,1958;Lietal.,1998). aqueous methylglyoxal, methylglyoxal/NaCl and
Saltscanalsodecreasethesolubilityoforganics,commonly methylglyoxal/(NH ) SO systems using I− or
4 2 4
referredtoas“saltingout”(Setschenow,1889),possiblyre- H O+.(H O) as the reagent ion are shown in Figs. 5–
3 2 n
sultinginsurfacefilmformation. Saltshavecommonlybeen 8. The spectra represented in these figures have mass
Atmos. Chem. Phys.,10,997–1016,2010 www.atmos-chem-phys.net/10/997/2010/
44
N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics 1005
resolutionof1amuexceptasnoted; peakassignmentswere
madeusing0.5amuresolutionspectra.
3.3.1 NegativeiondetectionwithI−
A summary of proposed peak assignments for the mass
spectra in Fig. 5 using I− as the reagent ion can be
found in Table 2. Significant signal in the aqueous
methylglyoxal control spectrum is observed at 173.0,
190.1, 217.3, and 273.5amu. Most of the peaks in
the methylglyoxal/(NH ) SO mass spectrum are also
4 2 4

found in the methylglyoxal/NaCl spectrum. Increased
signal appears at 271.5, 273.5, and 289.5amu in the FFigiugr.e 77.. PPoossitiitviev ieoni omnasms aspsesctsrpumec otfr uamerosooflizaeedr oaqsuoeloiuzse dsolauqtiuoneso iunistiaslolyl uctoinotanisning
mmeetthhyyllggllyyooxxaall//((NNHH44))22SSOO44 isnpcelcutdruem2.25.2Peaankds 2u7n5iq.6uaemtuo. mchioenertmhity(iicNlagalllHyl iyoo4xnca)izl2o aSantiloOotann 4iern e(ai(gngrreeegnydt)m.) T.oerh teHwh miy3tahOls gsN+-ltyao.Co-(clHx h(aab2rllgOueae ))lr anootnirow es( Nao(fHsg s4ret)el2heSyecO)t 4pco rh(orredewdum)ci.t ti Hphcea3aONlk+sai. (oaCHrne2lO ilz(a)bban etlwliueoadesn.) the
The peak at 217.3amu is consistent with the cluster of I− reagent. Themass-to-chargeratiosofselectproductpeaksarela-
beled.
withsinglyhydratedmethylglyoxal. Thepresenceofmulti-
plepeaks>217.3amuisindicativeofdimerformation.
with volatile species. The signal observed at 173.0amu is
DFTcalculations
consistentwithformicacid(I−.HCOOH).Gas-phaseformic
I− has previously been used as a reagent ion with Aerosol- acidhasbeenpreviouslyobservedtobeanoxidationproduct
oforganicacidsinaerosols;itwasdetectedatthismassusing
CIMS to detect organic acids in aerosols (McNeill et
Aerosol-CIMS with the same ionization scheme used here
al., 2007, 2008). Since the product species expected
(McNeill et al., 2008). The signal we observe at 190.1amu
to be present in this reactive system (methylglyoxal, ac-
is consistent with a molecular formula of I−.CH O N, but
etal/hemiacetal oligomers, aldol condensation oligomers) 5 2
have not been previously detected via the I− ionization it is more likely due to the water cluster of the formic acid
peak.
scheme,weperformedabinitiocalculationsinordertochar-
acterizetheinteractionofproposedproductspecieswithI−.
(Hemi)acetalsandaldolcondensationproducts
The results are summarized in Table 3. Optimized geome-
triesandcalculatedenergiesforeachspeciescanbefoundin
Thepeakat271.5amuisattributedtothemolecularformula
the Supplementary Material (http://www.atmos-chem-phys.
I−.C H O , which could correspond to the pathway (1) al-
net/10/997/2010/acp-10-997-2010-supplement.pdf). Our 6 8 4
calculations show that the formation of clusters between I− dolcondensationdimersorpathway(2)aldoladditionprod-45
ucts (ref. Scheme 1 and Table 2). 289.5amu is consistent
and several of the acetal and hemiacetal species proposed
by Zhao et al. (2006) via ligand switching with I−.H O is withI−.C6H10O5,andthereforecanbematchedtoacetaland
2
hemiacetal dimers proposed by others (Nemet et al., 2004;
thermodynamicallyfavorable,particularlywhentwoormore
hydroxyl moieties are available to interact with I− simulta- Loeffleretal.,2006;Zhaoetal.,2006)orpathway(1)aldol
addition products. Small amountsof signal at bothof these
neously. This is also the case for hydrated methylglyoxal
massesarepresentinthemethylglyoxal/NaClspectrum,and
species. Non-hydrated methylglyoxal is not predicted to
formstrongclusterswithorbeionizedbyI−,andtherefore temperature control experiments indicate that these species
aresemivolatile.
wedonotexpecttodetectitusingthisapproach. Aldolad-
Sincesuccinicacid,anorganicdiacid,isexpectedtoclus-
dition products from either pathway (e.g. species (d) or (h)
terstronglywithI− wemayassumethattheinstrumentsen-
from Table 3), if present, should be detected as their clus-
ters with I−. The only aldol condensation products which sitivity to succinic acid (100Hzppt−1) is an upper limit for
wepredicttoformstrongclusterswithI− arethosespecies thesensitivitytothesespecies.Usingthisassumption,wees-
timatelowerboundsfortheproductionratesofthespeciesat
which terminate in a carboxylic acid group (e.g. species (f)
271.5and289.5tobe≥10−3Mmin−1and≥10−2Mmin−1,
fromTable3). Wedonotexpecttoobserveproductsofaldol
respectively.
pathway(2)(ref.Scheme1)suchasspecies(c)fromTable3
Thepeaksat273.5and275.6amuareassignedthemolec-
withthisionizationscheme.
ular formulas I−.C H O and I−.C H O , respectively.
6 10 4 6 12 4
Volatilespecies These molecular formulas, since they each contain six car-
bons, are consistent with the addition of two methylgly-
The peaks at 173.0, 190.1, and 217.3amu were present in oxal monomers. Possible structures are shown in Ta-
thesamemagnitudewhetherthevolatilizationinletheatwas ble 2, but the formation mechanisms of these species in the
turned on or off, indicating that these signals are associated methylglyoxal/(NH ) SO systemarenotknown.
4 2 4
www.atmos-chem-phys.net/10/997/2010/ Atmos. Chem. Phys.,10,997–1016,2010
1006 N.Sareenetal.: SOAformedbymethylglyoxalinaqueousaerosolmimics
Table 2. Proposed peak assignments for Aerosol-CIMS mass spectra with I- as the reagent
−
Table2.Proposedpeaikonas. sSigene mteexnt tfsofro dreAtaeirlos.s o l-CIMSmassspectrawithI asthereagention.Seetextfordetails.
m/z (amu) Ion Molecular
Possible Structure(s)
± 0.5 amu formula formula
217.3
I-.CHO CHO 
3 6 3 3 6 3
225.2 CHOS- CH OS 
6 9 7 6 10 7

271.5 I-.CHO CHO
6 8 4 6 8 4 
273.5 I-.CH O CH O 
6 10 4 6 10 4

275.6 I-.CH O CH O 
6 12 4 6 12 4

289.5 I-.CH O CH O 
6 10 5 6 10 5

800 ratio of the stable isotopes of sulfur 32S and 34S (Fig. 6),
suggestingacompoundcontainingsulfur. Eitheramolecule
with a molecular weight of 226.2amu (in the case of pro-
) 600
.
b ton abstraction) or 98.3amu (in the case of a cluster with
r 536.6
(a 400 610.8 I−) would be consistent with the 225.2amu mass-to-charge
al 685.2 759.5 ratio. The species with m/z 225.2 was observed to be non-
n
g volatile at room temperature. One possible molecular for-
Si 200 mulaforthisspeciesisC H O S−. Theproposedstructure
6 9 7
fortheC H O S−organosulfatespeci3e4sisshowninTable1.
6 9 7
0 Our DFT calculations predict that proton abstraction from
500 550 600 650 700 750 800 thesulfategroupbyI−,ratherthanclusteringviatheligand-
switching reaction, is thermodynamically favorable for this
m/z (amu)
 species. Kinetics studies show that the signal at 225.2amu
developswithinapproximately30minofmixing. Assuming
Fig. 8. Detail of a positive ion mass spectrum of an aerosolized
Fsmmoigleautsuhtsriyomelng o8.l dy.S eoDpxweeacitlta/th(riNulH mHo3f 4O w)a2+ aSp.s(Oo Htsa4i2ktOsievon)eln u iitnaioso nhnt ih.gmehSar-psemesa cgastseprsnue tmmcitoorwnud.ames wtoafikt hean nH ia3neOrh+oi.gs(Hohl-2iOze)dn amacsna etnthuheepy sprltgeeialmrygloaeixtmneat ilait/o(spNner Hnosd4i)ut2icSvtOiitoy4n orfa1te00ofH≥z4p×pt1−01−f3oMrthmisins−p1e.cieswe
Sulfur-containingspecies H2SO4mayclusterwithI−orundergoprotonabstraction.
I−.H SO , if present, would also appear at 225.2amu and
2 4
Thepeakat225.2amufeaturesasatellitepeakat227.2amu displayasatellitepeakat227.2amu. Inordertotestforthis

withanabundanceroughlyconsistentwiththeexpected95:4

Atmos. Chem. Phys.,10,997–1016,2010 www.atmos-chem-phys.net/10/997/2010/
46