Organosulfate formation during the uptake of pinonaldehyde on acidic sulfate aerosols
John Liggio1and Shao-Meng Li1
Received2March2006;revid19May2006;accepted30May2006;published8July2006.
[1]Organosulfates are obrved in studies of pinonaldehyde reactions with acidic sulfate aerosols using aerosol mass spectrometry,during which a significant fraction of the pinonaldehyde reaction products were found to consist of organosulfate compounds that account for6–51%of the initial SO4=mass.Resultant aerosol mass spectra were consistent with propod sulfate ester mechanisms,which likely form stable products.The existence of organosulfates was also confirmed in studies of the reaction system in bulk solution.The formation of organosulfates suggests that conventional inorganic SO4= chemical analysis may underestimate total SO4=mass in ambient aerosols.Citation:Liggio,J.,and S.-M.Li(2006), Organosulfate formation during the uptake of pinonaldehyde on acidic sulfate aerosols,Geophys.Res.Lett.,33,L13808, doi:10.1029/2006GL026079.
1.Introduction
[2]Organic compound nucleation and condensation are important pathways leading to condary organic aerosols (SOA),which in turn have numerous global and local impacts.However,recent studies have now also implicated heterogeneous reactions as a route for SOA formation[Jang et al.,2003;Kroll et al.,2005].Such reactions may lead to a transformation of mi-volatile compounds to high molec-ular weight species or oligomers which can remain in aerosols,and are believed to ari via compounds contain-ing carbonyl functional ,aldehydes and ketones. Indeed,some aldehydes and ketones,when expod to ed aerosols,increa SOA yields,particularly on acidic aero-sols[Jang et al.,2003].Potential reaction mechanisms include hydration,hemi-acetal and acetal formation, aldol condensation[Noziere and Riemer,2003],and poly-merization[Jang and Kamens,2001].Products resulting from the mechanisms have been identified by mass-spectrometric techniques in laboratory studies[Kalberer et al.,2004].
[3]Whereas polymerization remained the focus of such studies,sulfate esters formed in the reaction with acidic sulfate have been largely unnoticed.This is partially due to difficulties detecting such species which may be somewhat non-volatile at typical GC injection temperatures,potentially thermally unstable,and not easily differentiated from inor-ganic sulfate.Mass spectra of sulfate esters from a complex aerosol matrix are likely ambiguous and easily confud with the spectra of other or
ganic products or inorganic sulfate.Hence,evidence for covalently bound sulfate in aerosols is a key step in understanding this reaction pathway.A recent study has in fact reported organo-sulfates in ambient aerosols[Romero and Oehme,2005]. The prent study strives to determine whether organo-sulfate formation occurs in aerosols,to elucidate reaction pathways,and to asss the importance of their formation as a heterogeneous mechanism.The uptake of pinonalde-hyde,a major mi-volatile product from a-pinene oxidation,on acidic SO4aerosols,is ud as a model process to demonstrate organosulfate formation and its implications with regard to SOA formation in the atmosphere.
2.Experiment
busy[4]The pinonaldehyde uptake experiments have been described by Liggio and Li[2006].Mono-disperd aero-sols310–360nm in diameter,consisting primarily of H2SO4but also(NH4)2SO4,were introduced into a2m3 Teflon bag at a given relative humidity,followed by addition of gaous pinonaldehyde.Aerosol inorganic and organic mass were quantified with an aerosol mass spec-trometer(AMS),which provided organic structural infor-mation[Jayne et al.,2000].Seed aerosols were generated via liquid atomization followed by sizing with a differential mobility analyzer.The particle acidity was controlled by the H2SO4concentration in an(NH4)2SO4solution and by the RH in the cha
mber(3–65%),thus altering the HSO4Àto SO4=molar ratios(Table1).
樱花国际日语怎么样[5]Pinonaldehyde vapor was introduced into the cham-ber via a zero air stream at40°C pasd through a spiked quartz filter and was measured by2,4-DNPH scrubbing followed by off-line HPLC-UV analysis[Liggio and Li, 2006].Pinonaldehyde was synthesized via an oxidative bond cleavage of pinandiol[Glasius et al.,1997]and was >90%pure.
[6]The pinonaldehyde reaction in a bulk sulfuric acid solution was studied to confirm the prence of sulfate esters.In each experiment20m L of pinonaldehyde was spiked into a concentrated acid solution(0.9ml H2SO4, 0.1ml H2O),followed by GC-MS analysis.An uncoated, deactivated fud silica column(0.25mm i.d.,5m)was ud,which retains H2SO4for about15min while other products elute immediately.In principle,once inorganic sulfate is removed from the sample matrix,sulfate esters can be confirmed by identifying fragments from the ioni-zation of organically bound SO4(m/z48,64,80,96)or its decomposition products(SO2).A1m L injection of the reaction solution was analyzed with the GC injector heated to250°C and a temperature program which began isother-mally at60°C for1min,followed by a ramp of10°C minÀ1
GEOPHYSICAL RESEARCH LETTERS,VOL.33,L13808,doi:10.1029/2006GL026079,2006 1Air Quality Rearch Division,Atmospheric Science&Technology
Directorate,Science&Technology Branch,Environment Canada.
Copyright2006by the American Geophysical Union.
0094-8276/06/2006GL026079
to 300°C.A 1m L H 2SO 4solution was analyzed to rule out interferences.Pure organic sulfate esters (Sigma-Aldrich)were also analyzed for comparison to the reaction products.
3.Results and Discussion
3.1.Inorganic Sulfate Conversion
[7]The uptake of pinonaldehyde to acidic sulfate aerosols was rapid,depending on particle acidity,and resulted in aerosols ranging from primarily inorganic to mostly organic in nature [Liggio and Li ,2006].The associated uptake coefficients were highly dependent upon particle acidity and spanned veral orders of magnitude (1.2Â10À5–1.3Â10À3).Small increas in the relative humidity reduced the aerosol acidity and decread the uptake coefficients by an order of magnitude,while no uptake was obrved on neutral (NH 4)2SO 4aerosols.To determine whether sulfate esters were formed during the reactive uptake of pinonaldehyde,the inorganic SO 4=and organic mass from the
AMS measure-ments were normalized to the aerosol number concentration determined concurrently by the AMS as discreet single particle ion puls [Jayne et al.,2000].In doing so,the effect of wall loss is removed and the mass per particle is obtained.The normalized mass for lected experiments are shown in Figure 1.Upon the addition of pinonaldehyde the aerosol organic mass increas dramatically indicating significant uptake of the aldehyde.This increa is accompanied by a decrea in the inorganic SO 4=mass (Figure 1).In comparison,the NH 4+
mass (from (NH 4)2SO 4)in Figure 1is not lost during the uptake,indicating that the SO 4=decrea is not a measure-ment artifact.Since quantification of SO 4=by the AMS is bad on its mass spectrum,a decrea in the measured SO 4
=indicates that a portion of the original SO 4=
is covalently bonded to the organic mass and not quantified as inorganic by the AMS.Thus,to some degree,the covalent C–O–S bonds in organosulfates are retained during AMS detection.How-ever,if the organosulfates partially revert to inorganic sulfate during detection,then the results in Figure 1reprent the minimum SO 4=conversion from inorganic to organic.Anal-ysis of commercial sulfate esters with the AMS suggests that this can occur,as 14–20%of the total measured mass is accounte
d for by SO 4=arising from the compounds.[8]The estimates of the minimum SO 4=conversion to organosulfate are given in Table 1as a percent of the initial SO 4=ed mass.Uncertainties are calculated from the standard deviation of the SO 4=mass per particle before and after the addition of pinonaldehyde.For tho experi-ments where minimal uptake is obrved,the uncertainties
are large,yet under the experimental conditions,<6–51%of the inorganic SO 4=was converted into organosulfates;the larger conversions being associated with high aerosol acid-ities as measured by large HSO 4À/SO 4=
ratios.However,variations in the SO 4=conversion were obrved in experi-ments with aerosols of similar acidities.This implies that other factors are important for this process,including the gaous pinonaldehyde concentration,water activity,parti-cle diameter,and exposure time.The type of experiments here precludes a more detailed analysis of the effect of the factors.The approximate fraction of the total AMS mea-sured organic mass accounted for by the organosulfates is also given in Table 1.This is calculated as the ratio of converted sulfate mass to total organic mass (Â100%)and could not be determined for veral experiments where little organic mass is added due to the uncertain SO 4=decreas.A preci mass balance is not possible due to uncertainties in (1)the SO 4=loss,(2)the relative ionization efficiency of organic species ud in the AMS algorithm to calculate
organic mass,(3)the amount of organosulfates detected as inorganic,and (4)the molecular weight of organosulfates.Nonetheless,the converted SO 4=mass accounts for a large fraction of the organic mass (22–65%,Table 1),suggesting important contributions of organosulfates in such a SOA formation mechanism.Although organosulfate formation appears to be the dominant reaction mechanism at the beginning of each experiment,it is accompanied by other reactions,such as aldol-condensation,as indicated by the continued slow increa in total organic mass at 40min during the SO 4=plateau (Figure 1).The slow organic mass increa after $40min implies that the alternate reaction is slower than the organosulfate formation,which may have reached chemical equilibrium by this point.
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3.2.Sulfate Ester Mechanism and Mass Spectra
[9]The organosulfates are likely prent as sulfate esters,although adducts such as sulfonates and sulfonic acids cannot be completely ruled out.Potential mechanisms leading to some sulfate esters of pinonaldehyde in sulfuric acid are given in Figure 2.Sulfate esters of pinonaldehyde may be formed via the protonation of carbonyl groups followed by nucleo-phillic attack of HSO 4À,or by the electrophilic addition of
Table 1.Inorganic to Organic SO 4=Conversion During Pinonal-dehyde Uptake跆拳道黑带九段
Exp.Initial a
HSO 4À/SO 4
=
Inorganic SO 4
=Conversion (%)(±s )
Fraction of Organic (%)(±s )
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5.7(72)-2 5.021.4(11.5)-39.415.5(32.2)-417.847.9(15.2)-518.521.9(14.6)-618.633.0(23.0)64.8(31.0)718.749.5(8.9)53.6(21.7)819.42
6.8(12.0)28.1(15.3)920.146.9(9.4)48.0(13.4)10luo是什么意思
27.5
51.1(11.1)
22.2
(14.3)
a
Equilibrium molar ratio bad on thermodynamic
calculation.
Figure 1.SO 4=,NH 4+and organic mass for lected aerosol chamber experiments as measured by the AMS.‘‘Relative Time’’is relative to pinonaldehyde addition (t =0).
H 2SO 4directly to olefin bonds.Given the hygroscopicity of H 2SO 4,the reactions are likely followed or preceded by dehydration,leading to olefins.Conquently,any number of ester products in addition to tho illustrated in Figure 2are possible,including compounds with multiple SO 4=or pino-naldehyde units.An aerosol mass spectrum of the organic mass,10minutes after the ont of uptake (to maximize the signal to noi ratio),is given in Figure 3.As a result of the 70eV ionization of the AMS,fragmentation is largely skewed toward smaller mass,although many large frag-ments of weak intensity are obrvable.A conclusive interpretation of the mass spectrum is not possible as most fragments can be formulated from high molecular weight aldol-condensation products of pinonaldehyde,or sulfate esters in Figure 2;both reactions result in compounds with a pinonaldehyde structural backbone.Nevertheless,some fragments in the spectrum are potentially unique to the esters in Figure 2,including m/z 107,141and 205(Figure 3),which are not easily formulated from aldol-condensation products of pinonaldehyde.The advent of high resolution aerosol mass spectrometry will make it possible to unequivocally differentiate sulfate ester frag-ments (Figure 3)fro
m other oligomeric products.Further-more,sulfate esters are known to produce obrvable M+1fragments [Lloyd and Porter ,1977].This is confirmed by analysis of pure sulfate esters with the AMS,which showed numerous fragments arising from the break down of the M+1or M+2ions,and by the study on the simpler reaction of glyoxal in sulfuric acid aerosols [Liggio et al.,2004].Similarly,for the reaction products of pinonaldehyde with sulfuric acid,many possible M+1fragments arising from the sulfate esters (Figure 2)are found in the spectrum,including ions 169,179and potentially many others (Figure 2).The formation of M+1ions is made more likely by the numerous oxygen atoms prent in the structures of Figure 2.Given the difficulty in distinguishing sulfate esters from other reaction products by commonly ud analytical methods,it is likely that this class of compound is formed under laboratory and ambient conditions and yet not identified.
3.3.Bulk Studies
[10]To confirm the existence of sulfate esters in the aerosol studies,the reaction of pinonaldehyde with
H 2SO 4/H 2O ($90%v/v)was examined in bulk solution ($1%pinonaldehyde wt%)during which the solution color clearly changed upon mixing of the reactants.GC-MS analysis of this solution did not r
esult in the elution of organic carbon from the un-coated column at the GC vaporization temperature of 250°C,suggesting that highly nonvolatile compounds were prent and only vaporized or thermally broken down at the higher temperature of the AMS oven (560°C).However,the formation of organo-sulfates was obrved by measuring the SO 2gas which evolved from the organic material upon heating in the GC injector.The pul of SO 2from the organic mass is shown in Figure 4as extracted m/z 48ion chromatograms and was reproducible.Injection of pure H 2SO 4solution reproduces much smaller amounts of SO 2(Figure 4).Thus,most of
the
Figure 2.Selected potential mechanisms for the formation of sulfate esters of pinonaldehyde in acidic sulfate
aerosols.Figure 3.Organic particle mass spectra resulting from the uptake of
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pinonaldehyde.
Figure 4.(a)Evolution of SO 2(as m/z 48)from bulk studies of the pinonaldehyde-H 2SO 4reaction analyzed by GC-MS.(b)Equivalent analysis of non-reactive oxalic acid and associated acid solution.(c)Analysis of pure sulfate ester compounds: 1.4,6-Dimethyl-2-Hydroxy-5-pyrimidinyl Sulfate 2.2-((3,5-dioxo-2,3,4,5-tetrahydro-1,2,4-triazin-6-yl)thio)ethyl hydrogen sulfate 3.2-Amino-2-methyl-1-propanol-hydrogen Sulfate.
SO2derived from the reaction solution must have arin from the decomposition of organically bound SO4=.Analysis of a H2SO4solution containing a dissolved non-reactive compound(oxalic acid,$1wt%)did not produce similar SO2puls(Figure4),suggesting that a physical process was not responsible for the previous SO2evolution.Injec-tions of the bulk solution as a function of reaction time (Figure4)implies that the incorporation of SO4=into the organic mass is fast(<5min)but may reach an equilibrium after approximately20–60min,consistent with the rapid SO4=conversion in the aerosol chamber(Figure1).A pha transition(liquid to solid)resulting in a reduced reaction rate,although unlikely,cannot be ruled out as a cau of the time dependence.As a further validation,three commer-cially available sulfate esters were similarly analyzed,and resulted in near identical SO2chromatograms as the reaction solution(Figure4),despite the fact that their formation m
echanism and structure are somewhat different than tho formed by reactions of carbonyls and sulfuric acid as propod here.This suggests that the evolution of SO2at high temperature is common to most sulfate esters.
3.4.Atmospheric Implications
[11]The experimental results indicate that organosulfates can form from reactions of acidic aerosol sulfate with carbonyls in the atmosphere.Liggio and Li[2006]illustrated that reactive uptake of pinonaldehyde can potentially form 7.2ng mÀ3of organic material in the ambient atmosphere, in3hours.Bad on this estimate and data in Table1,1.6–4.7ng mÀ3(22%–65%)may be attributed to organic SO4=. This SO4=conversion reprents a small fraction of the SO4= typically found in the atmosphere.However,given the multitude of ambient carbonyl species which may form organosulfates,this fraction may be much more significant. Indeed,sulfate esters have been obrved in the reactive uptake of glyoxal on SO4=aerosols[Liggio et al.,2004].The importance of organosulfates to atmospheric aerosols depends on their stability under ambient conditions,or during sampling and analysis.The conversion of inorganic SO4=obrved in the experiments suggests that the orga-nosulfates are stable during the vaporization or ionization of the AMS detection process.Although hydrolysis of the ester bond may also occur,it typically aris after inten heating
in H2O;it is known that even under harsh hydrolytic conditions many sulfate esters,including tho of steroids and simple alcohols,are highly resistant to hydrolysis [Goren and Kochansky,1973;Young and Maw,1958]. The apparent stability of organosulfates suggests that the conventional filter-bad inorganic SO4=measurements us-ing aqueous extraction methods probably underestimate the total SO4=burden in atmospheric aerosols.To support this conclusion,Romero and Oehme[2005]reported organo-sulfates in ambient aerosols by detecting SO4=associated with humic material using mass spectrometry following chromatographic paration of inorganic SO4=.It is possible that heterogeneous reactions of the kind described here are pathways leading to humic-like materials in ambient aero-sols.In addition,the hygroscopic and optical properties of this compound class are unknown,and hence their ability to suppress aerosol activation and their impact on radiation are unknown.Further study with respect to the chemical and physical properties of organosulfates and their contribution to ambient aerosols is warranted.
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S.-M.Li and J.Liggio,Air Quality Rearch Division,Atmospheric Science&Technology Directorate,Science&Technology Branch, Environment Canada.(john.a)
