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Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO3.. Bertram, Allan K.; Gross, Simone 2009

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Products and kinetics of the reactions of an alkane monolayerand a terminal alkene monolayer with NO3radicalsSimone Gross1and Allan K. Bertram1Received 14 August 2008; revised 4 November 2008; accepted 14 November 2008; published 27 January 2009.[1] The reactions of an alkanethiol and a terminal alkenethiol self-assembled monolayerwith NO3radicals (in the presence of NO2and O2) were studied. For the alkanemonolayer, infrared (IR) spectroscopy and time-of-flight secondary ion mass spectrometry(ToF-SIMS) confirmed the formation of organonitrates (RONO2). The observation oforganonitrates is in contrast to the recent X-ray photoelectron spectroscopy (XPS) data,which showed very little nitrogen-containing surface species. The identification oforganonitrates may help explain why significant volatilization of the organic chain was notobserved in recent studies of alkane monolayer oxidation by NO3radicals. The reactiveuptake coefficient (g)ofNO3on alkene monolayers determined in our study is higherthan the values obtained in a recent study using liquid and solid alkene bulk films. Apossible reason for this difference may be the location of the double bond at the interface.Using the g value determined in our studies, we show that under conditions where NO3is high the lifetime of an alkene monolayer in the atmosphere may be short (approximately20 min). XPS, IR, and ToF-SIMS were used to identify surface functional groups aftertheoxidationofthealkenemonolayersbyNO3.Theresultsareconsistentwiththeformationof C-O, aldehyde/ketone, carboxylic groups, and nitrogen containing species.Citation: Gross, S., and A. K. Bertram (2009), Products and kinetics of the reactions of an alkane monolayer and a terminal alkenemonolayer with NO3radicals, J. Geophys. Res., 114, D02307, doi:10.1029/2008JD010987.1. Introduction[2] Field measurements have shown that organic materialis abundant in the atmosphere, comprising 10–90% of thetotal fine particulate mass [Kanakidou et al., 2005]. Thisorganic material can be in the form of pure organic aerosolparticles, or alternatively the organic substances can bemixed with inorganic material. In the latter case, the organicmaterialcanformcoatingsonthesurfaceofaqueousparticles[Ellisonetal.,1999;Gilletal.,1983]orcoatingsadsorbedonthe surface of solid particles, such as mineral dust [Usher etal., 2003a].[3] Recently the oxidation of condensed phase organicmaterial by atmospheric radicals has attracted significantattention [Arens et al., 2002; Bertram et al., 2001; Bro¨skeet al., 2003; D’Andrea et al., 2008; Docherty and Ziemann,2006; Eliason et al., 2004; Ellison et al., 1999; Esteve etal., 2003, 2004, 2006; George et al., 2007; Gross andBertram,2008; Hearn etal.,2007;Hearn and Smith,2006;Hung et al., 2005; Inazu et al., 1997; Ishii et al., 2000;Kahan et al., 2006; Knopf et al., 2006; Lai and Finlayson-Pitts, 1991; Lambe et al., 2007; Mak et al., 2007; McNeill etal., 2007, 2008; Moise and Rudich, 2001; Moise et al., 2002;Molina et al., 2004; Perraudin et al., 2005; Robinson et al.,1995; Rudich, 2003; Rudich et al., 2007; Vlasenko et al.,2008; Wang et al., 2000]. Field measurements suggest thatthis chemistry may change the composition of atmosphericparticles [Robinson et al., 2006]. Some studies suggest thatthese heterogeneous reactions can lead to rapid volatilizationof organic particulate matter [McNeill et al., 2008; Molinaetal.,2004;Vlasenkoetal.,2008].Thereactionsmayalsobea significant sink for organic particles [Molina et al., 2004]and a major source of volatile organic carbon (VOC) in theatmosphere [Ellison et al., 1999; Kwan et al., 2006]. Otherstudies suggest that under certain conditions these radicalheterogeneous reactions may not be a significant sourceof VOC material [see, e.g., Docherty and Ziemann, 2006;George et al., 2007; Hearn et al., 2007; Knopf et al., 2006;Moise and Rudich, 2001].[4] Recently it has been suggested that radical-organicheterogeneous chemistry may vary significantly dependingon the phase of the organic, the organic structure, oxygenconcentration, and NO concentration [Ziemann, 2007].These factors are expected to influence the relative impor-tance of the different reaction pathways, and the importanceof volatilization. More work with a range of experimentalconditions is required to better understand the effect of thesevarious factors on heterogeneous radical-organic chemistrysothatlaboratoryresultscanaccuratelybeextrapolatedtotheatmosphere.[5] To improve the understanding of radical-organic het-erogeneous chemistry, we have studied the reaction betweenNO3radicals (in the presence of O2and NO2) and two typesof organic self-assembled monolayers (SAMs). Studies withthese surfaces enabled us to probe radical-organic reactionsJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D02307, doi:10.1029/2008JD010987, 2009ClickHereforFullArticle1Department of Chemistry, University of British Columbia, Vancouver,British Columbia, Canada.Copyright 2009 by the American Geophysical Union.0148-0227/09/2008JD010987$09.00D02307 1of14confined to the gas-surface interface, and separate surfaceand bulk processes [Rudich, 2003]. The results from thesestudies may provide insight into the reactivity of organicsadsorbed on solid substrates such as mineral dust particlesor urban surfaces [Diamond et al., 2000; Donaldson et al.,2005; Simpson et al., 2006; Usher et al., 2003a], and thereactivity of organic coatings on aqueous particles in theatmosphere [Ellison et al., 1999]. These results may alsoprovide insight into the reactivity of solid surfaces [Vieceliet al., 2004]. The self-assembled monolayers we studiedwere an alkane monolayer (1-octadecanethiol C18H38S),referred to as ODT; and a terminal alkene monolayer(undec-10-ene-1-thiol, C11H22S), referred to as UDT. Dif-ferences in behavior between ODT and UDT can beattributed to the presence of the terminal double bond inUDT and not to the difference in chain length, sinceproperties of hydrocarbon thiols on gold are thought tobe independent of chain length for chains of more than10 C atoms [Bain et al., 1989].[6]NO3radicals were chosen for these studies since NO3is an abundant radical in the troposphere with concentrationsinthepollutednighttimeof60–300ppt[Aldeneretal.,2006;Platt et al., 1980; Stutz et al., 2004] with measured extremesof up to 430 ppt [Finlayson-Pitts and Pitts, 2000]. Also it hasrecently been shownthat reactions between NO3radicals andorganic particles can be efficient and comparable in atmo-spheric importance to OH-organic heterogeneous reactionsundercertainconditions[GrossandBertram,2008;Karagulianand Rossi, 2007; Knopfetal., 2006; Mak et al., 2007; Moiseet al., 2002].[7] In our experiments, as mentioned above, the NO3reactions were performed in the presence of O2and NO2(NO2is a byproduct of the method of producing NO3). Thisshould complement several previous radical-organic hetero-geneous studies which were carried out free of NO2.Anunderstanding of the effect of NO2concentrations may helpexplain some of the discrepancies in the literature on radical-organic heterogeneous reactions and they are expected to beof atmospheric relevance, since many atmospheric condi-tions have high NO2concentrations [Finlayson-Pitts andPitts, 2000].[8] The following research expands on our previous studyofNO3radicals with ODT monolayers. Inour previous studywe used X-ray photoelectron spectroscopy (XPS) to investi-gate the oxidation of ODT monolayers [Knopf et al., 2006].Shown in Figure 1 are some of the results from this previouswork [Knopf et al., 2006]. The XPS data also show that lessthan 11% of the carbon chain was removed (i.e., volatilized)owing to NO3exposure. In addition, the XPS profiles wereconsistent with the formation of (1) C-O groups, (2) ketonesor aldehydes, and (3) carboxylic acid groups. However,nitrogen species were not detected on the surface. As notedpreviously, nitrogen species (such as organonitrates) mayhaveformedduringtheoxidationchemistry,butdecomposedduring the XPS measurements. In the following, we expandon our previous measurements by using infrared (IR) spec-troscopy and time-of-flight secondary ion mass spectrometry(ToF-SIMS) to determine if surface nitrogen species do formduring theoxidation ofODT monolayers by NO3. This infor-mation is needed to determine if radical-organic reactions onmonolayers are significantly different from the same reac-tions in the gas and liquid phase.[9] In addition to studying reactions of an alkane mono-layer, we also studied reactions of a terminal alkene mono-layer. In this case, we performed detailed studies, since therehave not been any previous studies of NO3radicals withalkene monolayers. Using a flow tube reactor, we measuredthe reactive uptake coefficientg of NO3on the alkene mono-layer as a function of NO3exposure. g is defined as the frac-tion of collisions of the gas-phase reactant with the surfacethat leads to reactive uptake. These data were then used todetermine how fast alkene surfaces and monolayers will beoxidized by NO3radicals in the atmosphere. We also carriedout detailed surface-product studies. The results from thesemeasurements were used to develop a mechanism for theNO3-alkene monolayer reaction.[10] Below we present theresults for thealkane and alkenemonolayers.Wediscussreactionmechanismsforbothmono-layers. One of the main conclusions from these studies is thatnitrogen containing species are significant products of theheterogeneous reactions studied.2. Experimental Section2.1. Chemicals[11] Octadecanethiol(98%,C18H38S,ODT)waspurchasedfrom Sigma-Aldrich and used without further purification.Undec-10-ene-1-thiol (C11H22S, UDT) was synthesizedaccording to the procedure described by Peanasky andFigure 1. XPS results of NO3exposed ODT from Knopf etal.[2006].(a)Atomicratiooftotaloxygen,O,tototalcarbon,Ctotal, as a function of NO3exposure. (b) Atomic ratio ofoxidized carbon, Cox, to total carbon, Ctotal, as a functionof NO3exposure. The bottom axis gives NO3exposure(atm sec) and the top axis gives total exposure time at 50 pptNO3(days). Both x axes are valid for Figures 1a and 1b.D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS2of14D02307McCarley[1998].NO2(99.5%)waspurchasedfromMatheson,N2(99.999%), O2(99.993%) and He (99.999%) were pur-chased from Praxair. N2O5was generated by reacting NO2withanexcessamountofO3inaflowsystemasdescribedbySchott and Davidson [1958]. The solid N2O5crystals werestored at 197 K. NO3radicals were obtained by thermalconversion of gaseous N2O5to NO3and NO2at 430 K in aTeflon-coated glass oven [Knopfetal., 2006]. This disso-ciation of N2O5was almost complete with residual N2O5concentrations in the flow cell of approximately 1–3% ofNO3concentrations.2.2. Monolayer Preparation[12] Monolayers were prepared on either gold coatedsilicon wafers for condensed phase product studies or acylindrical gold coated tube for measurements of reactiveuptake coefficients. These wafers and the cylindrical goldcoated tube (inner diameter 1.91 cm, length 15 cm) were firstcleanedinpiranhasolution(H2SO4(96%)/H2O2(30%)=3:1),then rinsed with Millipore water (18 MW) and distilledethanol.Thewafersorthetubeswerethenimmersedina1-mMsolutionof1-octadecanethiol(ODT)orundecenethiol(UDT)in distilled ethanol for C2124 h [Ishida et al., 1997]. Subse-quently, samples were cleaned in ethanol using an ultrasonicbath for 1 min and rinsed with Millipore water for approx-imately 3 min (5 min for gold tube). This cleaning procedurewas repeated two more times. SAM coated gold surfaceswere then dried under a stream of ultra-high-purity N2.2.3. Measurements of the Reactive Uptake Coefficient[13] Atemperaturecontrolled,cylindricalflowtubereactorcoupled to a chemical ionization mass spectrometer (CIMS)was employed for measurements of the reactive uptakecoefficient (g)ofNO3on alkene monolayers at 298 K. TheinnerwallofagoldcoatedtubewascoatedwithaUDTSAMand inserted into the flow reactor. NO3radicals were addedthrough a movable injector as previously described [Grossand Bertram, 2008; Knopf et al., 2006; Mak et al., 2007].NO3concentrations of (1–2) C2 1011molecule cmC03wereusedinthepresenceofO2((1.1–1.3)C21016moleculecmC03).The uncertainty in the NO3concentration was approximately±40%. Helium was used as a carrier gas for NO3. Totalpressure in the flow reactor was 2.3–2.5 torr. NO3wasdetected at the exit of the flow cell using chemical ionizationwith IC0(obtained by passing trace amounts of CH3IinN2througha210Posource).TheNO3signalwasmonitoredwhilethe injector was pulled back at equal increments, exposingUDTsurfacestoNO3radicals.Calculationproceduresforthedetermination of g from the depletion of the CIMS signal ofthe gas-phase reactant (here NO3) during exposure to anorganic surface have been described elsewhere [Knopf et al.,2005].2.4. Product Studies as a Function of Exposure[14] TheflowreactorwasalsousedtoexposeSAMsonAucoated Si plates to NO3for subsequent XPS, IR, H2O contactangle, and ToF-SIMS analysis. NO3exposure levels rangedfrom 0 to 9.06C210C05atm sec (an exposure of 0–21 days at50 pptNO3).Experiments wereperformed at298 K,at a pres-sure of 2.3–2.7 torr, using NO3concentrations of (1–5) C21011molecule cmC03and O2concentrations of (1.1–1.6) C21016molecule cmC03. Helium was used as carrier gas for NO3.XPS, IR and ToF-SIMS were then used to monitor theoxidation of the monolayer and identify surface products.H2O contact angle measurements were used to determine thehydrophilicity of the monolayer.[15] Note that in our experiments we were using relativelyhigh NO3concentrations (approximately 1 C21011moleculecmC03), whereas in the atmosphere NO3concentrations arelower (roughly 1 C2 109molecule cmC03depending on timeand location). In order to extrapolate our results to the atmo-sphere, we assume that the only important parameter is totalnumber of collisions between NO3radicals and the surface.This, however, requires verification in future experiments.2.5. XPS, IR, ToF-SIMS, and ContactAngle Measurements[16] XPS measurements were performed on a Leyboldinstrument using an achromatic Al Ka X-ray source at aphoton energy of 1486.6 eVand an electron take-off angle of90C176. Infrared measurements were performed with a Brukergrazing angle IR instrument in the wave number range7000C0500 cmC01(2048 scans per spectrum, 4 cmC01resolu-tion). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was done using a Physical Electronics, PHI TRIFT IIToF-SIMS instrument with a 15 keV Ga+primary ion beam(mass resolutionC219000). The pulse duration was 5ns with acurrent of 500 pA. Sample areas of 100 C2 100 mm2wereirradiated,totalMSacquisitiontimewas10minandnegativeion spectra were obtained. The total ion dose was well belowstatic limit. Contact angle estimates were obtained fromcamera images of a H2O droplet (Millipore, 18 MW, static)on unreacted monolayers (ODT and UDT) and SAMs ex-posed to NO3.3. Results and Discussion3.1. Reaction of NO3With an Octadecanethiol (ODT)Monolayer3.1.1. IR Spectroscopy[17] Grazing angle IR measurements were performed onODT SAMs that had been exposed to NO3radicals. Listed inTable 1 are possible nitrogen species that may form duringthe NO3chemistry (in the presence of O2and NO2) and thecorresponding IR transition frequencies. For our analysis weonly focused on the IR regions from 3000 to 2800 cmC01and1700 to 1200 cmC01. The 3000C02800 cmC01region corre-sponds to the C-H stretching region. The 1700–1200 cmC01regioncoversafrequencyrangewhereseveralofthepossiblenitrogen species have transitions. We did not include in thisanalysis the regions above 3000 cmC01, between 2800 and1700 cmC01and below 1200 cmC01because assigning thebaseline in these regions was often difficult and subjectiveowingtolargebaselinefluctuations,lowpeakintensities,andinterference by water vapor peaks in some areas. Shown inFigure 2 are typical results. NO3exposures used in this studywere3.6C210C05atmsec,whichisequalto50pptfor8.4days.50 ppt (24 h average) corresponds roughly to pollutedconditions. On the basis of the XPS data shown in Figure 1the amount of carbon oxidation with this exposure was about20%. The spectrum shown in Figure 2 corresponds to(IunoxidizedC0Ioxidized)/Iunoxidized, where Ioxidizedand Iunoxidizedcorrespond to the intensity of the reflected light from theoxidized and unoxidized film, respectively. The results fromD02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS3of14D02307Figure 2 show first that the NO3reaction led to the disap-pearance of the CH3and CH2groups, which is consistentwith the oxidation of the organic chain. Figure 2 also showsthe appearance of new peaks at 1645 and 1283 cmC01afterexposure to NO3. The presence of these peaks is consistentwith the formation of organonitrates (RONO2) (see Table 1).Organonitro (RNO2) groups, peroxynitrates (ROONO2) andperoxyacylnitratesurfacespecieswerenotidentifiedintheIRdata. Peroxynitrates and peroxyacylnitrates are expected tohaveashortlifetime.Hence,thesespeciesmayhavebeenlostwhen transferring the substrates to the analytical instrumentsfor surface analysis.[18] The observationof organonitratepeaks inthe IR spec-trum is in contrast to the XPS data by Knopf et al. [2006],which showed very little N(1s) signal. This suggests that theorganonitrates decomposed during the XPS measurements.To investigate this further we exposed an ODT monolayer toNO3(exposure level = 9.06 C2 10C05atm sec). Then werecorded an IR spectrum of the sample, followed by anXPS spectrum, followed by another IR spectrum. In the firstIR spectrum negative peaks in the region from 3000 to2800 cmC01were observed owing to the disappearanceof CH3and CH2surface species, and positive peaks at1645 cmC01and 1283 cmC01were observed due to theformation of alkylnitrates, similar to Figure 2. However, inthe second IR spectrum (after the XPS measurements) the1645cmC01and1283cmC01peakswerestronglydiminishedinintensity, while features for CH3and CH2remained un-changed. This provides further support that the N-containingfunctional groups decomposed during XPS analysis. Ingeneral, XPS is considered nondestructive, but some damageto sensitive material has been reported before [Laibinis et al.,1991; Ulman, 1995; Wasserman et al., 1989]. A comparisonbetween the average bond energies shows that O-N bonds(201 kJ molC01) are much weaker than C-O (358 kJ molC01)orC = O (799 kJ molC01)[Ebbing, 1993] and therefore organo-nitratesshouldbemuchmorepronetodecompositioninXPSthantheotherfunctionalgroupsexpectedinourexperiments.Alcohols, carbonyl and carboxyl groups have been analyzedsuccessfully by XPS in the past [Briggs and Beamson, 1992;Dicke et al., 2002; Wang et al., 2005] and are not expected todecompose owing to the bond strengths.3.1.2. ToF-SIMS Measurements[19] ToF-SIMS measurements were also carried out tofurther confirm the formation of nitrate functional groups.Figure 3 shows negative ion ToF-SIMS spectra of an ODTmonolayer prior to exposure to NO3(Figure 3a) and afterexposuretoNO3(Figure3b).Forthesestudiesanexposureof5.4C210C05atm sec (equivalent to 50 ppt NO3for 12.6 days)was used. On the basis of the XPS data shown in Figure 1,approximately 35% of the carbon is oxidized with thisexposure.[20] The Figure 3a spectrum, recorded before NO3expo-sure, shows mostly AuxSyclusters and low molecular weightFigure 2. ODT IR spectrum calculated from an ODT sam-ple exposed to 3.6C210C05atm sec of NO3and an unoxidizedODT SAM as reference sample (see text). Negative peakscorrespond to features only present or more prominent in theunexposed ODT SAM; positive peaks show features onlypresent or more prominent in the exposed ODT SAM.Table 1. Peak Assignment for IR Spectroscopy of Different Nitrogen Containing SpeciesaAssignmentPeak Position(cmC01) ReferenceRNO21580–1540 Allen et al. [1994], Hung et al. [2005], Jang and Kamens [2001],Lai and Finlayson-Pitts [1991], Tuazon et al. [1999], Williams and Fleming [1989]1390–1340 [Hung et al. [2005], Jang and Kamens [2001],Tuazon et al. [1999], Williams and Fleming [1989]RONO21666–1600 [Allen et al. [1994], Alvarado et al. [1999], Atkinson et al. [1998],Hallquist et al. [1999], Hung et al. [2005], Jang and Kamens [2001],Lai and Finlayson-Pitts [1991], Palen et al. [1992], Tuazon et al. [1999], Williams and Fleming [1989]1286C01250 Allen et al. [1994], Alvarado et al. [1999], Atkinson et al. [1998],Cassanelli et al. [2006], Hallquist et al. [1999], Hung et al. [2005],Palen et al. [1992], Tuazon et al. [1999], Williams and Fleming [1989]862–843 Allen et al. [1994], Alvarado et al. [1999], Atkinson et al. [1998],Cassanelli et al. [2006], Hallquist et al. [1999], Hung et al. [2005], Palen et al. [1992], Tuazon et al. [1999]ROONO21724–1721 Hallquist et al. [1999], Tuazon et al. [1999]1298–1296 Hallquist et al. [1999], Hung et al. [2005], Tuazon et al. [1999]797–789 Hallquist et al. [1999], Tuazon et al. [1999]PAN 1842–1830 Allen et al. [2005], Atkinson et al. [1998]1741–1738 Allen et al. [2005], Atkinson et al. [1998]1302–1300 Allen et al. [2005], Atkinson et al. [1998]794 Allen et al. [2005], Atkinson et al. [1998]aR, alkyl chain; PAN, peroxyacetylnitrate. Typical ranges of wave numbers (cmC01) and references are provided.D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS4of14D02307fragments of the alkanethiol chains (CC0,CHC0,C2HC0,SC0,SHC0), but also characteristic peaks for ODT. These peaksincluded the whole molecule of ODTand are labeled ‘‘MS’’in Figure 3 with ‘‘M’’ meaning C18H37. The observed peaksagreed well with those observed in previous ToF-SIMSmeasurements of alkanethiol SAMs on gold performed byOffordetal.[1994],SunandGardella[2002],andSohnetal.[2004].[21] Comparisons of ODT spectra in Figures 3a and 3bshowed that after NO3exposure, peaks characteristic of theunoxidized hydrocarbon SAMs (CC0,CHC0,C2HC0,SC0,SHC0,HAuMSC0, and Au2MSC0) decreased or disappeared. Newpeaks ofhigh intensities formedinthe m/zregion <100amu,which can be attributed to OC0,OHC0,NO2C0,NO3C0,SO3C0, andHSO4C0. Furthermore, a large number of additional peaksemerged in the m/z region > 100 amu, but with smallerintensities. These are most likely due to oxidized ODTmolecules that subsequently fragmented during the sputter-ing process. Owing to the high number of possible fragmentsin these long chain hydrocarbons, the wide variety ofdifferent oxidation levels and the variety of different oxida-tion products, it was not possible to identify these productsunambiguously.[22] TheidentificationofNO2C0andNO3C0peaksinToF-SIMSconfirmedtheformationofnitrogencontainingspeciesonthesurface, and these peaks are also consistent with the forma-tion of organonitrates, which fragment to NO2C0and NO3C0during the sputtering process.[23] The combined evidence for the presence of organo-nitrates found in IR and ToF-SIMS clearly showed that thesecompounds were formed during reaction of ODT with NO3but were not stable in XPS analysis.[24] The Figure 3b spectrum also shows peaks due to SO3C0and HSO4C0. Oxidized sulfur likely results from oxidization ofthe sulfur group in the monolayer by NO3at large exposures.This is consistent with previous XPS results that also showedoxidation of the sulfur group at NO3exposures greater than3 C2 10C05atm sec [Knopfetal., 2006].3.1.3. Contact Angle Measurements[25] Contact angles of H2O droplets on unexposed andNO3exposed ODT monolayers were obtained using cameraimages. The initial contact angle determined for ODT beforeNO3exposure was 101C176 ±4C176 (error represents one standarddeviation). Reported literature values for different long chainalkane monolayers (C8–C18) range from 93C176 to 119C176 [Bain etal., 1989; Bertram et al., 2001; Dubowski et al., 2004; InmanFigure 3. Negative ion ToF-SIMS spectra of ODT on gold in the m/z 1–700 region. (a) Spectrum wasobtained on an ODT sample without NO3exposure. (b) Spectrum was obtained after an NO3exposure of5.4 C2 10C05atm sec (equivalent to 50 ppt NO3for 12.6 days).D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS5of14D02307etal.,2004;Offordetal.,1994;Owensetal.,2004;Pazetal.,1992; Robinson et al., 1995; Rudich et al., 2000; Thomas etal., 2001; Wasserman et al., 1989]. These numbers show thatour contact angle for ODT is in general agreement withliterature values.[26] ODT samples were exposed to 9.06 C2 10C05atm secNO3(equivalent to 50 ppt for 21 days). This exposure can beseen as an extreme limit of aerosol exposure under pollutedconditions. After this exposure, the contact angle on ODTwas 60C176 ±6C176, a decrease by approximately 41C176. This result isconsistent with the monolayers becoming more hydrophilicowingtooxidation.PreviousstudiesinvestigatingchangesinH2O contact angles (q) on hydrocarbon monolayers (C8–C20) after exposure to gas-phase reactants reported a verybroad range of data for alkanes: after F radical exposure q =82C176[Robinsonetal.,1995],afterO3exposureq=71C176and99C176[Owens et al., 2004; Thomas et al., 2001], after OH exposureq=10C176[Bertrametal.,2001],andafterOexposureq=C2435–45C176 [Paz et al., 1992]. Part of the variability is likely due tothe different exposure levels used in these experiments andalso to different condensed phase products formed (e.g.,hydroxyl groups versus carbonyl groups).[27] To put our values in an atmospheric context, weconverted our contact angle results into cloud condensationnucleation activities. On the basis of data presented byPruppacherandKlett[1997],aninsoluble200nm(diameter)particle with q = 101C176 will have a critical supersaturation(Scrit=RHC0 100%) for water nucleation of > 150%. Incontrast,aninsoluble200nmparticlewithq=60C176willhavea critical supersaturation of approximately 100%. Thisshows that the oxidation by NO3radicals can decrease thecriticalsupersaturationrequiredforwaternucleation.How-ever, even after oxidation, the Scritis still much larger thanScritfor water soluble particles. For example, Scitfor a200 nm ammonium sulfate particle is less than 0.1%[Seinfeld and Pandis, 2006]. Also, typical values of super-saturations found in clouds are between about 0.2 and 2%[Finlayson-Pitts and Pitts, 2000].3.1.4. Proposed Reaction Mechanism[28] Shown in Figure 4 is the proposed mechanism foroxidation of an alkane monolayer by NO3radicals in thepresence of O2and NO2based on gas-phase chemistry. Weuse this as a starting point to discuss the previous XPS datafrom Knopf et al. [2006] and the current IR and ToF-SIMSdata.[29] As a reminder, XPS showed limited volatilization(i.e., loss of the carbon chain), and the formation of (1) C-Ogroups, (2) ketones or aldehydes, and (3) carboxylic groups.The IR spectra and ToF-SIMS showed the formation oforganonitrates.[30] The initial reaction step (1) in the oxidation process isthe abstraction of a hydrogen atom from a methyl or meth-ylene group of the alkyl chain to form HNO3and an alkylradical. The second reaction step is the transformation of analkyl radical into a peroxy radical in the presence of O2[Atkinson,1997].InthepresenceofNO2,peroxynitratesmayform which may subsequently thermally decompose back tothe reactants (step 3) [Atkinson, 1997]. Hence peroxynitratesrepresent a temporary reservoir of NO2. The peroxy radicalcan also react with NO3to form an alkoxy radical, NO2, andO2(step5).Alternatively,theperoxyradicalcanundergoselfreaction, leading to the formation of alkoxy radicals and O2(step 5) or an alcohol and carbonyl (step 4) [Atkinson, 1997].If the alkoxy radical forms, it can decompose by scission of aC-C bond (step 8), or can undergo isomerization to form ahydroxyl alkyl radical (step 9). Alternatively the alkoxyradical can react with NO2to form alkylnitrates (step 6) orreactwithO2toformketonesoraldehydes(step7)[Atkinson,1997].[31] The XPS data suggest that decomposition by scissionof a C-C bond is of minor importance (step 8). The presenceof C-O, ketones, and aldehydes can be explained by step 4,step 7 or step 9. Carboxylic functional groups observed inXPS spectra are probably due to C(=O)O formed in second-aryreactions,forexample,oxidationofaldehydestoacids(asreported by George et al. [2007]). The new results in thismanuscript (based on IR and ToF-SIMS data) show thatorganic nitrates are also a significant product of the hetero-geneous oxidation. Organonitrates can be formed by step 6.Alternatively ithasbeensuggestedthatorganoperoxynitratescan decompose in the condensed phase to form organo-nitrates or carbonyls [Docherty and Ziemann, 2006]. Thispathway(notshowninFigure4)mayalsobeimportantinourmonolayer studies for the production of organonitrates.[32] The identification of organonitrates in our oxidationstudies may be important for a few reasons: first, the for-mation of organonitrates in our experiment may explain whywedidnotseesignificantdecompositionofthecarbonchainsin our recent studies with NO3radicals. In our previous studywe showed that NO3(in the presence of O2and NO2) doesnot rapidly decompose an alkane monolayer by scission of aC-C bond: even under extremely polluted conditions, themaximumlossoftheorganiclayerwasonly10%[Knopfetal.,2006]. This is consistent with our current studies that showthat the formation of organonitrates is a significant pathway.Formation of organonitrates removes RO2and/or RO speciesFigure 4. Proposed reaction mechanism for an alkanesurface with NO3in the presence of NO2and O2based ongas-phase chemistry.D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS6of14D02307from the system, and hence will reduce the importance of thedecompositionchannel.Possiblythisisoneofthedifferencesbetween our work with NO3and alkane monolayers and therecent studies with OH and alkane SAMs where significantdecomposition was observed.[33] Second, the results suggest that perhaps under certainatmospheric conditions, radical-organic reactions may be asource of condensed phase organonitrates. It is well knownthat condensation of gas phase species is an important sourceof condensed phase organonitrates. Also another possiblesource of particle-bound organonitrates is condensed phasephotochemistry[Karagulian etal.,2008].Perhapsundercer-tain conditions of high NO3and NO2concentrations the for-mation of condensed phase organonitrates by radical-organicreactions may also contribute. Field measurements exploringthis topic would be interesting.[34] Third, our results may have implications for con-densed phase OH-organic reactions as well. Under manyatmospheric conditions, NO2concentrations are high. Per-haps for these situations, organonitrates will also form by asimilar mechanism to the mechanism in our experiments(either RO + NO2or RO2+NO2to form RO2NO2followedby decomposition toRONO2)and limit theimportanceofthedecompositionchannel(step8inFigure4)intheOH-organicreaction.However,Molina etal.[2004]studiedtheoxidationof an alkane monolayer by OH in the presence of NOx/O2/H2O in various proportions and they observed significantdecomposition of the monolayer. This suggests that theOH-organic reaction mechanism is less susceptible to thepresence of NO2.3.2. Reaction of NO3With Undecenethiol (UDT):A Terminal Alkene SAM3.2.1. Reactive Uptake Coefficient of NO3on an Unoxidized UDT SAM[35] The g value for a UDT SAM was determined to be3.4 C2 10C02(+ 4.4 C2 10C02/C01.8 C2 10C02) (the uncertaintyreported corresponds to the 95% confidence interval and anuncertainty in the diffusion coefficients of NO3in He andO2of 15%). In comparison, the value determined for ODTwas (8.8 ± 1.0) C2 10C04[Knopf et al., 2006], a factor ofapproximately 39 less. This enhancement of UDT reactivitycompared to ODT reactivity is in agreement with theenhancement observed for reaction rate constants of differ-ent gas-phase reactions of alkanes and alkenes with NO3.For example kpropene/kpropane= 18, k1-butene/kn-butane= 305,kcyclohexene/kcyclohexane= 237, where k represents differentgas-phase rate constants for NO3reactions [Atkinson, 1997;Finlayson-Pitts and Pitts, 2000].[36] The g value of the terminal alkene UDT obtained inthis study is higher than the values obtained by Moise et al.[2002] using liquid and solid alkene bulk films. Moise et al.[2002] measured (1.6 ± 0.3) C2 10C03and (1.4 ± 0.1) C2 10C03for liquid and solid 1-octadecene films; (2.3 ± 0.9) C2 10C03and (1.8 ± 0.3) C2 10C03for liquid and solid 1-hexadecenefilms; (5.8 ± 2.0) C2 10C03and (5.2 ± 2.0) C2 10C03for liquidand solid 7-tetradecene films, respectively. A possiblereason for this difference may be the location of the doublebond at the interface. In our experiments the double bond islocated at the outermost two carbon atoms and is probablymore easily accessible by NO3radicals than double bonds ina liquid or bulk solid. Vieceli et al. [2004], recently studiedthe structure of liquid 1-tetradecene (C14alkene with aterminal double bond) and a 1-octenethiol SAM usingmolecular dynamics simulations. They showed that at theair-liquid interface the molecular orientation becomes per-pendicular to the interface normal rather than random. Alsothese authors reported the percentage of the total accessiblesurface area that is due to double bonds as 28.5 and 99.7%for liquid 1-tetradecene and the terminal-alkene SAM,respectively. This trend is consistent with the differenceobserved between our study and the studies by Moise et al.[2002].[37] Another possible reason for the difference betweenour results and the results by Moise et al. [2002] may be theNO3concentrations used in the different experiments. Moiseet al. [2002] used a slightly higher concentration of NO3.They also used a rotating flow reactor which replenishes thesurfaces of the liquid and prevents the surface from beingrapidly oxidized.[38] Table2showsasummaryofstudiesonheterogeneousreactionsofdifferentgas-phaseoxidantswithterminalalkenemonolayers. Reactive uptake coefficients, g, and averageatmospheric concentrations for each oxidant are reported.Direct comparison of g values for each oxidant is of minoratmospheric relevance since the concentration of each oxi-dant is different in the atmosphere. Therefore the last columnin Table 2 shows the product of g and the average atmo-spheric concentrations of the gas-phase oxidants. Thesenumbers are more relevant parameters for assessing theimportance of the various gas-phase species to atmosphericoxidation since the number of molecules lost to an organicsurface should be proportional to g C2 concentration. Themost important process will generally be the process withthe largest g C2 [reactant concentration] and therefore thehighest oxidative power. The abundance of NO3in the atmo-sphere is highly variable, with a high spatial and seasonalvariability. For these calculations we use NO3concentrationsof 50 ppt and 5 ppt to roughly represent polluted urban andrural conditions [Atkinson et al., 1986; Finlayson-Pitts andPitts, 2000; Geyer et al., 2001]. However, keep in mind NO3concentrations vary significantly in urban and rural condi-tions. As can be seen from Table 2, the oxidative power ofNO3is significantly higher than that of OH, Cl, and Br. OnlyO3is of equal importance to NO3owing to its much highertropospheric concentrations.[39] Using the reactive uptake coefficient determined inthis study and the equation presented by Gross and Bertram[2008] and Moise and Rudich [2001], we can calculate anaverage lifetime of an alkene monolayer at the interface of anaerosol particle in the atmosphere. Assuming an NO3con-centration of 50 ppt, we obtain a value of 22.6 min. Thisshows that the lifetime is short in polluted environments andthat in regions where NO3concentrations are high, surfaceconcentrations of alkenes are expected to be low. A similarconclusion was reached by Moise and Rudich [2001] usingreactive uptake coefficients determined on bulk liquid andsolid alkene films.3.2.2. Measurements of the Reactive Uptake of NO3as a Function of Exposure Time[40] Measurements of the reactive uptake coefficient as afunction of time were carried out to determine if the surfaceswere catalytic (i.e., a reaction takes place at the surface butthe surface is not an active participant) or if the reaction rateD02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS7of14D02307decreased with time owing to oxidation of the monolayer.For these measurements an NO3concentration of 1.0C21011molecule cmC03was used. The change in g with exposure toNO3is shown in Figure 5. The initial fast uptake coefficientis most likely due to a fast reaction with the double bond.Once the double bond was oxidized, the uptake coefficientreached a value consistent with the uptake on an alkanemonolayer. The exposure at which the reactive uptakereached a near steady state value is consistent with the timeit would take to oxidize all the alkene functional groups onthe surface, based on calculations presented by Bertram etal. [2001].[41] NextXPS,IRandToF-SIMSstudiesofthereactionofNO3with alkene monolayers are presented. The purpose ofthese measurements is to identify surface products anddetermine the mechanism for the oxidation of the doublebond.Alltheseexperimentswerecarriedoutusingexposureslessthan2C210C05atmsec,whichissignificantlylessthantheexposures used in our previous alkane SAM studies. Asshown in Figure 1, an alkane monolayer is hardly oxidizedat exposures less than 2 C2 10C05atm sec. Using these lowexposurelevels,thereactionshouldbemostlyconfinedtothedouble bond.3.2.3. X-ray Photoelectron Spectroscopy[42] XPS measurements of alkene monolayers were car-ried out as a function of NO3exposure (ranging from 0 to1.8 C2 10C05atm sec, equivalent to 0 – 4.2 days at 50 pptNO3). Figure 6 shows the C(1s) region of the XPS spectraobtained using three different NO3exposures ranging from0to9.1C210C06atm sec (equivalent to 50 ppt for 50 h). Forthe unexposed sample only one peak at 285 eV wasobserved. This peak is due to vinyl or methylene functionalgroups. After exposure to NO3, the main peak at about285 eV remained, but a shoulder was observed at higherenergies within the C(1s) region indicating the oxidation ofthe organic surface. We fit the total C(1s) region with fouroverlapping Gaussian-Lorentzian peaks, similar to our pre-vious studies of alkane monolayers. The peaks used in thefit were centered at approx. 285 eV, 286.5 eV, 288 eV, and289.5 eV. The peaks at energies above 285 eV representoxidized C species (consistent with C-O, aldehyde/ketone,and carboxylic groups, respectively).[43] To determine the fraction of carbon oxidized as afunction of exposure time, we calculated the atomic ratio ofoxidized carbon (Cox, three peaks at higher binding energy)to the total carbon peak area (Ctotal). Figure 7c shows theseratios for NO3exposures of 0–1.8C210C05atm sec. The errorbars (which represent ±2s) were derived from four differentUDT SAMs on gold-coated Si wafers exposed to the sameamountofNO3.Ascanbeseen,afterexposuresofonly2.0C210C07atm sec (equivalent to 1.1 h at 50 ppt NO3), approxi-mately 12% of the C(1s) signal corresponded to oxidizedcarbon.[44] Figure 7b shows the increase in O(1s) due to increas-ing NO3exposure. The trend observed is qualitativelythe same as for Cox/Ctotalin Figure 7c. As with Cox/Ctotal,itis observed that for O/C after longer exposures (1.2 C210C06C01.8 C2 10C05atm sec) the reaction slowed down andO/C increased with a smaller slope than in the initial stage ofoxidation.[45] WealsoexposedthemonolayerstoO2andNO2,intheabsence of NO3. After exposure to just these species (usingconcentrations greater than those used in the NO3experi-ments) the Cox/Ctotalatomic ratio and O/C atomic ratio wereTable 2. Reactive Uptake Coefficients of Different Gas-Phase Oxidants on Terminal Alkene Monolayers, Average AtmosphericConcentrations of the Gas-Phase Species, and Calculated Oxidative PoweraOxidant Surface g [Oxidant] g C2 [Oxidant]NO3terminal C11alkeneb0.034 1.2 C2 1094.1 C2 1071.2 C2 1084.1 C2 106OH terminal C3alkenec0.6 1 C2 1066 C2 105Cl terminal C8alkened0.1–1 1 C2 1041 C2 103C0 1 C2 104Br terminal C8alkened0.05 1 C2 1065 C2 104O3terminal C3and C8alkenese9 C2 10C06–3C2 10C042.5 C2 10122.3 C2 107C0 7.5 C2 108aNotation: g, reactive uptake coefficients; [oxidant]/molecule cmC03, average atmospheric concentrations of the gas-phase species; and g C2 [oxidant],calculated oxidative power. For NO3radicals, calculations have been performed for both high NO3concentrations (50 ppt) and low concentrations (5 ppt) torepresent approximately a range of concentrations encountered in the atmosphere owing to spatial and seasonal fluctuations.bThis study.cBertram et al. [2001].dMoise and Rudich [2001].eDubowski et al. [2004], Moise and Rudich [2000], and Usher et al. [2003b].Figure 5. Changes in the reactive uptake coefficientg withincreasing NO3exposure of a UDT SAM. Bottom x axiscorrespondstoactualtimeoftheexperiment(min)(atanNO3concentration of 1.0 C2 1011molecule cmC03), and top x axisshows the corresponding total NO3exposure time (hours) ofthe SAM at an atmospheric concentration of 50 ppt.D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS8of14D02307within the error limits for the unexposed sample shown inFigure 7.[46] To assess the amount of decomposition of the UDTSAM,weplotted theatomic ratioofthetotalintegrated C(1s)signal to the total Au(4f) signal intensity (C/Au) (Figure 7d).No significant changes of this ratio could be observed withinexperimental certainty. However, owing to the large uncer-tainties, we can only conclude that less than 21% of thesurface volatilized during the longest exposure experiments.As detachment of the carbon chain is also possible owing tooxidationofthesulfurheadgroup,21%shouldbeconsideredan upper limit to the amount of carbon loss due to C-C bondscission.[47] Monitoring the S(2p) signal of UDT SAMs afterexposure to NO3showed only unoxidized sulfur (peak max-imum at 163.1 ± 0.2 eV) for exposuresC201.2C210C06atm secandbothunoxidizedsulfur(peakmaximumat163.1±0.2eV)and oxidized sulfur (peak maximum at 168.0 ± 0.2 eV) forexposures C219.1 C210C06atm sec.[48] N(1s) signals were below detection limit in all mea-surements of exposed and unexposed UDT SAMs. However,the nitrogen species may have formed during the oxidationchemistry but decomposed during XPS measurements, sim-ilar to the alkane monolayer experiments. To determine ifnitrogen species were formed in these reactions, we carriedout IR and ToF-SIMS measurements (see below).[49] ToinvestigatethebehaviorofUDTafterlongexposuresto NO3radicals, we also performed a few experimentswith large NO3exposures. After an exposure of 4.5 C210C05atm sec the monolayers reached oxidized C fractions(Cox/Ctotal) of 0.37%, and an O/C ratio of 0.42% (data notFigure 6. XPS spectra of the C(1s) region for a UDTmonolayer (top) before exposure to NO3(0 atm sec) andafter exposures of (middle) 1.2 C2 10C06atm sec (equivalentto 50 ppt for 6.7 h) and (bottom) 9.1 C2 10C05atm sec(equivalent to 50 ppt for 50 h).Figure 7. Summary of processing study and XPS resultsfor UDT. (a) Changes in g with increasing NO3exposure.Changes in atomic ratios obtained in XPS as a function ofNO3exposure for (b) atomic ratio of total oxygen, O, to totalcarbon, C; (c) atomic ratio of oxidized carbon, Cox, to totalcarbon, Ctotal; and (d) atomic ratio of total carbon, Ctotal,tototal gold, Au. The two scales on the top and bottom x axesare valid for Figures 7aC07d and provide information on NO3exposurelevelsinatmsec(bottomaxis)andintotalexposuretime (hours) at an atmospheric NO3concentration of 50 ppt(top axis).D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS9of14D02307shown). These values are similar to the values obtained onODT monolayers after long NO3exposures.3.2.4. IR Spectroscopy[50] First, spectra were taken of an unexposed UDT sam-ple with perdeuterated ODT as reference to confirm thepresence of methylene groups in the original UDT SAMs(data not shown). Then IR measurements were performedon samples that had been exposed to NO3. For this analysiswe only focused on the IR regions from 3000 to 2800 cmC01and 1370 to 1200 cmC01. The wavelength range used in theseexperiments is slightly smaller than the range used in thealkane monolayers experiments. This is because the signal tonoise is smaller in the alkene experiments owing to the lowNO3exposures and smaller degree of oxidation of the films.The spectrum shown in Figure 8 was obtained the same wayasFigure2forODT.Anexposurelevelof1.21C210C06atmsec(50 ppt for 6.7 h) was used in this experiment. As with ODT,NO3exposure of UDT resulted in negative peaks in the range3000–2800 cmC01due to the decrease of the symmetric andasymmetric stretching modes of CH2. At this short exposureto NO3we do not expect significant oxidation of the meth-ylene groups in UDT. As peak intensities depend on theorientation of the different vibrational modes [Ulman, 1991],we assume that the negative peaks appearing in the region of3000–2800 cmC01are due to increased disorder within themonolayer due to reaction of the double bond. This isconsistent with results by Moise and Rudich [2001] whoobserved almost complete disappearance of the CH2stretch-ing peaks in IR, while XPS indicated only a 20% reduction intotalcarbonandasignificantfractionofremainingunoxidizedC(1s). The IR spectra clearly showed the formation of a peakat 1281 cmC01. On the basis of Table 1, this could be due to anorganonitrate, organoperoxynitrate, or peroxyacylnitrate. Itseems unlikely that the identified surface species areorganoperoxynitrates or peroxyacylnitrates, since thesespecies typically have a short lifetime and are expectedto decompose while the substrates are transferred to the IRspectrometer for analysis, as discussed above. We can con-clude that nitrogen species are formed, contrary to theconclusion one may reach from the XPS data.3.2.5. ToF-SIMS Measurements[51]ShowninFigure9arenegativeionToF-SIMSspectra for UDT before (Figure 9a) and after exposure toNO3(Figure 9b). As with ODT,spectra before NO3exposureshowed AuxSyclusters, low molecular weight fragments ofthe monolayer chains (CC0,CHC0,C2HC0,SC0,SHC0), andcharacteristic peaks for the whole UDT molecule. Thesepeaks are labeled M, which indicates C11H21. All peakscharacteristic of the unoxidized hydrocarbon SAMs de-creased or disappeared upon NO3exposure (CC0,CHC0,C2HC0,SC0,SHC0,MSC0, and Au(MS)2C0). New peaks of highintensities formed especially in the m/z region < 100, whichcould be attributed to OC0,OHC0,NO2C0,NO3C0,SO3C0, andHSO4C0. As with ODT, peaks for NO2C0and NO3C0led us toconclude that nitrogen containing functional groups formedduring the oxidization of UDT by NO3, but decomposedduring XPS measurements.[52] Again,aswithODT,NO3exposed UDT SAMsshowed peaks of SO3C0and SO4C0. This is consistent with theappearance of a S(2p) peak of oxidized sulfur in XPS spectraof UDT exposed to C219.1 C210C06atm sec.3.2.6. Contact Angle Measurements[53] The initial contact angle determined before NO3exposure was 92C176 ±5C176 for UDT (number given here is theaverage value and standard deviation). Literature values formonolayers of terminal alkenes (C8–C19) were reported at89C176C0107C176 [Bain et al., 1989; Bertram et al., 2001; Dubowskiet al., 2004; Robinson et al., 1995; Rudich et al., 2000;Thomas et al., 2001; Wasserman et al., 1989]. The largecontact angles observed for the unoxidized monolayersindicate the presence of well-ordered SAMs in this study.NO3exposure of UDT samples was 9.06 C2 10C05atm sec(equivalent to 50 ppt NO3for 3 weeks). After this exposure,the contact angle on UDT had decreased to 59C176 ±6C176,adecrease of 33C176. This gives further support that the mono-layers were oxidized leading to a more hydrophilic surface.Previous studies investigating changesinH2Ocontact angleson hydrocarbon monolayers (C8–C19) after exposure to gas-phase reactants reported the following data for terminalalkene monolayers: after F radical exposure q =75C176[Robinson et al., 1995], and after O3exposure q =74C176and C24 70C176 [Dubowski et al., 2004; Thomas et al., 2001].3.2.7. Proposed Reaction Mechanism[54] Shown in Figure 10 is the proposed mechanism foroxidation of an alkene monolayer by NO3radicals in thepresence of O2and NO2based on gas-phase chemistry[Atkinson, 1991; Atkinson, 1997, 2000; Berndt and Bo¨ge,1995;Gongetal.,2005;Kwoketal.,1996;Nodaetal.,2000;Tuazon et al., 1999]. The gas-phase reaction of NO3with alkenes is assumed to proceed via addition of NO3tothe C-C double bond (Figure 10, step 1). This leads to aC-ONO2functional group on one of the two C atoms of thedouble bond and leaves an alkyl radical at the other C atom.Figure 8. UDT IR spectrum calculated from an UDT sam-ple exposed to 1.21C210C06atm sec of NO3(50 ppt for 6.7 h)and an unoxidized UDT SAM as reference sample (see text).Negative peaks correspond to features only present or moreprominent in the unexposed UDT SAM; positive peaksshow features only present or more prominent in the exposedUDT SAM.D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS10 of 14D02307This radical compound is expected to react further with O2toan alkylperoxy radical (RO2,step2)[Finlayson-Pitts andPitts, 2000].[55] Reactions of the RO2radical are believed to occur inthe same way as those of the RO2in the alkane reactionscheme (Figure 4), except that the neighboring C atom con-tains an ONO2functional group. An additional difference isthe fact that the scission channel leads to the elimination ofNO2and the formation of two aldehydes (step 9).[56] Not shown in Figure 10 are additional reactions thatthe alkyl nitrate radical may undergo at low pressures incompetition with step 2. When the nitrate radical adds to thedouble bond, an excited state results. If this excited state isnot collisionally stabilized, it can decompose to NO2and analdehyde or ketone or alternatively eliminate NO2and forman epoxide that reacts further to a carbonyl [Atkinson, 1991;Dlugokencky and Howard, 1989]. The importance of thesesteps on a surface however is unclear.[57] As a reminder, the XPS results are consistent with theformation of C-O, C = O, and C(= O)O functional groups. IRand ToF-SIMS analysis showed the formation of nitrogen-containing species, likely organonitrates. Scission of thecarbon chain at the double bond could not be confirmed,but could not be ruled out either. The formation of C-O andC = O functional groups could be explained by step 4, theRussell mechanism. Formation of C-O could also beexplained by step 7, and the formation of C = O could alsobeexplainedbysteps8and9.Carboxylicfunctionalgroupsdetermined using XPS were probably formed in secondaryreactions,forexample,oxidationofaldehydes[George etal.,2007].[58] Our experiments are the first to investigate heteroge-neous reactions between radicals and alkene monolayers inthe presence of O2and NO2, so our product analysis shouldbe helpful for understanding this heterogeneous mechanism.D’Andrea et al. [2008] investigated the reaction between OHand an alkene monolayer, but in the absence of NO2and O2,so the results are not directly comparable. In addition, bothHungetal.[2005]andDochertyandZiemann[2006]studiedthe reaction between NO3(in the presence of O2and NO2)and liquid alkene particles. Hung et al. [2005] observedorganonitrate, peroxynitrate, organonitrite,and carbonylfunc-tional groups using infrared spectroscopy and high molecularweight products were also observed using LC-MS. Dochertyand Ziemann [2006], using mass spectrometry, identifiedhydroxyl nitrates, carbonyl nitrates, dinitrates, hydroxydini-trates, and possibly more highly nitrated products. It isinteresting to note that several of the functional groupsidentified in these previous liquid reactions were also iden-tified in our current monolayer studies. It therefore appearsFigure 9. Negative ion ToF-SIMS spectra of UDT on gold in the m/z 1–600 region. (a) Spectrum wasobtained on a UDT sample without NO3exposure. (b) Spectrum was obtained after an NO3exposure of9.1 C2 10C06atm s (equivalent to 50 ppt for 50 h).D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3RADICALS11 of 14D02307that the reaction pathways for the alkene monolayers aresimilar to the reaction pathways for the liquid, although therelative importance of the different channels may differ.[59] Also, when studying the reaction between NO3andliquidalkeneparticles,DochertyandZiemann[2006]observedsubstantialorganonitrateformationafterthedoublebondwascompletely reacted, which was attributed to either the reac-tion of N2O5with a hydroxyl group or H-atom abstraction.This latter explanation is consistent with the alkane mono-layer results we presented above.4. Conclusions and Atmospheric Implications[60] For the alkane SAM, IR spectroscopy and ToF-SIMSconfirmed the formation of organonitrates (RONO2). TheobservationoforganonitratesisincontrasttotheXPSdatabyKnopf et al. [2006], which showed very little N(1s) signal.The formation of organonitrates in our experiment mayexplain why we did not see significant decomposition ofthe carbon chains in our recent studies with NO3radicals.Formation of organonitrates removes alkyl peroxy (RO2)and/or alkoxy (RO) species from the system, and hence willreduce the importance of the decomposition channel. Possi-bly this is one of the differences between our work with NO3and alkane monolayers and some of the recent studies withOH and alkane SAMs where significant decomposition wasobserved. In the case of alkene SAMs, the g value of theterminal alkene UDTobtained in this study is higher than thevalues obtained by Moise et al. [2002] using liquid and solidalkene bulk films. A possible reason for this difference maybethelocationofthedoublebondattheinterface.Usingthegvalue determinedinourstudies,weshowthat oxidationofanalkene monolayer by NO3can be significantly more impor-tant than oxidation by OH, Cl, and Br in the atmosphere.Only O3is of equal importance to NO3owing to its muchhigher tropospheric concentration. We also used our g valuefor the alkene SAM to show that the lifetime of an alkenemonolayer in the atmosphere will be short (approximately23 min) assuming an NO3concentration of 50 ppt.[61] XPS, IR and ToF-SIMS were used to identify surfacefunctional groups after the oxidation of the alkene mono-layers by NO3. The results are consistent with the formationof C-O, aldehyde/ketone, carboxylic groups, and nitrogenspecies. It is interesting to note that several of the functionalgroups identified in previous studies of NO3radicals (in thepresence of O2and NO2) with liquid alkene films were alsoidentified in our current monolayer studies.[62] Acknowledgments. The authors would like to acknowledge thehelp of D. A. Knopf and J. Mak in the early stages of NO3and monolayerexperiments. K. C. Wong is acknowledged for XPS measurements,D. Bizzotto for assistance with IR, M. J. Wheeler for help with contact angleimages.Aportionofthisresearch(ToF-SIMSmeasurements)wasperformedusing EMSL, a national scientific user facility sponsored by the Departmentof Energy’s Office of Biological and Environmental Research located atPacific Northwest National Laboratory. The authors thank Z. Zhu for per-forming the ToF-SIMS analysis. The authors are very grateful to L. Fieglandand T. Morris at Virginia Tech, Blacksburg, Virginia, for synthesizing andproviding undecenethiol. 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