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Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO3.. Bertram, Allan K. 2011

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Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO3 radicals Simone Gross1 and Allan K. Bertram1 Received 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 monolayer with NO3 radicals (in the presence of NO2 and O2) were studied. For the alkane monolayer, infrared (IR) spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirmed the formation of organonitrates (RONO2). The observation of organonitrates is in contrast to the recent X-ray photoelectron spectroscopy (XPS) data, which showed very little nitrogen-containing surface species. The identification of organonitrates may help explain why significant volatilization of the organic chain was not observed in recent studies of alkane monolayer oxidation by NO3 radicals. The reactive uptake coefficient (g) of NO3 on alkene monolayers determined in our study is higher than the values obtained in a recent study using liquid and solid alkene bulk films. A possible 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 NO3 is high the lifetime of an alkene monolayer in the atmosphere may be short (approximately 20 min). XPS, IR, and ToF-SIMS were used to identify surface functional groups after the oxidation of the alkenemonolayers by NO3. The results are consistent with the formation of 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 alkene monolayer with NO3 radicals, J. Geophys. Res., 114, D02307, doi:10.1029/2008JD010987. 1. Introduction [2] Field measurements have shown that organic material is abundant in the atmosphere, comprising 10–90% of the total fine particulate mass [Kanakidou et al., 2005]. This organic material can be in the form of pure organic aerosol particles, or alternatively the organic substances can be mixed with inorganic material. In the latter case, the organic material can form coatings on the surface of aqueous particles [Ellison et al., 1999;Gill et al., 1983] or coatings adsorbed on the surface of solid particles, such as mineral dust [Usher et al., 2003a]. [3] Recently the oxidation of condensed phase organic material by atmospheric radicals has attracted significant attention [Arens et al., 2002; Bertram et al., 2001; Bröske et al., 2003; D’Andrea et al., 2008; Docherty and Ziemann, 2006; Eliason et al., 2004; Ellison et al., 1999; Esteve et al., 2003, 2004, 2006; George et al., 2007; Gross and Bertram, 2008; Hearn et al., 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 et al., 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 that this chemistry may change the composition of atmospheric particles [Robinson et al., 2006]. Some studies suggest that these heterogeneous reactions can lead to rapid volatilization of organic particulate matter [McNeill et al., 2008; Molina et al., 2004; Vlasenko et al., 2008]. The reactions may also be a significant sink for organic particles [Molina et al., 2004] and a major source of volatile organic carbon (VOC) in the atmosphere [Ellison et al., 1999; Kwan et al., 2006]. Other studies suggest that under certain conditions these radical heterogeneous reactions may not be a significant source of 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-organic heterogeneous chemistry may vary significantly depending on the phase of the organic, the organic structure, oxygen concentration, and NO concentration [Ziemann, 2007]. These factors are expected to influence the relative impor- tance of the different reaction pathways, and the importance of volatilization. More work with a range of experimental conditions is required to better understand the effect of these various factors on heterogeneous radical-organic chemistry so that laboratory results can accurately be extrapolated to the atmosphere. [5] To improve the understanding of radical-organic het- erogeneous chemistry, we have studied the reaction between NO3 radicals (in the presence of O2 and NO2) and two types of organic self-assembled monolayers (SAMs). Studies with these surfaces enabled us to probe radical-organic reactions JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D02307, doi:10.1029/2008JD010987, 2009 Click Here for Full Article 1Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JD010987$09.00 D02307 1 of 14 confined to the gas-surface interface, and separate surface and bulk processes [Rudich, 2003]. The results from these studies may provide insight into the reactivity of organics adsorbed on solid substrates such as mineral dust particles or urban surfaces [Diamond et al., 2000; Donaldson et al., 2005; Simpson et al., 2006; Usher et al., 2003a], and the reactivity of organic coatings on aqueous particles in the atmosphere [Ellison et al., 1999]. These results may also provide insight into the reactivity of solid surfaces [Vieceli et al., 2004]. The self-assembled monolayers we studied were 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 be attributed to the presence of the terminal double bond in UDT and not to the difference in chain length, since properties of hydrocarbon thiols on gold are thought to be independent of chain length for chains of more than 10 C atoms [Bain et al., 1989]. [6] NO3 radicals were chosen for these studies since NO3 is an abundant radical in the troposphere with concentrations in the polluted nighttime of 60–300 ppt [Aldener et al., 2006; Platt et al., 1980; Stutz et al., 2004] with measured extremes of up to 430 ppt [Finlayson-Pitts and Pitts, 2000]. Also it has recently been shown that reactions between NO3 radicals and organic particles can be efficient and comparable in atmo- spheric importance to OH-organic heterogeneous reactions under certain conditions [Gross andBertram, 2008;Karagulian and Rossi, 2007; Knopf et al., 2006;Mak et al., 2007;Moise et al., 2002]. [7] In our experiments, as mentioned above, the NO3 reactions were performed in the presence of O2 and NO2 (NO2 is a byproduct of the method of producing NO3). This should complement several previous radical-organic hetero- geneous studies which were carried out free of NO2. An understanding of the effect of NO2 concentrations may help explain some of the discrepancies in the literature on radical- organic heterogeneous reactions and they are expected to be of atmospheric relevance, since many atmospheric condi- tions have high NO2 concentrations [Finlayson-Pitts and Pitts, 2000]. [8] The following research expands on our previous study of NO3 radicals with ODTmonolayers. In our previous study we 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 previous work [Knopf et al., 2006]. The XPS data also show that less than 11% of the carbon chain was removed (i.e., volatilized) owing to NO3 exposure. In addition, the XPS profiles were consistent with the formation of (1) C-O groups, (2) ketones or aldehydes, and (3) carboxylic acid groups. However, nitrogen species were not detected on the surface. As noted previously, nitrogen species (such as organonitrates) may have formed during the oxidation chemistry, but decomposed during the XPS measurements. In the following, we expand on 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 form during the oxidation of ODTmonolayers by NO3. This infor- mation is needed to determine if radical-organic reactions on monolayers 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 there have not been any previous studies of NO3 radicals with alkene monolayers. Using a flow tube reactor, we measured the reactive uptake coefficient g of NO3 on the alkene mono- layer as a function of NO3 exposure. g is defined as the frac- tion of collisions of the gas-phase reactant with the surface that leads to reactive uptake. These data were then used to determine how fast alkene surfaces and monolayers will be oxidized by NO3 radicals in the atmosphere. We also carried out detailed surface-product studies. The results from these measurements were used to develop a mechanism for the NO3-alkene monolayer reaction. [10] Below we present the results for the alkane and alkene monolayers.We discuss reactionmechanisms for bothmono- layers. One of the main conclusions from these studies is that nitrogen containing species are significant products of the heterogeneous reactions studied. 2. Experimental Section 2.1. Chemicals [11] Octadecanethiol (98%, C18H38S, ODT) was purchased from Sigma-Aldrich and used without further purification. Undec-10-ene-1-thiol (C11H22S, UDT) was synthesized according to the procedure described by Peanasky and Figure 1. XPS results of NO3 exposed ODT from Knopf et al. [2006]. (a) Atomic ratio of total oxygen, O, to total carbon, Ctotal, as a function of NO3 exposure. (b) Atomic ratio of oxidized carbon, Cox, to total carbon, Ctotal, as a function of NO3 exposure. The bottom axis gives NO3 exposure (atm sec) and the top axis gives total exposure time at 50 ppt NO3 (days). Both x axes are valid for Figures 1a and 1b. D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 2 of 14 D02307 McCarley [1998].NO2 (99.5%)was purchased fromMatheson, N2 (99.999%), O2 (99.993%) and He (99.999%) were pur- chased from Praxair. N2O5 was generated by reacting NO2 with an excess amount of O3 in a flow system as described by Schott and Davidson [1958]. The solid N2O5 crystals were stored at 197 K. NO3 radicals were obtained by thermal conversion of gaseous N2O5 to NO3 and NO2 at 430 K in a Teflon-coated glass oven [Knopf et al., 2006]. This disso- ciation of N2O5 was almost complete with residual N2O5 concentrations in the flow cell of approximately 1–3% of NO3 concentrations. 2.2. Monolayer Preparation [12] Monolayers were prepared on either gold coated silicon wafers for condensed phase product studies or a cylindrical gold coated tube for measurements of reactive uptake coefficients. These wafers and the cylindrical gold coated tube (inner diameter 1.91 cm, length 15 cm) were first cleaned in piranha solution (H2SO4 (96%)/H2O2 (30%) = 3:1), then rinsed with Millipore water (18 MW) and distilled ethanol. Thewafers or the tubes were then immersed in a 1-mM solution of 1-octadecanethiol (ODT) or undecenethiol (UDT) in distilled ethanol for 24 h [Ishida et al., 1997]. Subse- quently, samples were cleaned in ethanol using an ultrasonic bath for 1 min and rinsed with Millipore water for approx- imately 3 min (5 min for gold tube). This cleaning procedure was repeated two more times. SAM coated gold surfaces were then dried under a stream of ultra-high-purity N2. 2.3. Measurements of the Reactive Uptake Coefficient [13] A temperature controlled, cylindrical flow tube reactor coupled to a chemical ionization mass spectrometer (CIMS) was employed for measurements of the reactive uptake coefficient (g) of NO3 on alkene monolayers at 298 K. The inner wall of a gold coated tube was coated with a UDT SAM and inserted into the flow reactor. NO3 radicals were added through a movable injector as previously described [Gross and Bertram, 2008; Knopf et al., 2006; Mak et al., 2007]. NO3 concentrations of (1–2)  1011 molecule cm3 were used in the presence of O2 ((1.1–1.3) 1016molecule cm3). The uncertainty in the NO3 concentration was approximately ±40%. Helium was used as a carrier gas for NO3. Total pressure in the flow reactor was 2.3–2.5 torr. NO3 was detected at the exit of the flow cell using chemical ionization with I (obtained by passing trace amounts of CH3I in N2 through a 210Po source). TheNO3 signal wasmonitoredwhile the injector was pulled back at equal increments, exposing UDTsurfaces to NO3 radicals. Calculation procedures for the determination of g from the depletion of the CIMS signal of the gas-phase reactant (here NO3) during exposure to an organic surface have been described elsewhere [Knopf et al., 2005]. 2.4. Product Studies as a Function of Exposure [14] The flow reactor was also used to expose SAMs onAu coated Si plates to NO3 for subsequent XPS, IR, H2O contact angle, and ToF-SIMS analysis. NO3 exposure levels ranged from 0 to 9.06  105 atm sec (an exposure of 0–21 days at 50 ppt NO3). Experiments were performed at 298 K, at a pres- sure of 2.3–2.7 torr, using NO3 concentrations of (1–5)  1011 molecule cm3 and O2 concentrations of (1.1–1.6)  1016 molecule cm3. Helium was used as carrier gas for NO3. XPS, IR and ToF-SIMS were then used to monitor the oxidation of the monolayer and identify surface products. H2O contact angle measurements were used to determine the hydrophilicity of the monolayer. [15] Note that in our experiments we were using relatively high NO3 concentrations (approximately 1  1011 molecule cm3), whereas in the atmosphere NO3 concentrations are lower (roughly 1  109 molecule cm3 depending on time and location). In order to extrapolate our results to the atmo- sphere, we assume that the only important parameter is total number of collisions between NO3 radicals and the surface. This, however, requires verification in future experiments. 2.5. XPS, IR, ToF-SIMS, and Contact Angle Measurements [16] XPS measurements were performed on a Leybold instrument using an achromatic Al Ka X-ray source at a photon energy of 1486.6 eVand an electron take-off angle of 90. Infrared measurements were performed with a Bruker grazing angle IR instrument in the wave number range 7000500 cm1 (2048 scans per spectrum, 4 cm1 resolu- tion). Time-of-flight secondary ion mass spectrometry (ToF- SIMS) was done using a Physical Electronics, PHI TRIFT II ToF-SIMS instrument with a 15 keV Ga+ primary ion beam (mass resolution 9000). The pulse duration was 5 ns with a current of 500 pA. Sample areas of 100  100 mm2 were irradiated, total MS acquisition time was 10 min and negative ion spectra were obtained. The total ion dose was well below static limit. Contact angle estimates were obtained from camera images of a H2O droplet (Millipore, 18 MW, static) on unreacted monolayers (ODT and UDT) and SAMs ex- posed to NO3. 3. Results and Discussion 3.1. Reaction of NO3 With an Octadecanethiol (ODT) Monolayer 3.1.1. IR Spectroscopy [17] Grazing angle IR measurements were performed on ODT SAMs that had been exposed to NO3 radicals. Listed in Table 1 are possible nitrogen species that may form during the NO3 chemistry (in the presence of O2 and NO2) and the corresponding IR transition frequencies. For our analysis we only focused on the IR regions from 3000 to 2800 cm1 and 1700 to 1200 cm1. The 30002800 cm1 region corre- sponds to the C-H stretching region. The 1700–1200 cm1 region covers a frequency range where several of the possible nitrogen species have transitions. We did not include in this analysis the regions above 3000 cm1, between 2800 and 1700 cm1 and below 1200 cm1 because assigning the baseline in these regions was often difficult and subjective owing to large baseline fluctuations, low peak intensities, and interference by water vapor peaks in some areas. Shown in Figure 2 are typical results. NO3 exposures used in this study were 3.6 105 atm sec, which is equal to 50 ppt for 8.4 days. 50 ppt (24 h average) corresponds roughly to polluted conditions. On the basis of the XPS data shown in Figure 1 the amount of carbon oxidation with this exposure was about 20%. The spectrum shown in Figure 2 corresponds to (Iunoxidized  Ioxidized)/Iunoxidized, where Ioxidized and Iunoxidized correspond to the intensity of the reflected light from the oxidized and unoxidized film, respectively. The results from D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 3 of 14 D02307 Figure 2 show first that the NO3 reaction led to the disap- pearance of the CH3 and CH2 groups, which is consistent with the oxidation of the organic chain. Figure 2 also shows the appearance of new peaks at 1645 and 1283 cm1 after exposure to NO3. The presence of these peaks is consistent with the formation of organonitrates (RONO2) (see Table 1). Organonitro (RNO2) groups, peroxynitrates (ROONO2) and peroxyacylnitrate surface species were not identified in the IR data. Peroxynitrates and peroxyacylnitrates are expected to have a short lifetime. Hence, these species may have been lost when transferring the substrates to the analytical instruments for surface analysis. [18] The observation of organonitrate peaks in the 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 the organonitrates decomposed during the XPS measurements. To investigate this further we exposed an ODT monolayer to NO3 (exposure level = 9.06  105 atm sec). Then we recorded an IR spectrum of the sample, followed by an XPS spectrum, followed by another IR spectrum. In the first IR spectrum negative peaks in the region from 3000 to 2800 cm1 were observed owing to the disappearance of CH3 and CH2 surface species, and positive peaks at 1645 cm1 and 1283 cm1 were observed due to the formation of alkylnitrates, similar to Figure 2. However, in the second IR spectrum (after the XPS measurements) the 1645 cm1 and 1283 cm1 peaks were strongly diminished in intensity, while features for CH3 and CH2 remained un- changed. This provides further support that the N-containing functional groups decomposed during XPS analysis. In general, XPS is considered nondestructive, but some damage to sensitive material has been reported before [Laibinis et al., 1991; Ulman, 1995;Wasserman et al., 1989]. A comparison between the average bond energies shows that O-N bonds (201 kJ mol1) are much weaker than C-O (358 kJ mol1) or C = O (799 kJ mol1) [Ebbing, 1993] and therefore organo- nitrates should be much more prone to decomposition in XPS than the other functional groups expected in our experiments. Alcohols, carbonyl and carboxyl groups have been analyzed successfully by XPS in the past [Briggs and Beamson, 1992; Dicke et al., 2002;Wang et al., 2005] and are not expected to decompose owing to the bond strengths. 3.1.2. ToF-SIMS Measurements [19] ToF-SIMS measurements were also carried out to further confirm the formation of nitrate functional groups. Figure 3 shows negative ion ToF-SIMS spectra of an ODT monolayer prior to exposure to NO3 (Figure 3a) and after exposure to NO3 (Figure 3b). For these studies an exposure of 5.4  105 atm sec (equivalent to 50 ppt NO3 for 12.6 days) was used. On the basis of the XPS data shown in Figure 1, approximately 35% of the carbon is oxidized with this exposure. [20] The Figure 3a spectrum, recorded before NO3 expo- sure, shows mostly AuxSy clusters and low molecular weight Figure 2. ODT IR spectrum calculated from an ODT sam- ple exposed to 3.6 105 atm sec of NO3 and an unoxidized ODT SAM as reference sample (see text). Negative peaks correspond to features only present or more prominent in the unexposed ODT SAM; positive peaks show features only present or more prominent in the exposed ODT SAM. Table 1. Peak Assignment for IR Spectroscopy of Different Nitrogen Containing Speciesa Assignment Peak Position (cm1) Reference RNO2 1580–1540 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] RONO2 1666–1600 [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] 12861250 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] ROONO2 1724–1721 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 (cm1) and references are provided. D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 4 of 14 D02307 fragments of the alkanethiol chains (C, CH, C2H , S, SH), but also characteristic peaks for ODT. These peaks included the whole molecule of ODT and are labeled ‘‘MS’’ in Figure 3 with ‘‘M’’ meaning C18H37. The observed peaks agreed well with those observed in previous ToF-SIMS measurements of alkanethiol SAMs on gold performed by Offord et al. [1994], Sun and Gardella [2002], and Sohn et al. [2004]. [21] Comparisons of ODT spectra in Figures 3a and 3b showed that after NO3 exposure, peaks characteristic of the unoxidized hydrocarbon SAMs (C, CH, C2H , S, SH, HAuMS, and Au2MS ) decreased or disappeared. New peaks of high intensities formed in the m/z region < 100 amu, which can be attributed to O, OH, NO2 , NO3 , SO3 , and HSO4 . Furthermore, a large number of additional peaks emerged in the m/z region > 100 amu, but with smaller intensities. These are most likely due to oxidized ODT molecules that subsequently fragmented during the sputter- ing process. Owing to the high number of possible fragments in these long chain hydrocarbons, the wide variety of different oxidation levels and the variety of different oxida- tion products, it was not possible to identify these products unambiguously. [22] The identification ofNO2  andNO3  peaks in ToF-SIMS confirmed the formation of nitrogen containing species on the surface, and these peaks are also consistent with the forma- tion of organonitrates, which fragment to NO2  and NO3  during the sputtering process. [23] The combined evidence for the presence of organo- nitrates found in IR and ToF-SIMS clearly showed that these compounds were formed during reaction of ODT with NO3 but were not stable in XPS analysis. [24] The Figure 3b spectrum also shows peaks due to SO3  and HSO4 . Oxidized sulfur likely results from oxidization of the sulfur group in the monolayer by NO3 at large exposures. This is consistent with previous XPS results that also showed oxidation of the sulfur group at NO3 exposures greater than 3  105 atm sec [Knopf et al., 2006]. 3.1.3. Contact Angle Measurements [25] Contact angles of H2O droplets on unexposed and NO3 exposed ODT monolayers were obtained using camera images. The initial contact angle determined for ODT before NO3 exposure was 101 ± 4 (error represents one standard deviation). Reported literature values for different long chain alkane monolayers (C8–C18) range from 93 to 119 [Bain et al., 1989; Bertram et al., 2001;Dubowski et al., 2004; Inman Figure 3. Negative ion ToF-SIMS spectra of ODT on gold in the m/z 1–700 region. (a) Spectrum was obtained on an ODT sample without NO3 exposure. (b) Spectrum was obtained after an NO3 exposure of 5.4  105 atm sec (equivalent to 50 ppt NO3 for 12.6 days). D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 5 of 14 D02307 et al., 2004;Offord et al., 1994;Owens et al., 2004; Paz et al., 1992; Robinson et al., 1995; Rudich et al., 2000; Thomas et al., 2001;Wasserman et al., 1989]. These numbers show that our contact angle for ODT is in general agreement with literature values. [26] ODT samples were exposed to 9.06  105 atm sec NO3 (equivalent to 50 ppt for 21 days). This exposure can be seen as an extreme limit of aerosol exposure under polluted conditions. After this exposure, the contact angle on ODT was 60 ± 6, a decrease by approximately 41. This result is consistent with the monolayers becoming more hydrophilic owing to oxidation. Previous studies investigating changes in H2O contact angles (q) on hydrocarbon monolayers (C8– C20) after exposure to gas-phase reactants reported a very broad range of data for alkanes: after F radical exposure q = 82 [Robinson et al., 1995], after O3 exposure q = 71 and 99 [Owens et al., 2004; Thomas et al., 2001], after OH exposure q = 10 [Bertram et al., 2001], and after O exposure q =35– 45 [Paz et al., 1992]. Part of the variability is likely due to the different exposure levels used in these experiments and also to different condensed phase products formed (e.g., hydroxyl groups versus carbonyl groups). [27] To put our values in an atmospheric context, we converted our contact angle results into cloud condensation nucleation activities. On the basis of data presented by Pruppacher and Klett [1997], an insoluble 200 nm (diameter) particle with q = 101 will have a critical supersaturation (Scrit = RH  100%) for water nucleation of > 150%. In contrast, an insoluble 200 nm particle with q = 60will have a critical supersaturation of approximately 100%. This shows that the oxidation by NO3 radicals can decrease the critical supersaturation required for water nucleation. How- ever, even after oxidation, the Scrit is still much larger than Scrit for water soluble particles. For example, Scit for a 200 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 for oxidation of an alkane monolayer by NO3 radicals in the presence of O2 and NO2 based on gas-phase chemistry. We use this as a starting point to discuss the previous XPS data from Knopf et al. [2006] and the current IR and ToF-SIMS data. [29] As a reminder, XPS showed limited volatilization (i.e., loss of the carbon chain), and the formation of (1) C-O groups, (2) ketones or aldehydes, and (3) carboxylic groups. The IR spectra and ToF-SIMS showed the formation of organonitrates. [30] The initial reaction step (1) in the oxidation process is the abstraction of a hydrogen atom from a methyl or meth- ylene group of the alkyl chain to form HNO3 and an alkyl radical. The second reaction step is the transformation of an alkyl radical into a peroxy radical in the presence of O2 [Atkinson, 1997]. In the presence of NO2, peroxynitrates may form which may subsequently thermally decompose back to the reactants (step 3) [Atkinson, 1997]. Hence peroxynitrates represent a temporary reservoir of NO2. The peroxy radical can also react with NO3 to form an alkoxy radical, NO2, and O2 (step 5). Alternatively, the peroxy radical can undergo self reaction, 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 a C-C bond (step 8), or can undergo isomerization to form a hydroxyl alkyl radical (step 9). Alternatively the alkoxy radical can react with NO2 to form alkylnitrates (step 6) or react with O2 to form ketones or aldehydes (step 7) [Atkinson, 1997]. [31] The XPS data suggest that decomposition by scission of a C-C bond is of minor importance (step 8). The presence of C-O, ketones, and aldehydes can be explained by step 4, step 7 or step 9. Carboxylic functional groups observed in XPS spectra are probably due to C(=O)O formed in second- ary reactions, for example, oxidation of aldehydes to acids (as reported by George et al. [2007]). The new results in this manuscript (based on IR and ToF-SIMS data) show that organic nitrates are also a significant product of the hetero- geneous oxidation. Organonitrates can be formed by step 6. Alternatively it has been suggested that organoperoxynitrates can decompose in the condensed phase to form organo- nitrates or carbonyls [Docherty and Ziemann, 2006]. This pathway (not shown in Figure 4) may also be important in our monolayer studies for the production of organonitrates. [32] The identification of organonitrates in our oxidation studies may be important for a few reasons: first, the for- mation of organonitrates in our experiment may explain why we did not see significant decomposition of the carbon chains in our recent studies with NO3 radicals. In our previous study we showed that NO3 (in the presence of O2 and NO2) does not rapidly decompose an alkane monolayer by scission of a C-C bond: even under extremely polluted conditions, the maximum loss of the organic layerwas only 10% [Knopf et al., 2006]. This is consistent with our current studies that show that the formation of organonitrates is a significant pathway. Formation of organonitrates removes RO2 and/or RO species Figure 4. Proposed reaction mechanism for an alkane surface with NO3 in the presence of NO2 and O2 based on gas-phase chemistry. D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 6 of 14 D02307 from the system, and hence will reduce the importance of the decomposition channel. Possibly this is one of the differences between our work with NO3 and alkane monolayers and the recent studies with OH and alkane SAMs where significant decomposition was observed. [33] Second, the results suggest that perhaps under certain atmospheric conditions, radical-organic reactions may be a source of condensed phase organonitrates. It is well known that condensation of gas phase species is an important source of condensed phase organonitrates. Also another possible source of particle-bound organonitrates is condensed phase photochemistry [Karagulian et al., 2008]. Perhaps under cer- tain conditions of high NO3 and NO2 concentrations the for- mation of condensed phase organonitrates by radical-organic reactions may also contribute. Field measurements exploring this topic would be interesting. [34] Third, our results may have implications for con- densed phase OH-organic reactions as well. Under many atmospheric conditions, NO2 concentrations are high. Per- haps for these situations, organonitrates will also form by a similar mechanism to the mechanism in our experiments (either RO + NO2 or RO2 + NO2 to form RO2NO2 followed by decomposition to RONO2) and limit the importance of the decomposition channel (step 8 in Figure 4) in the OH-organic reaction. However,Molina et al. [2004] studied the oxidation of an alkane monolayer by OH in the presence of NOx/O2/ H2O in various proportions and they observed significant decomposition of the monolayer. This suggests that the OH-organic reaction mechanism is less susceptible to the presence of NO2. 3.2. Reaction of NO3 With Undecenethiol (UDT): A Terminal Alkene SAM 3.2.1. Reactive Uptake Coefficient of NO3 on an Unoxidized UDT SAM [35] The g value for a UDT SAM was determined to be 3.4  102 (+ 4.4  102/1.8  102) (the uncertainty reported corresponds to the 95% confidence interval and an uncertainty in the diffusion coefficients of NO3 in He and O2 of 15%). In comparison, the value determined for ODT was (8.8 ± 1.0)  104 [Knopf et al., 2006], a factor of approximately 39 less. This enhancement of UDT reactivity compared to ODT reactivity is in agreement with the enhancement 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 different gas-phase rate constants for NO3 reactions [Atkinson, 1997; Finlayson-Pitts and Pitts, 2000]. [36] The g value of the terminal alkene UDT obtained in this 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)  103 and (1.4 ± 0.1)  103 for liquid and solid 1-octadecene films; (2.3 ± 0.9)  103 and (1.8 ± 0.3)  103 for liquid and solid 1-hexadecene films; (5.8 ± 2.0)  103 and (5.2 ± 2.0)  103 for liquid and solid 7-tetradecene films, respectively. A possible reason for this difference may be the location of the double bond at the interface. In our experiments the double bond is located at the outermost two carbon atoms and is probably more easily accessible by NO3 radicals than double bonds in a liquid or bulk solid. Vieceli et al. [2004], recently studied the structure of liquid 1-tetradecene (C14 alkene with a terminal double bond) and a 1-octenethiol SAM using molecular dynamics simulations. They showed that at the air-liquid interface the molecular orientation becomes per- pendicular to the interface normal rather than random. Also these authors reported the percentage of the total accessible surface 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 difference observed between our study and the studies by Moise et al. [2002]. [37] Another possible reason for the difference between our results and the results by Moise et al. [2002] may be the NO3 concentrations used in the different experiments.Moise et al. [2002] used a slightly higher concentration of NO3. They also used a rotating flow reactor which replenishes the surfaces of the liquid and prevents the surface from being rapidly oxidized. [38] Table 2 shows a summary of studies on heterogeneous reactions of different gas-phase oxidants with terminal alkene monolayers. Reactive uptake coefficients, g, and average atmospheric concentrations for each oxidant are reported. Direct comparison of g values for each oxidant is of minor atmospheric relevance since the concentration of each oxi- dant is different in the atmosphere. Therefore the last column in Table 2 shows the product of g and the average atmo- spheric concentrations of the gas-phase oxidants. These numbers are more relevant parameters for assessing the importance of the various gas-phase species to atmospheric oxidation since the number of molecules lost to an organic surface should be proportional to g  concentration. The most important process will generally be the process with the largest g  [reactant concentration] and therefore the highest oxidative power. The abundance of NO3 in the atmo- sphere is highly variable, with a high spatial and seasonal variability. For these calculations we use NO3 concentrations of 50 ppt and 5 ppt to roughly represent polluted urban and rural conditions [Atkinson et al., 1986; Finlayson-Pitts and Pitts, 2000; Geyer et al., 2001]. However, keep in mind NO3 concentrations vary significantly in urban and rural condi- tions. As can be seen from Table 2, the oxidative power of NO3 is significantly higher than that of OH, Cl, and Br. Only O3 is of equal importance to NO3 owing to its much higher tropospheric concentrations. [39] Using the reactive uptake coefficient determined in this study and the equation presented by Gross and Bertram [2008] and Moise and Rudich [2001], we can calculate an average lifetime of an alkene monolayer at the interface of an aerosol particle in the atmosphere. Assuming an NO3 con- centration of 50 ppt, we obtain a value of 22.6 min. This shows that the lifetime is short in polluted environments and that in regions where NO3 concentrations are high, surface concentrations of alkenes are expected to be low. A similar conclusion was reached by Moise and Rudich [2001] using reactive uptake coefficients determined on bulk liquid and solid alkene films. 3.2.2. Measurements of the Reactive Uptake of NO3 as a Function of Exposure Time [40] Measurements of the reactive uptake coefficient as a function of time were carried out to determine if the surfaces were catalytic (i.e., a reaction takes place at the surface but the surface is not an active participant) or if the reaction rate D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 7 of 14 D02307 decreased with time owing to oxidation of the monolayer. For these measurements an NO3 concentration of 1.0  1011 molecule cm3 was used. The change in g with exposure to NO3 is shown in Figure 5. The initial fast uptake coefficient is most likely due to a fast reaction with the double bond. Once the double bond was oxidized, the uptake coefficient reached a value consistent with the uptake on an alkane monolayer. The exposure at which the reactive uptake reached a near steady state value is consistent with the time it would take to oxidize all the alkene functional groups on the surface, based on calculations presented by Bertram et al. [2001]. [41] Next XPS, IR and ToF-SIMS studies of the reaction of NO3 with alkene monolayers are presented. The purpose of these measurements is to identify surface products and determine the mechanism for the oxidation of the double bond. All these experiments were carried out using exposures less than 2 105 atm sec, which is significantly less than the exposures used in our previous alkane SAM studies. As shown in Figure 1, an alkane monolayer is hardly oxidized at exposures less than 2  105 atm sec. Using these low exposure levels, the reaction should be mostly confined to the double bond. 3.2.3. X-ray Photoelectron Spectroscopy [42] XPS measurements of alkene monolayers were car- ried out as a function of NO3 exposure (ranging from 0 to 1.8  105 atm sec, equivalent to 0 – 4.2 days at 50 ppt NO3). Figure 6 shows the C(1s) region of the XPS spectra obtained using three different NO3 exposures ranging from 0 to 9.1  106 atm sec (equivalent to 50 ppt for 50 h). For the unexposed sample only one peak at 285 eV was observed. This peak is due to vinyl or methylene functional groups. After exposure to NO3, the main peak at about 285 eV remained, but a shoulder was observed at higher energies within the C(1s) region indicating the oxidation of the organic surface. We fit the total C(1s) region with four overlapping Gaussian-Lorentzian peaks, similar to our pre- vious studies of alkane monolayers. The peaks used in the fit were centered at approx. 285 eV, 286.5 eV, 288 eV, and 289.5 eV. The peaks at energies above 285 eV represent oxidized C species (consistent with C-O, aldehyde/ketone, and carboxylic groups, respectively). [43] To determine the fraction of carbon oxidized as a function of exposure time, we calculated the atomic ratio of oxidized carbon (Cox, three peaks at higher binding energy) to the total carbon peak area (Ctotal). Figure 7c shows these ratios for NO3 exposures of 0–1.8 105 atm sec. The error bars (which represent ±2s) were derived from four different UDT SAMs on gold-coated Si wafers exposed to the same amount of NO3. As can be seen, after exposures of only 2.0 107 atm sec (equivalent to 1.1 h at 50 ppt NO3), approxi- mately 12% of the C(1s) signal corresponded to oxidized carbon. [44] Figure 7b shows the increase in O(1s) due to increas- ing NO3 exposure. The trend observed is qualitatively the same as for Cox/Ctotal in Figure 7c. As with Cox/Ctotal, it is observed that for O/C after longer exposures (1.2  1061.8  105 atm sec) the reaction slowed down and O/C increased with a smaller slope than in the initial stage of oxidation. [45] We also exposed the monolayers to O2 andNO2, in the absence of NO3. After exposure to just these species (using concentrations greater than those used in the NO3 experi- ments) the Cox/Ctotal atomic ratio and O/C atomic ratio were Table 2. Reactive Uptake Coefficients of Different Gas-Phase Oxidants on Terminal Alkene Monolayers, Average Atmospheric Concentrations of the Gas-Phase Species, and Calculated Oxidative Powera Oxidant Surface g [Oxidant] g  [Oxidant] NO3 terminal C11 alkene b 0.034 1.2  109 4.1  107 1.2  108 4.1  106 OH terminal C3 alkene c 0.6 1  106 6  105 Cl terminal C8 alkene d 0.1–1 1  104 1  103  1  104 Br terminal C8 alkene d 0.05 1  106 5  104 O3 terminal C3 and C8 alkenes e 9  106–3  104 2.5  1012 2.3  107  7.5  108 aNotation: g, reactive uptake coefficients; [oxidant]/molecule cm3, average atmospheric concentrations of the gas-phase species; and g  [oxidant], calculated oxidative power. For NO3 radicals, calculations have been performed for both high NO3 concentrations (50 ppt) and low concentrations (5 ppt) to represent 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 coefficient g with increasing NO3 exposure of a UDT SAM. Bottom x axis corresponds to actual time of the experiment (min) (at an NO3 concentration of 1.0  1011 molecule cm3), and top x axis shows the corresponding total NO3 exposure time (hours) of the SAM at an atmospheric concentration of 50 ppt. D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 8 of 14 D02307 within the error limits for the unexposed sample shown in Figure 7. [46] To assess the amount of decomposition of the UDT SAM, we plotted the atomic ratio of the total integrated C(1s) signal to the total Au(4f) signal intensity (C/Au) (Figure 7d). No significant changes of this ratio could be observed within experimental certainty. However, owing to the large uncer- tainties, we can only conclude that less than 21% of the surface volatilized during the longest exposure experiments. As detachment of the carbon chain is also possible owing to oxidation of the sulfur head group, 21% should be considered an upper limit to the amount of carbon loss due to C-C bond scission. [47] Monitoring the S(2p) signal of UDT SAMs after exposure to NO3 showed only unoxidized sulfur (peak max- imum at 163.1 ± 0.2 eV) for exposures 1.2 106 atm sec and both unoxidized sulfur (peakmaximum at 163.1 ± 0.2 eV) and oxidized sulfur (peak maximum at 168.0 ± 0.2 eV) for exposures 9.1  106 atm 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 oxidation chemistry but decomposed during XPS measurements, sim- ilar to the alkane monolayer experiments. To determine if nitrogen species were formed in these reactions, we carried out IR and ToF-SIMS measurements (see below). [49] To investigate the behavior of UDTafter long exposures to NO3 radicals, we also performed a few experiments with large NO3 exposures. After an exposure of 4.5  105 atm sec the monolayers reached oxidized C fractions (Cox/Ctotal) of 0.37%, and an O/C ratio of 0.42% (data not Figure 6. XPS spectra of the C(1s) region for a UDT monolayer (top) before exposure to NO3 (0 atm sec) and after exposures of (middle) 1.2  106 atm sec (equivalent to 50 ppt for 6.7 h) and (bottom) 9.1  105 atm sec (equivalent to 50 ppt for 50 h). Figure 7. Summary of processing study and XPS results for UDT. (a) Changes in g with increasing NO3 exposure. Changes in atomic ratios obtained in XPS as a function of NO3 exposure for (b) atomic ratio of total oxygen, O, to total carbon, C; (c) atomic ratio of oxidized carbon, Cox, to total carbon, Ctotal; and (d) atomic ratio of total carbon, Ctotal, to total gold, Au. The two scales on the top and bottom x axes are valid for Figures 7a7d and provide information on NO3 exposure levels in atm sec (bottom axis) and in total exposure time (hours) at an atmospheric NO3 concentration of 50 ppt (top axis). D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 9 of 14 D02307 shown). These values are similar to the values obtained on ODT monolayers after long NO3 exposures. 3.2.4. IR Spectroscopy [50] First, spectra were taken of an unexposed UDT sam- ple with perdeuterated ODT as reference to confirm the presence of methylene groups in the original UDT SAMs (data not shown). Then IR measurements were performed on samples that had been exposed to NO3. For this analysis we only focused on the IR regions from 3000 to 2800 cm1 and 1370 to 1200 cm1. The wavelength range used in these experiments is slightly smaller than the range used in the alkane monolayers experiments. This is because the signal to noise is smaller in the alkene experiments owing to the low NO3 exposures and smaller degree of oxidation of the films. The spectrum shown in Figure 8 was obtained the same way as Figure 2 for ODT.An exposure level of 1.21 106 atm sec (50 ppt for 6.7 h) was used in this experiment. As with ODT, NO3 exposure of UDT resulted in negative peaks in the range 3000–2800 cm1 due to the decrease of the symmetric and asymmetric stretching modes of CH2. At this short exposure to NO3 we do not expect significant oxidation of the meth- ylene groups in UDT. As peak intensities depend on the orientation of the different vibrational modes [Ulman, 1991], we assume that the negative peaks appearing in the region of 3000–2800 cm1 are due to increased disorder within the monolayer due to reaction of the double bond. This is consistent with results by Moise and Rudich [2001] who observed almost complete disappearance of the CH2 stretch- ing peaks in IR, while XPS indicated only a 20% reduction in total carbon and a significant fraction of remaining unoxidized C(1s). The IR spectra clearly showed the formation of a peak at 1281 cm1. On the basis of Table 1, this could be due to an organonitrate, organoperoxynitrate, or peroxyacylnitrate. It seems unlikely that the identified surface species are organoperoxynitrates or peroxyacylnitrates, since these species typically have a short lifetime and are expected to decompose while the substrates are transferred to the IR spectrometer for analysis, as discussed above. We can con- clude that nitrogen species are formed, contrary to the conclusion one may reach from the XPS data. 3.2.5. ToF-SIMS Measurements [51] Shown in Figure 9 are negative ion ToF-SIMS spectra for UDT before (Figure 9a) and after exposure to NO3 (Figure 9b). As with ODT, spectra before NO3 exposure showed AuxSy clusters, low molecular weight fragments of the monolayer chains (C, CH, C2H , S, SH), and characteristic peaks for the whole UDT molecule. These peaks are labeled M, which indicates C11H21. All peaks characteristic of the unoxidized hydrocarbon SAMs de- creased or disappeared upon NO3 exposure (C , CH, C2H , S, SH, MS, and Au(MS)2 ). New peaks of high intensities formed especially in the m/z region < 100, which could be attributed to O, OH, NO2 , NO3 , SO3 , and HSO4 . As with ODT, peaks for NO2  and NO3  led us to conclude that nitrogen containing functional groups formed during the oxidization of UDT by NO3, but decomposed during XPS measurements. [52] Again, as with ODT, NO3 exposed UDT SAMs showed peaks of SO3  and SO4 . This is consistent with the appearance of a S(2p) peak of oxidized sulfur in XPS spectra of UDT exposed to 9.1  106 atm sec. 3.2.6. Contact Angle Measurements [53] The initial contact angle determined before NO3 exposure was 92 ± 5 for UDT (number given here is the average value and standard deviation). Literature values for monolayers of terminal alkenes (C8–C19) were reported at 89107 [Bain et al., 1989; Bertram et al., 2001;Dubowski et al., 2004; Robinson et al., 1995; Rudich et al., 2000; Thomas et al., 2001; Wasserman et al., 1989]. The large contact angles observed for the unoxidized monolayers indicate the presence of well-ordered SAMs in this study. NO3 exposure of UDT samples was 9.06  105 atm sec (equivalent to 50 ppt NO3 for 3 weeks). After this exposure, the contact angle on UDT had decreased to 59 ± 6, a decrease of 33. This gives further support that the mono- layers were oxidized leading to a more hydrophilic surface. Previous studies investigating changes in H2O contact angles on hydrocarbon monolayers (C8–C19) after exposure to gas- phase reactants reported the following data for terminal alkene monolayers: after F radical exposure q = 75 [Robinson et al., 1995], and after O3 exposure q = 74 and  70 [Dubowski et al., 2004; Thomas et al., 2001]. 3.2.7. Proposed Reaction Mechanism [54] Shown in Figure 10 is the proposed mechanism for oxidation of an alkene monolayer by NO3 radicals in the presence of O2 and NO2 based on gas-phase chemistry [Atkinson, 1991; Atkinson, 1997, 2000; Berndt and Böge, 1995;Gong et al., 2005;Kwok et al., 1996;Noda et al., 2000; Tuazon et al., 1999]. The gas-phase reaction of NO3 with alkenes is assumed to proceed via addition of NO3 to the C-C double bond (Figure 10, step 1). This leads to a C-ONO2 functional group on one of the two C atoms of the double bond and leaves an alkyl radical at the other C atom. Figure 8. UDT IR spectrum calculated from an UDT sam- ple exposed to 1.21 106 atm 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 more prominent in the unexposed UDT SAM; positive peaks show features only present or more prominent in the exposed UDT SAM. D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 10 of 14 D02307 This radical compound is expected to react further with O2 to an alkylperoxy radical (RO2, step 2) [Finlayson-Pitts and Pitts, 2000]. [55] Reactions of the RO2 radical are believed to occur in the same way as those of the RO2 in the alkane reaction scheme (Figure 4), except that the neighboring C atom con- tains an ONO2 functional group. An additional difference is the fact that the scission channel leads to the elimination of NO2 and the formation of two aldehydes (step 9). [56] Not shown in Figure 10 are additional reactions that the alkyl nitrate radical may undergo at low pressures in competition with step 2. When the nitrate radical adds to the double bond, an excited state results. If this excited state is not collisionally stabilized, it can decompose to NO2 and an aldehyde or ketone or alternatively eliminate NO2 and form an epoxide that reacts further to a carbonyl [Atkinson, 1991; Dlugokencky and Howard, 1989]. The importance of these steps on a surface however is unclear. [57] As a reminder, the XPS results are consistent with the formation of C-O, C = O, and C(= O)O functional groups. IR and ToF-SIMS analysis showed the formation of nitrogen- containing species, likely organonitrates. Scission of the carbon chain at the double bond could not be confirmed, but could not be ruled out either. The formation of C-O and C = O functional groups could be explained by step 4, the Russell mechanism. Formation of C-O could also be explained by step 7, and the formation of C = O could also be explained by steps 8 and 9. Carboxylic functional groups determined using XPS were probably formed in secondary reactions, for example, oxidation of aldehydes [George et al., 2007]. [58] Our experiments are the first to investigate heteroge- neous reactions between radicals and alkene monolayers in the presence of O2 and NO2, so our product analysis should be helpful for understanding this heterogeneous mechanism. D’Andrea et al. [2008] investigated the reaction between OH and an alkene monolayer, but in the absence of NO2 and O2, so the results are not directly comparable. In addition, both Hung et al. [2005] andDocherty and Ziemann [2006] studied the reaction between NO3 (in the presence of O2 and NO2) and liquid alkene particles. Hung et al. [2005] observed organonitrate, peroxynitrate, organonitrite, and carbonyl func- tional groups using infrared spectroscopy and high molecular weight products were also observed using LC-MS. Docherty and Ziemann [2006], using mass spectrometry, identified hydroxyl nitrates, carbonyl nitrates, dinitrates, hydroxydini- trates, and possibly more highly nitrated products. It is interesting to note that several of the functional groups identified in these previous liquid reactions were also iden- tified in our current monolayer studies. It therefore appears Figure 9. Negative ion ToF-SIMS spectra of UDT on gold in the m/z 1–600 region. (a) Spectrum was obtained on a UDT sample without NO3 exposure. (b) Spectrum was obtained after an NO3 exposure of 9.1  106 atm s (equivalent to 50 ppt for 50 h). D02307 GROSS AND BERTRAM: HETEROGENEOUS REACTIONS OF NO3 RADICALS 11 of 14 D02307 that the reaction pathways for the alkene monolayers are similar to the reaction pathways for the liquid, although the relative importance of the different channels may differ. [59] Also, when studying the reaction between NO3 and liquid alkene particles,Docherty and Ziemann [2006] observed substantial organonitrate formation after the double bond was completely reacted, which was attributed to either the reac- tion of N2O5 with 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-SIMS confirmed the formation of organonitrates (RONO2). The observation of organonitrates is in contrast to the XPS data by Knopf et al. [2006], which showed very little N(1s) signal. The formation of organonitrates in our experiment may explain why we did not see significant decomposition of the carbon chains in our recent studies with NO3 radicals. Formation of organonitrates removes alkyl peroxy (RO2) and/or alkoxy (RO) species from the system, and hence will reduce the importance of the decomposition channel. Possi- bly this is one of the differences between our work with NO3 and alkane monolayers and some of the recent studies with OH and alkane SAMs where significant decomposition was observed. In the case of alkene SAMs, the g value of the terminal alkene UDTobtained in this study is higher than the values obtained byMoise et al. [2002] using liquid and solid alkene bulk films. A possible 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 oxidation of an alkene monolayer by NO3 can be significantly more impor- tant than oxidation by OH, Cl, and Br in the atmosphere. Only O3 is of equal importance to NO3 owing to its much higher tropospheric concentration. We also used our g value for the alkene SAM to show that the lifetime of an alkene monolayer in the atmosphere will be short (approximately 23 min) assuming an NO3 concentration of 50 ppt. [61] XPS, IR and ToF-SIMS were used to identify surface functional groups after the oxidation of the alkene mono- layers by NO3. The results are consistent with the formation of C-O, aldehyde/ketone, carboxylic groups, and nitrogen species. It is interesting to note that several of the functional groups identified in previous studies of NO3 radicals (in the presence of O2 and NO2) with liquid alkene films were also identified in our current monolayer studies. [62] Acknowledgments. The authors would like to acknowledge the help of D. A. Knopf and J. Mak in the early stages of NO3 and monolayer experiments. K. C. Wong is acknowledged for XPS measurements, D. Bizzotto for assistance with IR, M. J. 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