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Does atmospheric processing of saturated hydrocarbon surfaces by NO3 lead to volatilization? Knopf, Daniel A. 2011

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Does atmospheric processing of saturated hydrocarbon surfaces by NO3 lead to volatilization? D. A. Knopf,1 J. Mak,1 S. Gross,1 and A. K. Bertram1 Received 11 May 2006; revised 12 July 2006; accepted 31 July 2006; published 14 September 2006. [1] The heterogeneous oxidation of a saturated hydrocarbon monolayer by NO3 was studied. A flow tube reactor coupled to chemical ionization mass spectrometry was used to determine the reactive uptake coefficient of NO3 on these surfaces, and X-ray photoelectron spectroscopy (XPS) was used to investigate surface oxidation and to determine if exposure to NO3 leads to volatilization of the organic substrate. The uptake coefficient of NO3 by an alkane monolayer is about (8.8 ± 2.5)  104, which may lead to competitive oxidation compared with OH, due to the higher atmospheric abundance of NO3 under certain conditions. The XPS results are consistent with the formation of 1) C-O groups, 2) ketones or aldehydes, and 3) carboxylic groups. The XPS results also suggest that NO3 does not rapidly volatilize the organic surface: even under extremely polluted conditions, maximum 10% of the organic layer is volatilized. Citation: Knopf, D. A., J. Mak, S. Gross, and A. K. Bertram (2006), Does atmospheric processing of saturated hydrocarbon surfaces by NO3 lead to volatilization?, Geophys. Res. Lett., 33, L17816, doi:10.1029/2006GL026884. 1. Introduction [2] Field measurements have shown that organic material is abundant in the atmosphere, comprising 10–70% of the total fine particulate mass. This organic material can be in the form of pure organic particles, or alternatively the organic material can be mixed with inorganic material. In the latter case the organic material can form organic coat- ings on the surface of aqueous particles [Gill et al., 1983; Ellison et al., 1999], or organic coatings adsorbed on the surface of solid particles, such as mineral dust [Usher et al., 2003]. While in the atmosphere, these organic particles and organic surfaces can be modified by reactions with atmo- spheric oxidants such as O3, NO3, and OH [Ellison et al., 1999; Rudich, 2003]. These heterogeneous reactions are expected to change the hygroscopicity and toxicity of these organic particles and organic surfaces. Due to the potential atmospheric importance of these heterogeneous reactions, they need to be understood and quantified. [3] Over the past few years the heterogeneous oxidation of unsaturated aliphatic particles and substrates have been the focus of many studies [see, e.g., de Gouw and Lovejoy, 1998; Moise and Rudich, 2000; Wadia et al., 2000; Thomas et al., 2001; Smith et al., 2002; Eliason et al., 2003; Thornberry and Abbatt, 2004; Hung et al., 2005; Knopf et al., 2005; Docherty and Ziemann, 2006]. In contrast, there have been considerably fewer studies on the heterogeneous oxidation of saturated aliphatic particles and substrates by atmospheric oxidants [Bertram et al., 2001; Moise and Rudich, 2001; Eliason et al., 2004; Molina et al., 2004], although a large fraction of condensed-phase organic mate- rial in the atmosphere consists of these organic species. In the following we investigate the heterogeneous reactions between NO3 radicals and a self-assembled organic mono- layer of octadecanethiol (C18H38S) on gold. Octadecane- thiol on gold is a methyl-terminated, C-18 alkane monolayer. Here we use this self-assembled organic mono- layer (SAM) as a proxy for organics adsorbed on solid substrates such as mineral dust particles or urban surfaces in the atmosphere [Diamond et al., 2000; Usher et al., 2003]. The SAM surface may also serve as a proxy for solid organic particles in the atmosphere and a rough model for some types of organic coatings on aqueous particles in the atmosphere. Studies with these surfaces also enabled us to probe the reactive processes that are confined to the gas- surface interface, and separate surface and bulk processes [Rudich, 2003]. [4] Two types of experiments were carried out: in the first set of experiments, we used a flow tube reactor coupled to chemical ionization mass spectrometry (CIMS) [Knopf et al., 2005] to determine the reactive uptake coefficient () of NO3 on the saturated hydrocarbon surfaces (the reactive uptake coefficient is defined as the fraction of collisions with the surface that result in a reaction). In the second set of experiments X-ray photoelectron spectroscopy (XPS) was used to investigate surface oxidation and to determine if exposure to NO3 (in the presence of O2) leads to volatilization of the organic substrate. The last set of experi- ments was motivated by a recent study by Molina et al. [2004]. Using atmospherically relevant exposures of OH, these authors showed that OH-initiated oxidation of alkane self-assembled monolayers (in the presence of O2, NOx, and H2O) leads to rapid volatilization of the organic substrate. Moise and Rudich also studied heterogeneous reactions on self-assembled alkane monolayers and observed partial loss of the organic surface after exposure to Cl and Br radicals in the presence of O2 [Moise and Rudich, 2001]. In the gas phase, the mechanisms of the reactions between NO3, OH, Cl, and Br radicals with alkanes are similar (all involve alkyl, alkoxy, and alkylperoxy radicals). However, the NO3 + alkane reaction in the gas phase is slower compared to gas-phase reactions between OH + alkanes and Cl + alkanes. If the NO3 + alkane reaction rate on and in the organic monolayer is significantly enhanced compared to equivalent reactions in the gas phase, it seems reason- able to speculate that NO3 will lead to volatilization of GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L17816, doi:10.1029/2006GL026884, 2006 Click Here for Full Article 1Chemistry Department, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL026884$05.00 L17816 1 of 5 alkane monolayers using atmospherically relevant expo- sures, if OH and Cl lead to volatilization. 2. Experimental [5] A description of the preparation methods of ODT on gold is given by Ishida et al. [1997]. All chemicals were purchased from Aldrich. N2O5 was generated by reacting NO2 with an excess amount of O3 in a flow system as described previously in the literature [Schott and Davidson, 1958]. [6] Measurements of the reactive uptake coefficient were carried out with a cylindrical flow reactor coupled to a chemical ionization mass spectrometer [Knopf et al., 2005]. SAMs were prepared on a gold tube with an inside diameter of about 20 mm, and this tube was located in the flow tube reactor. NO3 radicals were produced by passing N2O5 through a Teflon coated glass oven held at 423 K. At the exit of the flow cell, N2O5 and NO3 were detected using chemical ionization with either I or SF6  as the reagent ion. The NO3 concentrations used in the uptake experiments (as well as NO3 concentrations used in the measurements of the extent of surface oxidation and volatilization) ranged from 2  1011 to 4  1011 molecule cm3, which is higher than typical atmospheric concentrations. Due to experimental constraints it was not possible to use lower NO3 concen- trations in our experiments. Further work is needed to verify that the reactive uptake coefficient and reaction products do not change at lower NO3 concentrations. [7] For the measurements of the extent of surface oxida- tion and volatilization using XPS, SAMs were prepared on gold coated silicone (100) wafers as mentioned above. These surfaces were exposed in the flow tube reactor to NO3 exposures ranging from 0 to 0.093 atm sec/1000 in the presence of O2 (concentrations of approximately 10 16 molecule cm3). XPS measurements were performed using an achromatic Al K X-ray source at a photon energy of 1486.6 eV and using an electron take-off angle of 90. 3. Results and Discussion 3.1. Measurements of the Reactive Uptake Coefficient [8] The reactive uptake coefficient was derived from the first order loss rate of NO3, which was corrected for concentration gradients that form close to the flow-tube wall because of uptake by the organic surface [Knopf et al., 2005]. The gas-phase diffusion constant of NO3 in He was taken as 345 cm2 s1 [Rudich et al., 1996]. Based on our measurements, the reactive uptake coefficient of NO3 by a fresh ODT surface is (8.8 ± 2.5)  104. The experimental conditions were chosen in a way that at most 25% of the methyl groups of the monolayer were oxidized during the uptake measurements. The uptake of NO3 by Teflon is about one order of magnitude slower than the uptake by ODT. [9] Table 1 represents a summary of our reactive uptake measurements as well as previous measurements employing various alkane substrates and atmospherically relevant rad- icals. The uptake of NO3 by the ODT monolayer is slightly faster than the uptake by the frozen organic liquid. This small difference could be due to a difference in surface concentration of reactive sites or may be due to a temper- ature dependence of . Table 1 also shows that the reactivity of Cl, Br, and OH radicals with octadecyltrichlorosilane, another type of C-18 alkane monolayer, is significantly higher compared to the reactivity of NO3 with ODT. [10] In Table 1, we have included atmospheric concen- trations of the various radical species. These values will vary with location and environmental conditions and hence should be considered as approximate numbers. In addition we have included in Table 1   [radical], where [radical] is the approximate atmospheric concentration in units of molecule cm3.   [radical] is a more relevant parameter for assessing the importance of the various radicals to atmospheric oxidation, compared with just , since the number of radicals lost to an organic surface will be proportional to   [radical]. The   [radical] values for NO3 are similar to OH, which suggests that heterogeneous oxidation by NO3 could potentially compete with OH under certain atmospheric conditions. Moise and Rudich [2001] previously reached this conclusion based on their measure- ments with liquid and frozen organic surfaces. 3.2. Studies of Surface Oxidation and Volatilization [11] Figure 1 shows the C(1s) region of the XPS spectra obtained for different NO3 exposures. The increasing shoul- der at higher binding energies within the C(1s) region with increasing NO3 exposure indicates the oxidation of the organic surface. For the unexposed sample only one peak at 285 eV was observed. This peak is due to methyl or Table 1. Reactive Uptake Coefficients, , of Different Atmospheric Radicals by Organic Surfacesa Oxidant Surface  Atmospheric Concentration   [radical]/molecule cm3 NO3 ODT (C18H38S) (8.8 ± 2.5)  104b 50 pptc 1.1  106 liquid n-hexadecane (2.6 ± 0.8)  103d 50 pptc 3.25  106 frozen n-hexadecane (3.8 ± 1.0)  104d 50 pptc 4.75  105 Cl OTS (C18H37Cl3Si) >0.1 e 4  104 pptc 1.0  103 Br OTS (C18H37Cl3Si) (3.0 ± 1.0)  102e 0.04 ppte 3.0  104 OH OTS (C18H37Cl3Si) >0.2 f 0.04 pptc 2.0  105 OTS (C18H37Cl3Si) 0.290.15 +0.71g 0.04 pptc 2.9  105 aIn addition, the product of  and the atmospheric radical concentration ([radical]) is given to assess the importance of the radicals to atmospheric oxidation. (The table has been adapted from Moise and Rudich [2001].) bThis work. cFinlayson-Pitts and Pitts [2000]. dMoise et al. [2002]. eMoise and Rudich [2001]. fMolina et al. [2004]. gBertram et al. [2001]. L17816 KNOPF ET AL.: PROCESSING OF ORGANIC SURFACES BY NO3 L17816 2 of 5 methylene functional groups. After exposure to NO3, the main peak at about 285 eV remains, but a shoulder was observed at higher energies due to the oxidation of the alkane monolayer. We fit the total C(1s) region with four overlapping Gaussian-Lorentzian peaks. In Figure 1b an enlarged view of the C(1s) region obtained with a NO3 exposure of 0.093 atm sec/1000 is shown as well as the Gaussian-Lorentzian fits to the data. Note that using four peaks gave superior fitting results compared to using three peaks. The chemical shifts discussed here were obtained from the fitting procedure of all XPS spectra and are given as average values with standard deviations. The chemical shift of about 1.5 ± 0.5 eV is consistent with the formation of C-O or C-O-O functional groups on the surface [Briggs and Seah, 1990]. The chemical shift at about 3.1 ± 0.6 is consistent with ketones or aldehydes, and the shift at approximately 4.7 ± 0.5 eV is consistent with carboxylic groups [Briggs and Seah, 1990]. [12] To assess the amount of volatilization of the organic surface, we have plotted in Figure 2a the atomic ratio of the total carbon (Ctotal) to the total gold (Au) derived from the integrated C(1s) and Au(4f) intensities and the necessary atomic sensitivity factors. The error bars (which represent ±1) were derived from 4 independent exposure experi- ments of ODT using equal NO3 concentrations. The vertical line in Figure 2 indicates an exposure of 0.0302 atm sec/ 1000, which is equivalent to exposing the surface to an NO3 concentration of 50 ppt NO3 (24 hour average) for one week. A linear fit to the data shown in Figure 2a gives a maximum carbon loss of about 7.5% (within 95% of confidence) for NO3 exposures of 0.0302 atm sec/1000. Based on this, we suggest that NO3 does not lead to rapid volatilization of organic monolayers under atmospheric conditions. [13] To determine the fraction of the carbon oxidized as a function of time, we calculated the atomic ratio of the oxidized carbon (Cox) to Ctotal, both determined from the fits shown in Figure 1a. These ratios are plotted in Figure 2b. Figure 2b suggests that approximately 17% of the alkyl chain is oxidized for NO3 exposures of 0.0302 atm sec/ 1000. It should be noted that exposure of the organic layer solely to O2 concentrations of about 10 16 molecule cm3 did not lead to oxidation of the monolayer. [14] From the XPS data, we also calculated the atomic ratio of total oxygen (O) to Ctotal as a function of NO3 exposure, determined from the integrated O(1s) and C(1s) intensities and the respective atomic sensitivity factors. The results of these calculations are plotted in Figure 2c and suggest that the atomic O/Ctotal ratio is approximately 0.2 after an exposure of 0.0302 atm sec/1000. Both Figures 2b and 2c show that NO3 leads to oxidation of the organic surface using atmospherically relevant exposures. Figure 1. (a) XPS spectra of the C(1s) region for an ODT monolayer exposed to NO3 concentrations of 0–0.093 atm sec/1000. Also included are the Gaussian-Lorentzian peaks from the fitting procedure. The spectra are shifted vertically for better visibility. (b) An enlarged view of the best fit of the C(1s) region for an ODT monolayer exposed to NO3 concentrations of 0.093 atm sec/1000. Figure 2. A summary of the XPS results. (a) The atomic ratio of the total carbon, Ctotal, to total gold, Au. The solid line is a linear fit to the data, and the dotted lines correspond to the 95% confidence limit. (b) The atomic ratio of oxidized carbon, Cox, to Ctotal determined by fitting the C(1s) spectrum with Gaussian-Lorentzian peaks (see text for further details). (c) The atomic ratio of total oxygen, O, to Ctotal as a function of NO3 exposure. The dashed line indicates atmospheric NO3 concentrations of 50 ppt (24 hour average) persisting for one week. L17816 KNOPF ET AL.: PROCESSING OF ORGANIC SURFACES BY NO3 L17816 3 of 5 [15] In addition to monitoring the C(1s) and O(1s) signals, we also monitored the N(1s) signal; however, the XPS measurements only show a small increase in the N(1s) signal. About 1% of the total surface elemental composition has been assigned as nitrogen based on the N(1s) signal at NO3 exposures higher than 0.032 atm sec/1000. The bind- ing energy of the N(1s) signal could be attributed to a nitrate group. We have also monitored the S(2p) signal from the monolayer. At exposures less than 0.03 atm sec/1000, the sulfur is not oxidized. At long exposures of approximately 0.08 atm sec/1000, the sulfur is oxidized to sulfonate due to reactions with NO3. This, however, can account for only a small amount of the O(1s) signal in the XPS spectrum. 3.3. Oxidation Mechanism [16] Shown in Figure 3 is the proposed mechanism for oxidation of an alkane monolayer by NO3 radicals based on gas-phase chemistry. We assume that the same reactions can occur on the SAM as in the gas-phase, although the relative importance of the pathways can be significantly different on and in the organic monolayer compared to the gas-phase. We use the XPS data to speculate on the importance of the different reaction pathways for the saturated hydrocarbon surfaces. [17] The initial reaction step in the oxidation process is the abstraction of a hydrogen atom from a methyl or methylene group of the alkyl chain to form HNO3 and an alkyl radical. Our experiments were not optimized to measure HNO3, and as a result we were not able to determine if this product remained on the surface or evaporated during NO3 exposures. The XPS measurements indicated that only 1% of the total surface elemental composition after exposure to NO3 was due to nitrogen. However, the HNO3 could have evaporated during the XPS measurements. The second reaction step is the transforma- tion of an alkyl radical into a peroxy radical in the presence of O2 [Atkinson, 1997]. In the presence of NO2, peroxyni- trates may form (which may subsequently thermally de- compose) [Atkinson, 1997]. The peroxy radical can also react with NO3 to form an alkoxy radical, NO2, and O2. Alternatively the peroxy radical can undergo self reaction, leading to the formation of alkoxy radicals and O2 or an alcohol and carbonyl [Atkinson, 1997]. In the gas phase, the branching ratio for the alcohol + carbonyl channel ranges from about 0.3 to 0.8 for primary and secondary radicals [Atkinson, 1997]. In the solution phase, however, the self reaction appears to proceed only through the alcohol + carbonyl channel for both primary and secondary radicals [Russell, 1957]. For tertiary radicals the alcohol + carbonyl channel is not accessible [Russell, 1957; Atkinson, 1997]. If the alkoxy radical forms, it can decompose by scission of a C-C bond, or can undergo isomerization to form a hydrox- yalkyl radical. Alternatively the alkoxy radical can react with O2 or NO2 to form ketones and aldehydes or alkylni- trates [Atkinson, 1997]. [18] The XPS data suggests that the C-C scission channel was of minor importance compared to other reaction chan- nels. Also, the XPS data show that the surface contained only a small amount of nitrogen after exposure. This suggests that the formation of peroxynitrates or alkynitrates were not important under our conditions. On the other hand, these nitrates may have formed on the surface and subse- quently thermally decomposed during the XPS measure- ments. In a future study IR spectroscopy will be used to further explore the possibility that nitrates resulted from NO3-initiated oxidation. [19] As mentioned above the C(1s) spectrum is consistent with the formation of 1) C-O groups, 2) ketones or alde- hydes, and 3) carboxylic groups. The presence of C-O, ketones, and aldehydes, can be explained by the RO2 self reaction leading to an alcohol and carbonyl (see Figure 3). As mentioned above the branching ratio for the alcohol + carbonyl channel ranges from about 0.3 to 0.8 in the gas- phase, but in the solution phase the RO2 self reaction appears to proceed only through the alcohol + carbonyl channel. Perhaps in our experiments, the self reaction also proceeds mainly through the alcohol + carbonyl channel. The presence of C-O, ketones, and aldehydes, could also be explained by the formation of an alkoxy radical followed by isomerization to form hydroxyalkyl radicals and reaction with O2 to form carbonyls. In terms of isomerization, alkoxy radicals could abstract a hydrogen from a neighbor- ing carbon chain to form C-OH groups. [20] The presence of a carboxylic group may be due to the formation of an aldehyde that forms at the terminal carbon followed by reactions with NO3, O2, and NO2 to form a peroxyacetylnitrate (PAN) [Finlayson-Pitts and Pitts, 2000], which then decomposes during the XPS measurements to form carboxylic groups. Further work is needed to explore this possibility. This may be especially important as PAN is a toxic species. [21] The XPS results and mechanism presented above suggest that the reactive sites in and on the organic monolayer will decrease with time, and hence the reactive uptake coefficient will decrease with exposure time. Pre- liminary measurements in our laboratory have confirmed this trend. In the future we will carry out detailed measure- ments of the uptake coefficient as a function of exposure. [22] We observed at most limited volatilization of organic monolayers when employing NO3, while Molina et al. [2004] observed complete volatilization when using OH radicals. The reason for this difference is not clear. In our experiments, we assume that HNO3 was produced in the initial step of the reaction. It is possible that this species remains on the surface and somehow influences the reaction mechanism. Another issue is possible contamination of the Figure 3. Proposed oxidation mechanism of alkane surfaces by NO3 in presence of NO2 and O2 and based on gas-phase chemistry. L17816 KNOPF ET AL.: PROCESSING OF ORGANIC SURFACES BY NO3 L17816 4 of 5 monolayer by hydrocarbons from the ambient laboratory. For the unoxidized monolayers, the C/Au ratio is in the range expected for C18 monolayers, and only one peak was observed in the C (1s) region, due to methyl or methylene groups. This suggests that contamination of the unoxidized monolayers was minor. Also, it has previously been shown that alkanethiol monolayers replace organic contaminates from gold substrates [Ishida et al., 1996]. We have also carried out measurements with a C11 alcohol terminated monolayer, and the observed C/Au ratios and O/C ratios agreed with theoretical predictions within experimental uncertainty, which suggests that contamination even for a more hydrophilic monolayer is minor. However, contami- nation cannot be completely ruled out. Moise and Rudich observed partial volatilization (approximately 20%) of the saturated hydrocarbon monolayer when using Cl and Br radicals [Moise and Rudich, 2001]. Perhaps the presence of HNO3 can also explain the difference between NO3, Cl, and Br. 4. Summary and Conclusions [23] The results suggest that even under extremely polluted conditions, i.e. NO3 concentrations of 100 ppt (24 hour average) persisting for one week [Finlayson-Pitts and Pitts, 2000], maximum 10% of the organic monolayer is volatil- ized. Our results are relevant to organics adsorbed on solid substrates such as mineral dust particles or urban surfaces in the atmosphere. Our results may also be relevant for solid organic particles in the atmosphere and for organic coatings on aqueous particles. In terms of liquid organic aerosols, there have only been a few studies that have investigated reactions between liquid organic aerosols and films and atmospher- ically relevant radicals [Eliason et al., 2004; Hung et al., 2005; Docherty and Ziemann, 2006]. The recent studies by Hung et al. [2005] and Docherty and Ziemann [2006] appear to suggest that decomposition and volatilization by radical-initiated oxidation may not be important for un- saturated and saturated liquid organic particles in low NOx environments. (See the work by Docherty and Ziemann [2006] for a full discussion on this topic.) More work is still needed, however, to completely understand radical-initiated oxidation of atmospherically relevant par- ticles and films. [24] Acknowledgments. The authors thank J. Thornton, I. Suh, R. Atkinson, and P. J. 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