Article pubs.acs.org/JPCA
Pressure-Dependent Criegee Intermediate Stabilization from Alkene Ozonolysis Jani P. Hakala†,‡ and Neil M. Donahue*,† †
Center for Atmospheric Particle Studies, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Gustaf Hällströminkatu 2 A, 00560 Helsinki, Finland
‡
ABSTRACT: We explored the pressure dependence of acetone oxide (stabilized Criegee Intermediate, sCI) formation from 2,3-dimethyl-2-butene ozonolysis between 50 and 900 Torr using a new, highly accurate technique. We exploited the ability of the sCI to oxidize SO2 to H2SO4, which we measured with a chemical ionization mass spectrometer. We produced the Criegee intermediates (CI) in a high-pressure flow reactor via ozonolysis of 2,3-dimethyl-2-butene (tetramethyl ethylene, TME) and measured the relative H2SO4 concentrations with and without an added OH scavenger. Because the TME reaction with ozone forms acetone oxide (a syn-CI) with unit efficiency, we directly calculated the sCI yields at different pressures from the precisely measured ratio of the uncalibrated H2SO4 signal with and without the scavenger. We observed a linear pressure dependence between 50 and 900 Torr with a minimum stabilization of 12.7 ± 0.6% at 50 Torr and a maximum stabilization of 42 ± 2% at 900 Torr. A linear fit to the measured data points shows a zero-pressure intercept of 15 ± 2%, constraining the fraction of CI formed below the barrier for acetone oxide isomerization.
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INTRODUCTION
organic compounds (ELVOC), which are precursors for secondary organic aerosols (SOA).8 There are two major CI conformers: in syn-CIs, the terminal carbonyl-oxide oxygen atom faces an organic substituent, while in anti-CIs, the terminal oxygen faces a H atom.9 Alkene ozonolysis is highly exothermic; thus, in the gas phase, CIs are formed with significant excess energy.9−11 They can either immediately isomerize or stabilize. Stabilization occurs either when the CI is formed with internal energy below the isomerization energy barrier or via collisions with a third body. If not stabilized, the syn conformer forms a vinyl hydroperoxide (VHP) which, if not also stabilized, rapidly decomposes to produce an OH radical.9,12,13 At room temperature, stabilized syn-CIs will also thermally decompose rapidly to produce VHP and then OH.9,14 The anti conformer forms a dioxirane, which has a more complex decomposition pathway.15 Collisional stabilization makes sCI formation dependent on pressure and the properties of the colliding molecules. We used N2 carrier gas
Ozonolysis is a major pathway for the removal of atmospheric alkenes, which are the most abundant organic compounds in the atmosphere after methane.1 Biogenic alkenes include isoprene and a vast selection of different terpenes.2 Anthropogenic alkene sources include biomass burning, transportation, and industry.3 Alkene ozonolysis produces a range of reactive species, including hydroxyl radical (OH), hydroperoxides, and Criegee intermediates (CI), or carbonyl-oxides. The mechanism for condensed-phase alkene ozonolysis was studied in depth by Rudolph Criegee,4 but the importance of stabilized Criegee intermediates (sCIs) as atmospheric oxidants has been realized only recently.5 Production of gas-phase H2SO4 is especially important in the atmosphere, and sCIs can readily oxidize SO2 to form H2SO4.5 H2SO4 has a very low vapor pressure and a high hygroscopicity, and it is one of the most important contributors to atmospheric nucleation and particle formation.6,7 In some cases, sCIs may drive up to 50% of the gas-phase H2SO4 production in the atmosphere.5 Alkene ozonolysis through the Criegee pathway is also an extremely important source of extremely low volatile © XXXX American Chemical Society
Received: February 14, 2016 Revised: March 24, 2016
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DOI: 10.1021/acs.jpca.6b01538 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Scheme 1. TME Ozonolysis and Formation of H2SO4 in the Presence of SO2 and Watera
a
Abbreviations: POZ is the primary ozonide, CI is the Criegee intermediate, and sCI is the stabilized CI. VHP and sVHP are the vinyl hydroperoxide and stabilized VHP, respectively.
YsCI+OH = 1.0,9,13,17 and thus the yield of sCI can be written as in eq 3:
because it adequately mimics the properties of colliding molecules in the atmosphere. Because the reactions are highly exothermic, direct gas-phase measurements of sCIs produced by alkene ozonolysis have only recently been reported.16 Because the stabilization process and overall gas-phase production of H2SO4 are important to atmospheric chemistry, we set up a flow system focusing on pressure-dependent stabilization following ozonolysis. We used SO2 oxidation by either OH radicals or sCI produced by alkene ozonolysis to probe oxidant production. SO2 oxidation produces H2SO4 in the presence of oxygen and water vapor, and we measured the resulting H2SO4 with a chemical ionization mass spectrometer (CIMS). By adding an OH scavenger, we separated H2SO4 production via sCI from the total H2SO4 production and assigned the sCI yield based on the total (thermal + prompt) OH yield.9 We present the formation and fate of the CI by TME ozonolysis in the presence of SO2 and water in Scheme 1.
[H 2SO4 ]OH + sCI = [OH] + [sCI]
(1)
[H 2SO4 ]sCI = [sCI]
(2)
YsCI = [sCI]/([OH] + [sCI])
(3)
We carried out these measurements in the pressure-controlled Carnegie Mellon University high-pressure flow system (HPFS).18,19 We present a schematic of the HPFS in Figure 1. In the HPFS, we mixed into an N2 carrier gas 50 ppm SO2, 550 ppm water, and a minute amount (less than 100 ppb) of TME. We then injected a small amount (less than 500 ppb) of ozone mixed in N2 into this gas mixture. Although we did not measure the exact concentrations of TME and ozone, their concentrations were minimal compared to those of the other reactants, and thus they did not compete as sinks for OH or sCI. We measured the resulting H2SO4 with a chemical ionization mass spectrometer (CIMS, THS Instruments LLC). The extremely high sensitivity of the CIMS to H2SO4 (detection limit ∼105 cm−3 at atmospheric pressure) allowed us to operate at very low reagent concentrations and thus minimize secondary chemistry. We modified the CIMS to operate at a pressure range from 50 to 900 Torr by making a series of six pinhole plates between the ion source/CI inlet and the mass spectrometer. The pinholes had diameters of 0.005, 0.006, 0.008, 0.010, 0.013, and 0.020 in. We selected the pinhole sizes to give the same sample flow as with the default pinhole plate at atmospheric pressure. We also set the sheath, sample, and HNO3 reactant so
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EXPERIMENTAL METHODS We measured the pressure dependence of sCI production indirectly by measuring the H2SO4 produced from SO2 oxidation by both OH and sCI and by sCI only, when OH was scavenged by propane. If we use sufficient SO2, the sCI and OH are completely consumed by the reaction (via titration). If we use sufficient propane, all of the OH is scavenged. The completeness of the titration and OH scavenging was tested by doubling the amount on SO2 and propane; no change was observed in measured sCI yields. Here, we focused on tetramethyl ethylene ozonolysis. TME reacts exclusively via the syn-CI pathway (via symmetrical acetone oxide). Thus, we can assume that B
DOI: 10.1021/acs.jpca.6b01538 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 1. CMU high-pressure flow system (HPFS) for Criegee intermediate pressure stabilization measurements using nitrate-ion chemical ionization mass spectrometry measurements of H2SO4.
Figure 2. Measurement cycle for sCI determination at 760 Torr. The four stages are the signal background (with ozone and SO2 present), the signal with an added OH scavenger (propane), the signal with an added reagent (tetramethyl ethylene, TME), and the signal with the OH scavenger removed.
pressure. We present an example of the H2SO4 signal measured by the CIMS in Figure 2. We normalized the H2SO4 signal with the reagent ion (NO3) signal and calculated the sCI yield from the ratio of the average sCI and sCI + OH signals after correcting for the corresponding background signals (propane and background, respectively): YsCI = (sCI − propane)/(sCI + OH − background).
that the residence time in the drift tube section of the ion source was the same as for the operation with default settings at atmospheric pressure. The alkene injection system shown in Figure 1 contains TME in a vial with a small flow of nitrogen over the headspace. We injected TME as vapor from this headspace, taking special care to get low enough concentrations in the flow tube for TME to not act as a competing sink for OH or the sCI but high enough concentrations to get good results. One measurement cycle consisted of background, scavenger only (propane), sCI, and sCI + OH measurements. The background measurement gas mix consisted of N2 carrier gas, ozone, water, and SO2; for the scavenger measurement, we mixed 500 ppm propane into the carrier-gas flow. We then added TME to measure the sCI. Finally, to measure the sum sCI + OH, we turned off the OH scavenger. We set the gas flows so that the residence time in the flow system after the ozone injection was 9 s at each
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RESULTS AND DISCUSSION We present sCI yields from TME ozonolysis as a function of pressure ranging from 50 to 900 Torr in Figure 3. We observed a linear pressure dependence for the sCI yields down to 100 Torr, below which the pressure dependence deviates from linearity toward lower yields. We calculated the error bars in Figure 3 from the standard deviations of the mean values in eq 3. The results show significant stabilization of the CI at each measured pressure. The highest sCI yield was 0.42 ± 0.02 at C
DOI: 10.1021/acs.jpca.6b01538 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 3. Pressure dependence of sCI formation from TME ozonolysis. The colors of the circles indicate different pinhole diameters. A linear fit to all data gives a zero-pressure intercept of 15 ± 2%. The data from Drozd et al.,18 using the same flow system but FTIR measurement of the reaction products, show systematically higher yields, while two measurements from IFT Leipzig (Berndt et al.21,21) fall near each of the two pressure-dependent data sets at 760 Torr. The lower IFT data point was obtained using a method similar to the one used herein.
900 Torr, and the lowest sCI yield was 0.127 ± 0.06 at 50 Torr. We also explored the suitable pressure range over which each pinhole can be used. In Figure 3, each circle color identifies the pinhole diameter. The pinholes we used above 200 Torr have a relatively wide usable range. Drozd et al.18 measured the pressure dependence of sCI formation from TME ozonolysis by observing the production of a secondary ozonide using Fourier transform infrared spectroscopy (FTIR). We modified the flow reactor used in that study to better suit the CIMS measurements, but the essential features of the two flow systems were the same. As can be seen in Figure 3, the results differ significantly, even though the earlier results have large uncertainty. However, both studies show a linear pressure dependence with slopes differing by a factor of roughly 2 with very similar intercepts. There are many possible explanations for the difference in the results. First, the methods were different. Drozd et al.18 calculated the sCI yield from the depletion of a selective sCI scavenger, hexafluoroacetone (HFA, 1,1,1,3,3,3-hexafluoro-2-propanone), and the corresponding production of a secondary ozonide (SOZ). The quoted yields were based on the ratio of SOZ production to ozone removal as the reagent (TME) was modulated on and off.18 Determining relative concentration changes from FTIR spectra is much more complicated and prone to uncertainty compared to our method, and the difference between the results could easily be due to the difficulties in the calibration of the FTIR. A second explanation is potential secondary chemistry. To bias the FTIR yield determinations high, the flow tube would need to have an unaccounted secondary source of ozone or an unaccounted source of SOZ. Finally, we note that the apparent linear pressure dependence up to nearly 70% CI stabilization in the Drozd et al.18 results is puzzling from a theoretical perspective, as the general expectation for a broad energy distribution in nascent CI would be a very broad pressure falloff curve. Thus, the linear or slightly sublinear pressure dependence we show in Figure 3 is at first blush easier to explain. Berndt et al.20,21 also measured the sCI yield from TME ozonolysis, but only at atmospheric pressure. As in this study, they also measured the H2SO4 produced via SO2 oxidation by the sCI with a CIMS, using two slightly different techniques. In their first study, they let TME react with ozone in a laminar flow tube in the presence of propane as an OH scavenger. They injected SO2 at the outlet of the laminar flow tube, using the steady-state sCI concentration to determine the sCI yield in a manner
similar to that employed by Donahue et al.22 and Kroll et al.9,23 to measure OH yields. They measured the H2SO4 produced as a function of SO2 and reaction time after SO2 injection, and from the measured H2SO4 concentration they determined the sCI yield along with other kinetic parameters associated with the sCI steady state.20 The resulting yield was 0.62 ± 0.28, which is very close to what Drozd et al.18 measured at atmospheric pressure (see Figure 3). In their second study, Berndt et al.21 measured the sCI yield from TME ozonolysis in the same way as conducted herein. Instead of adding SO2 at the outlet of the laminar flow tube, they let it react in the flow tube itself and measured the ratio of H2SO4 produced with and without propane used as an OH scavenger. They obtained almost exactly the same ratio at atmospheric pressure as found in this study: 0.389 ± 0.002, compared to ours at 0.373 ± 0.014. They calculated the sCI yield to be 0.45 ± 0.20 after accounting for possible sCI losses, but this led to a sCI + OH yield of 1.20 ± 0.54. This suggests that the real yield may be closer to the measured [H2SO4]sCI/[H2SO4]sCI+OH ratio. The differences in the measured sCI yields of the two flow systems (CMU and IFT) highlight intrinsic uncertainties associated with indirect yield determinations. Three of the measurements (Drozd et al.,18 Berndt et al.,21 and this study) are titrations, where the experimental design is to overwhelm the system with a scavenger and thus intercept all of the short-lived intermediate (in this case the sCI). The determination is then based on measured stoichiometry, generally the scavenger production vs loss of a limiting reagent (the alkene or ozone). One is a steady-state determination based on the production rate rather than loss of a limiting reagent. Given the complexity of the system, an agreement to a factor of better than 2 is tolerable, especially considering the high uncertainty associated with the FTIR measurements. Two of the titrations (Berndt et al.21 and ours) relied on yield ratios, specifically the sum of the sCI and OH yields vs the sCI yield alone. Overall, we regard this approach as the most accurate because it exploits the high precision of CIMS H2SO4 measurement without requiring an absolute calibration or an accurate determination of the precursor concentrations. For this test system (TME + ozone and acetone oxide), there are strong indications that the overall yield of sCI and OH is unity;9,19,24,25 therefore, we regard the yield ratio as an accurate measure of the (pressure-dependent) acetone oxide stabilization. D
DOI: 10.1021/acs.jpca.6b01538 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A The data in Figure 3 are very nearly linear in pressure with the obvious exception of the two lowest pressure points. Including those points, the data are suggestive of a modest falloff behavior. For every case except the two lowest pressure points, we were able to obtain data with two different pinholes at overlapping pressures, and the agreement was always within the measurement precision. We have no reason to suspect that the largest pinhole behaved any differently; however, with high diffusive losses and low absolute concentrations at low pressure, we cannot rule out the possibility that these points are systematically biased low. A linear fit to all of the data gives an intercept of 15 ± 2%, though given the falloff at the lowest measured pressures, the intercept could be as low as 10%. The zero-pressure intercept of the CI stabilization is an important constraint for reaction dynamics calculations because it indicates the fraction of CI that are formed with insufficient energy to pass over any unimolecular decomposition barrier (in this case, the acetone oxide to vinyl hydroperoxide isomerization17). This value is difficult to predict theoretically because energy is deposited in three reservoirs when the primary ozonide formed from TME + ozone decomposes: the acetone oxide, an acetone coproduct, and external translation and rotation (external modes).9 Especially because of the high POZ cycloreversion barrier, the energy distribution among these three reservoirs is probably not statistically significant. Direct measurements of pressure-dependent OH yields have given hits of this intercept as the difference between a large measured yield and the inferred zero-pressure limit, which is intrinsically uncertain.23 The earlier measurements from the CMU flow tube using FTIR provided a direct measurement of the zero-pressure limit, but with low accuracy. Given the high degree of chemical activation and the multiple wells in the alkene ozonolysis system, it is possible that the pressure dependence of sCI production has an unexpectedly complicated functional form. For example, stabilization into a succession of wells could produce a falloff curve with multiple pressure regimes. However, for this system, we do not expect any significant stabilization in the POZ,11 and the sCI is the next well in the reaction sequence. The modest pressure dependence we observed for sCI formation in conjunction with the very strong pressure dependence for prompt OH formation (measured via LIF23) strongly suggests that further stabilization follows the acetone oxide on the reaction coordinate, ultimately yielding OH, and the only plausible candidate for that stabilization is vinyl hydroperoxide.12,13 While developing our technique, we first operated the CIMS chemical ionization inlet at a constant pressure of 50 Torr and placed a pinhole between the flow tube and the CIMS inlet instead of between the chemical ionization inlet and the mass spectrometer. With this approach, we had to use much higher precursor concentrations (∼100 times more TME and ozone); they were sufficient to trigger particle formation. The particles acted as a sink for H2SO4. In fact, it seemed that H2SO4 was not detected at all unless it was carried into the chemical ionization inlet in the particle phase. This led to nonsensical apparent sCI pressure dependence, which was highly dependent on the precursor concentrations, especially with respect to water concentration. The effect of increased water concentration is twofold. Water vapor enhanced nucleation directly. Also, by adding more water, we increased the amount of H2SO 4 produced, because water is needed to turn SO3 made by SO2 oxidation into H2SO4. Thus, by adding water, we increased both of the components needed for water-H2SO4 nucleation.6,7
After we developed the technique described in this work, we held the precursor concentrations so low that we found no evidence of nucleation, and the measured sCI yields were not dependent on water concentration.
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CONCLUSIONS We have described a new pressure-dependent flow system to measure the pressure dependence of Criegee intermediate stabilization. We exploited the high sensitivity and precision of chemical ionization mass spectrometry for H2SO4 measurements to obtain pressure falloff curves with unprecedented precision and demonstrated the system using the well-studied and highly symmetrical reaction of ozone with tetramethyl ethylene (TME). The sCI yields depend nearly linearly on pressure, rising from a zero-pressure limit of 15 ± 2% to 37.3 ± 1.4% at atmospheric pressure, though they are also consistent with a modest falloff curvature and somewhat lower zero-pressure intercept.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a grant from NASA (NNX12AE54G) with instrumentation provided by an NSF MRI grant (CBET0922643) and Academy of Finland Centre of Excellence Grants 1118615 and 272041.
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REFERENCES
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