Hydroxyl Radical Quantum Yields from Isopropyl Nitrite Photolysis in

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Environ. Sci. Technol. 2010, 44, 8150–8155

is transferred to the O-N bond. The lifetime of the S1 state is 125 fs (9), too short for collisional quenching to occur at 1 atm pressure. In the case of photolysis of pure isopropyl nitrite, the isopropoxy radical formed in reaction 2 can decompose via C-C bond scission to form acetaldehyde and a methyl radical (reaction 3) (11, 12, 14) or react with NO formed in the photodissociation to regenerate i-C3H7ONO (11, 14-16):

Hydroxyl Radical Quantum Yields from Isopropyl Nitrite Photolysis in Air JONATHAN D. RAFF AND BARBARA J. FINLAYSON-PITTS* Department of Chemistry, University of California, Irvine, California 92697-2025

Received July 1, 2010. Revised manuscript received September 6, 2010. Accepted September 9, 2010.

Alkyl nitrites photolyze in air to yield alkoxy radicals and NO which, through secondary reactions, generate OH radicals. This photochemistry is important in the atmosphere and in laboratory studies where nitrites are often used as a source of OH. The overall quantum yield for hydroxyl radical formation from irradiation of isopropyl nitrite (i-C3H7ONO) between 300 and 425 nm in 1 atm air at 296 ( 2 K is reported for the first time. The OH radical was scavenged by reaction with CF3CFdCF2 and the formation of CF3CFO and CF2O monitored as a function of time using Fourier transform infrared spectrometry. The quantum yield was found to be 0.54 ( 0.07 (2σ) and is independent of whether or not NO was added (up to 3 × 1014 molecules cm-3) prior to photolysis to increase NO concentrations above those due to the photolysis of the nitrite. Ultraviolet-visible and infrared cross sections of i-C3H7ONO are also reported. These data on the OH quantum yields as well as the UV-visible and infrared cross sections for isopropyl nitrite are critical for quantitatively interpreting the results of laboratory studies where i-C3H7ONO is employed as an OH source as well as for assessing the role of alkyl nitrites in the chemistry of the troposphere.

Introduction Nitrites (RONO, where R ) H, or an alkyl group) have been used to probe the fundamental mechanisms and bondbreaking processes that occur during the photodissociation of polyatomic molecules. For the most extensively studied molecules in this class, HONO (1-3) and CH3ONO (4-10), the photodissociation dynamics of the primary processes have been explored in detail. Near-UV excitation of alkyl nitrites, including isopropyl nitrite, occurs via an n f π* transition that leads to photodissociation into alkoxy radical and NO (4, 5, 8-12): i-C3H7ONO (S0) + hν (λ ) 300-450 nm) f i-C3H7ONO (S1) i-C3H7ONO (S1) f i-C3H7O + NO

(1) (2)

The S1 state is localized on the NdO group and possesses a shallow potential well with associated vibrational states that are responsible for the vibrational structure in the nearUV spectrum (Figure 1) (6, 7, 13). Excitation to the S1 state results in several vibrations of the NdO bond before energy * Corresponding author phone: (949) 824-7670; fax: (949) 8242420; e-mail: [email protected]. 8150

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i-C3H7O f CH3CHO + CH3

(3)

i-C3H7O + NO f i-C3H7ONO

(4)

Previous studies of the photolysis of i-C3H7ONO alone or with added gases such as NO or CO2 reported primary quantum yields (i.e., the fraction of excited isopropyl nitrite that decomposes to i-C3H7O and NO) between 0.31 and 0.86 for photolysis in the 254-400 nm region (4, 11, 12). Less attention has been paid to the secondary reactions that occur when alkyl nitrites are photolyzed in air and at the higher pressures commonly encountered in combustion or atmospheric chemistry studies. In this case, the alkoxy radicals can also react with oxygen and in the presence of nitric oxide (NO) ultimately form the hydroxyl radical (OH): i-C3H7O + O2 f CH3C(O)CH3 + HO2

(5)

HO2 + NO f OH + NO2

(6)

In addition to this source of OH in this system, a minor channel that directly produces OH has been reported (10, 17). The chemistry of alkyl nitrites is important not only in the atmosphere but also because they are convenient sources of OH radicals for kinetics and mechanistic studies. For example, photolysis of CH3ONO in the presence of NO has been used extensively as a source of OH in laboratory studies (18-20). However, there are some limitations to this OH source. First, methyl nitrite is not commercially available and hence must be synthesized. Second, the formaldehyde generated as a product itself reacts rapidly with OH (kOH ) 8.5 × 10-12 cm3 molecule-1 s-1 at 298 K) (21), reducing the steady-state concentration of OH radicals available for reaction. This limits the types of compounds that can be studied to those with rate constants greater than ∼10-13 cm3 molecule-1 s-1. Finally, the presence of formaldehyde and its OH reaction product, formic acid, may interfere with the determination of other products with overlapping absorptions when Fourier transform infrared (FTIR) spectroscopy is used to follow the chemistry. Isopropyl nitrite and its perfluorinated (22) and perdeuterated analogs are alternatives to CH3ONO, since their byproducts (acetone, hexafluoroacetone, and acetone-d6) react relatively slowly with OH radical. Isopropyl nitrite is also commercially available, easy to handle, and, if desired, is readily synthesized by the reaction of 2-propanol and HONO in solution (23, 24). Although photodissociation of isopropyl nitrite has been used as a source of OH (25-28) in a number of laboratory studies, its efficiency, i.e., the overall quantum yield for OH production, has not been reported. These data are essential for quantification in studies where isopropyl nitrite is used as an OH source and also sheds light onto the atmospheric reactions of alkyl nitrites. The goal of this work is to determine the overall quantum yield for OH production from the photodissociation of i-C3H7ONO in air for use in quantifying OH radical concentrations in laboratory environmental chamber studies. This 10.1021/es102218d

 2010 American Chemical Society

Published on Web 09/29/2010

products due to losses to chamber walls; no losses were evident within the time frame of a typical experiment. Calculation of the Overall OH Quantum Yield. The rate of formation of OH radicals from i-C3H7ONO photolysis was determined using a scavenger technique whereby the OH generated reacts with a large excess of the scavenger molecule hexafluoropropene (CF3CFdCF2) to form trifluoroacetyl fluoride (CF3CFO) and carbonyl fluoride (CF2O) (30, 31). Hexafluoropropene (99.5%) was obtained from Matheson Tri-Gas and used without further purification. With NO (which is always present due to the photodissociation of the nitrite, reaction 2), the mechanism consists of multiple steps involving various alkoxy and peroxy radical intermediates (31) that are summarized in the Supporting Information. The overall quantum yield for OH formation (ΦOH) is derived using FIGURE 1. Ultraviolet-visible spectrum of isopropyl nitrite vapor (black line) at 296 K and the emission spectrum of the black lamp (cross-hatched area, arbitrary units) used in the photolysis experiments. is done by measuring the rates of product formation when OH is scavenged by reaction with CF3CFdCF2. Hexafluoropropene is, in principle, an ideal scavenger because (1) it reacts rapidly with OH [kOH ) (2.1 ( 0.2) × 10-12 cm-3 s-1 at 296 K] (29), (2) its reaction mechanisms are wellcharacterized (30, 31), (3) it does not absorb the light used to photolyze i-C3H7ONO, (4) it is commercially available and relatively safe to use, (5) the products CF3CFO and COF2 are formed quantitatively, and (6) the products do not photolyze or react appreciably with OH radical under the conditions of the experiment, obviating the need for corrections for secondary reactions. In addition, we present measurements of the UV-visible and infrared absorption cross sections of i-C3H7ONO, which are also useful for its quantification in laboratory studies.

Experimental Section UV-Visible Spectrum of Isopropyl Nitrite. Absorption cross sections of i-C3H7ONO in the 250-500 nm region were measured at 294.0 ( 0.5 K with a resolution of 0.3 nm using an Ocean Optics spectrometer (Model HR 4000 CG-UV-NIR) and a combined deuterium-tungsten light source. Absolute wavelength calibration is based on a mercury lamp spectrum collected at the start of the experiment. Isopropyl nitrite (Karl Industries Inc., 97%) was subjected to repeated freezepump-thaw cycles prior to each experiment and protected from light; when not used, it was stored under vacuum at -20 °C. The vapor (0.5-26 Torr) was introduced from a glass storage tube into a Pyrex cell with quartz windows (10.50 cm path length) via an attached vacuum manifold. Chamber Experiments. Infrared cross sections of reference gases and experiments to determine quantum yields were carried out in an evacuable 100 L environmental chamber interfaced to a Thermo-Nicolet Nexus 670 Fourier transform infrared spectrometer and a HgCdTe detector (32). The total path length of the IR beam was 31.2 ( 0.1 m. All spectra were obtained at a resolution of 0.5 cm-1 and derived from 160-250 coadded interferograms. Uniform illumination of the entire chamber was provided by a single black lamp (Sylvania, 40 W, F40/350BL) housed in a quartz tube located along the center of the chamber, spanning its entire length. Reagent gases were prepared and introduced into the chamber via an external vacuum manifold. Experiments were carried out in synthetic air (Ultrapure Air, Scott-Marrin Inc., total hydrocarbons as CH4 < 0.01 ppm; CO < 0.01 ppm; NOx < 0.001 ppm; SO2 < 0.001 ppm) at 1 atm total pressure. The temperature of the gas mixture during the photochemical experiments was 297 ( 2 K. Control experiments were conducted to investigate the decay of all reactants and

ΦOH )

molecules of OH formed cm-3 s-1 ) photons absorbed by i-C3H7ONO cm-3 s-1 rOH [i-C3H7ONO]0

∑ σ(λ)I(λ)

(I)

where rOH is the rate of formation of OH radicals, [i-C3H7ONO]0 is the initial concentration of isopropyl nitrite determined from the infrared band at 1664 cm-1, σ(λ) is the wavelength-dependent absorption cross section (base e), and I(λ) is the lamp intensity at a given wavelength determined from measurements of the photolysis rate of NO2 (∼2 × 1014 molecules cm-3) in nitrogen (Oxygen Service Co., ultra high purity, 99.999%) at 1 atm total pressure (33, 34). Nitrogen dioxide was synthesized by reacting NO (Matheson Tri-Gas, 99%) with excess oxygen (Oxygen Service Co., 99.993%), followed by trap-to-trap purification. Nitric oxide was purified by passage through an acetone/dry ice bath trap at 195 K before use. The rate of OH radical formation (rOH) is measured by monitoring the appearance of CF3CFO rOH )

rCF3CFO

(II)

YCF3CFO f

where rCF3CFO is the initial rate of formation of CF3CFO determined from a polynomial fit to the [CF3CFO] vs time data (35),YCF3CFO is the yield of CF3CFO from the reaction determined in separate experiments, and f is the fraction of OH radicals scavenged by CF3CFdCF2. Trifluoroacetyl fluoride formation was used as a surrogate for OH radical production in these reactions because its band at 1098 cm-1 is not overlapped by peaks from reactants or other products. On the basis of rate constants for the reaction of OH with CF3CFdCF2 and i-C3H7ONO and the initial concentrations, f > 95% for all experiments. The rate constant for the reaction of OH with i-C3H7ONO has not been measured. The measured rate constants for five different alkyl nitrites whose kinetics have been studied experimentally (36, 37) were used to estimate the OH-isopropyl nitrite rate constant in the following way. First, the measured rate constants (37) for ethyl- and n-propyl nitrite were used with structure-reactivitybased rate constants for alkanes (38, 39) to obtain neighboring group factors for -ONO, F(ONO) ) 0.726, and for CH2ONO, F(CH2ONO) ) 0.159. Application of these neighboring group factors to n-butyl and n-pentyl nitrite gave predicted rate constants within 10% of the measured values (37). Extrapolation to the isopropyl nitrite reaction and assuming F(CH2ONO) ) F(CHONO) gives a rate constant for the OH-isopropyl nitrite reaction of 7.2 × 10-13 cm3 molecule-1 s-1. FTIR Reference Spectra. The concentrations of i-C3H7ONO, CF3CFdCF2, CF2O, and CF3CFO were determined using VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Absorption Cross Sections (σ) for i-C3H7ONO in the near-UV and Mid-infrared Regionsa σ (10-19 cm2 molecule-1), base eb

band position 335 nm 347 nm 359 nm 374 nm 390 nm

UV-Visible 0.91 ( 0.02 1.33 ( 0.02 1.67 ( 0.03 1.61 ( 0.03 0.94 ( 0.02

1664 cm-1 781 cm-1

Mid-infrared 16.1 ( 0.3 19.8 ( 0.4

a Error represents the 95% confidence limits. b Although we report here absorption cross sections to the base e, values were converted to base 10 for use with absorbance values (base 10) measured using FTIR.

reference spectra of the pure gases. Calibrations for i-C3H7ONO and CF3CFdCF2 were carried out by introducing selected pressures into the chamber in 1 atm of nitrogen at 296 ( 1 K and recording spectra after sequential measured dilutions with nitrogen. The absorption cross sections for CF3CFdCF2 are in excellent agreement (to better than 5%) with those reported by Acerboni et al. (40) and with those in the Pacific Northwest National Laboratory Vapor-Phase Infrared Spectral Library (41). The infrared spectrum of CF2O was obtained from the Pacific Northwest National Laboratory’s Vapor-Phase Infrared Spectral Library (41) and corrected to match the resolution, data spacing, and apodization function used in this study. Cross sections for CF3CFO were obtained by reacting Cl atoms with CF3CFdCF2 in air, which generates equal amounts of CF2O and CF3CFO in 100% yields (30, 31). A mixture of CF3CFdCF2 (1.7 × 1014 molecules cm-3) and Cl2 (∼1015 molecules cm-3; Matheson Tri-Gas, 99.5%) was irradiated at 296 ( 1 K while the infrared spectra were recorded. Bands due to CF3CFdCF2 and CF2O were subtracted from each spectrum, and the resulting reference spectrum of CF3CFO was used to produce a set of 11 calibration points at different concentrations. This gives a peak cross section (base e) at 1098 cm-1of (2.69 ( 0.18) × 10-18 cm2 molecule-1, in good agreement with (2.90 ( 0.09) × 10-18 cm2 molecule-1 reported at 2 cm-1 resolution by Francisco et al. (42).

Results and Discussion UV-Visible Cross Sections of i-C3H7ONO. Figure 1 shows the measured absorption cross sections in the UV-visible region. In agreement with published spectra of alkyl nitrites (4, 6, 7, 11), the spectrum consists of vibrational fine-structure superimposed on a broad feature. The absorption cross sections at the peaks of the vibrational structure, determined from three independent measurements by a least-squares fit to a Beer-Lambert plot of optical density vs l × [i-C3H7ONO], are tabulated in Table 1. The absorption cross section of the peak at 359 nm is (1.67 ( 0.03) × 10-19 cm2 molecule-1 (reported as base e, which is used in the quantum yield calculations of eq I), where the error represents two standard deviations after considering error from the leastsquares fit, optical path length, and pressure measurements. This is in good agreement with the cross sections reported by Ludwig and McMillan (11). Infrared Cross Sections of i-C3H7ONO. Figure 2 shows the infrared spectra of the two most intense bands at 1664 and 781 cm-1, assigned to NdO and O-N stretches, and which are most useful for quantification. As is the case for other nitrites, i-C3H7ONO exists as a mixture of distinct interconverting rotational isomers, anti and syn conformers with respect to the R-ONdO dihedral angle (43-47). The 8152

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FIGURE 2. Mid-infrared spectrum of isopropyl nitrite vapor at 296 K in 1 atm of N2 showing (a) the NdO stretch of the anti (1664 cm-1) and syn (1613 cm-1) conformers and (b) the O-N stretch of the anti conformer (781 cm-1).

FIGURE 3. Formation of CF3CFO generated during the reaction of CF3CFdCF2 [(2.4-4.0) × 1015 molecules cm-3] with OH radicals from the photolysis of i-C3H7ONO [(2-3) × 1014 molecules cm-3] in the presence (O) and absence (0) of additional NO [(2-3) × 1014 molecules cm-3] in air. Also shown is the formation of CF3CFO when a mixture of CF3CFdCF2 (3.9 × 1015 molecules cm-3) and NO (2.8 × 1014 molecules cm-3) in air is irradiated (3). anti conformer dominates (76%) at room temperature (44) and is the origin of the intense peaks at 1664 and 781 cm-1. The weaker feature centered at 1613 cm-1 is due to the NdO stretch of the syn conformer. The absorption cross sections (0.5 cm-1 resolution, base e) of the 1664 and 781 cm-1 peaks are (1.61 ( 0.03) × 10-18 and (1.98 ( 0.04) × 10-18 cm2 molecule-1, respectively (see Table 1), where the errors are 2σ based on errors from the least-squares fit, optical path length, and pressure measurements. Quantum Yield for OH Radical Formation. The quantum yields for OH production were determined from the rates of formation of CF3CFO using eqs I and II. Figure 3 shows the formation of CF3CFO in typical experiments when i-C3H7ONO and CF3CFdCF2 in air were irradiated in the absence or presence of additional NO (i.e., NO added in excess of what is expected from reaction 2). The initial rate of CF3CFO formation was determined by fitting the data to the polynomial function, [CF3CFO] ) [CF3CFO]0 + r0t + r1t2 + r2t3, where r0 is the initial rate (rCF3CFO in eq II) and the initial concentration, [CF3CFO]0 is set to zero (35). The initial rates and measurement conditions for all experiments are summarized in Table 2. As seen in Figure 3, the rate of formation of CF3CFO is ∼20% higher in the presence of additional NO. To test whether this apparent increase in photooxidation rate is due to NOx

TABLE 2. Experimental Conditions and Overall Yields for OH Radical Formation (ΦOH) from the Photolysis of Isopropyl Nitrite in 1 atm of Air in the Presence and Absence of Additional NOa initial concentrations (1014 molecules cm-3) i-C3H7ONO

CF3CFdCF2

3.2 2.3 3.1 2.9 2.9

24 31 38 39 39

Errors represent 2σ.

fb

3.2 0.95 3.1 0.95 3.1 0.97 2.9 0.97 2.9 0.97 average ΦOH in presence of excess NO 31 0 0.96 39 0 0.97 39 0 0.97 39 0 0.97 average ΦOH in absence of additional NO overall average

3.2 3.1 2.9 2.9

a

NO

b

rCF3CFO (1011 molecules cm-3 s-1)

ΦOHa

1.61 ( 0.06 1.50 ( 0.07 1.57 ( 0.07 1.45 ( 0.07 1.54 ( 0.05

0.53 ( 0.07 0.49 ( 0.06 0.52 ( 0.07 0.52 ( 0.07 0.55 ( 0.08 0.52 ( 0.04 0.61 ( 0.08 0.56 ( 0.07 0.55 ( 0.07 0.52 ( 0.06 0.56 ( 0.07 0.54 ( 0.07

1.38 ( 0.07 1.24 ( 0.04 1.15 ( 0.04 1.09 ( 0.03

f is the fraction of OH radicals scavenged by CF3CFdCF2; see the text.

FIGURE 4. Formation of CF3CFO (O) and CF2O (0) during photolysis of i-C3H7ONO [(1-3) × 1014 molecules cm-3] as a function of the amount of CF3CFdCF2 reacted (a) in the presence and (b) absence of additional NO [(1-2) × 1014 molecules cm-3]. photochemistry alone, a control experiment was performed using a mixture of CF3CFdCF2 and NO irradiated under identical conditions. As shown by the trace at the bottom of Figure 3, CF3CFO formation is slow, demonstrating that NOx photooxidation chemistry alone cannot account for the difference between the two experiments where i-C3H7ONO is the OH radical source. The discrepancy in CF3CFO formation rates seen in Figure 3 is explained by a difference in the product yields for the CF3CFdCF2 + OH reaction in the presence and absence of NOx. Figure 4 compares results from two experiments designed to measure the product yield of both CF3CFO and CF2O when mixtures containing i-C3H7ONO and CF3CFdCF2 in air were irradiated in the presence and absence of additional NO. When an excess of NO is added to the reaction mixture, the yields of CF3CFO and CF2O are 0.95 ( 0.01 and 0.88 ( 0.01 (2σ), respectively. Without the additional NO, the yields drop to 0.70 ( 0.01 and 0.58 ( 0.03, respectively. The dramatic differences in product yields likely reflect differences in RO2 + HO2 and RO2 + RO2 pathways favored in the lowNOx situation versus the RO2 + NO pathways that dominate under high-NOx conditions (19). In addition, the curvature in the plot of [CF2O] versus ∆[CF3CFdCF2] in the absence of additional NO suggests that secondary losses of CF2O may be occurring in this system. While it is outside the scope of

this paper to discuss the mechanistic origins of these discrepancies, it is sufficient to use the measured yields to correct the observed rate of OH formation in eq II. Table 2 summarizes the overall OH quantum yields, ΦOH ) 0.52 ( 0.04 (2σ) in the presence of added NO and 0.56 ( 0.07 (2σ) without additional NO, with an average ΦOH ) 0.54 ( 0.07 (2σ). Although this appears to be the first measurement of the overall quantum yield for OH production, the primary quantum yield for photodissociation of isopropyl nitrite (reactions 1 and 2) has been reported to be 0.36 at 366 nm and 299 K (11). This should set an upper limit on the OH production through secondary chemistry. However, this primary quantum yield was stated to be subject to large uncertainty (without quantification of the possible error). We note that we were able to fit well the [i-C3H7ONO] vs time profiles observed during photolysis experiments using the function, [i-C3H7ONO] ) [i-C3H7ONO]0 exp[-(∑σΦI)t] and by assuming a primary quantum yield of 1.0 between 290 and 450 nm. Thus, the actual primary quantum yield for photodissociation of isopropyl nitrite (reactions 1 and 2) is likely closer to unity, consistent with primary quantum yields reported for methyl and tert-butyl nitrite (4, 48, 49). The overall quantum yield for OH production from i-C3H7ONO photolysis is less than for HONO, where ΦOH ) 1.0 at λ < 400 nm (3). This is not surprising since HONO photolysis generates OH directly, while in the case of isopropyl nitrite photolysis, OH formation stems from secondary chemistry. Thus, as discussed above, the reaction of O2 with i-C3H7O radicals forms HO2, which subsequently reacts with NO (reactions 5 and 6). The alkoxy radical reaction with O2 is in competition with its decomposition (reaction 3) and recombination with NO (reaction 4). We also considered the reaction of i-C3H7O with NO2 as a secondary reaction that could reduce the amount of HO2 formed in this system i-C3H7O + NO2 f i-C3H7ONO2

(7)

where a second-order rate constant of k7 ) 4 × 10-12 cm3 molecule-1 s-1 has been previously measured (15). This must be only a minor reaction channel, since we did not observe the expected IR stretches of the product i-C3H7ONO2 at 1280 and 1630 cm-1 (50). An upper limit for the quantum yield of OH in the presence of additional NO can be estimated by considering all the pathways that compete with reaction 5, assuming that every HO2 radical formed reacts with NO to yield OH. The overall quantum yield for OH production is then given by eq III: VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ΦOH )

k5[O2] k3 + k4[NO] + k5[O2] + k7[NO2]

(III)

The unimolecular decomposition of i-C3H7O radicals has been measured by Devolder et al. (14) over a range of pressures and temperatures between 330 and 408 K. Extrapolation to 296 K for the high-pressure limit provides a first-order rate constant of 689 s-1. The reactions of i-C3H7O radicals with NO and O2 have second-order rate constants of k4 ) 3.4 × 10-11 cm3 molecule-1 s-1 and k5 ) 6.5 × 10-15 cm3 molecule-1 s-1 at 296 K (15). At NO, NO2, and O2 concentrations of 2.5 × 1014, 3.0 × 1014, and 4.9 × 1018 molecules cm-3, respectively, eq III gives a maximum overall quantum yield for OH formation of 0.76 ( 0.28. Given the errors on this estimate and on our overall OH quantum yield and the simplified mechanism on which the estimate is based, the agreement is satisfactory. The overall quantum yield for OH production from the photolysis of i-C3H7ONO over the wavelength range from 300 to 425 nm at atmospheric pressure in air and at 296 ( 2 K has been measured for the first time to be 0.54 ( 0.07 (2σ), independent of whether or not NO was added. Despite having a less than unit yield for OH production, isopropyl nitrite is a very useful photolytic source of OH radicals in laboratory studies due to its commercial availability, ease of preparation and handling, and the relatively low reactivity of its major oxidation product, CH3C(O)CH3. The overall quantum yield measured here, along with the UV-visible and infrared cross sections, are useful for quantifying isopropyl nitrite, for calculating OH radical formation rates and concentrations in photochemical experiments, and for assessing photolysis lifetimes of isopropyl nitrite under atmospheric conditions. For example, on the basis of the absorption cross sections measured here and assuming a primary quantum yield of 1 for the initial dissociation step, its lifetime at the Earth’s surface at a solar zenith angle of 0° on July 1 is calculated (19) to be 3 min. As discussed with respect to the chemistry occurring in the experimental system, the rate of OH generation from this photolysis will depend on the fates of the alkoxy and HO2 radicals in air, but under most continental conditions where the major fate of HO2 is reaction with NO, significant yields of OH will result.

Acknowledgments We thank Samar Moussa for helpful discussions and Timothy Wallington and Michael Hurley for providing a spectrum of CF2O for comparison. J.D.R. is grateful to the National Science Foundation for fellowship support under CHE-0836070. This work was carried out at AirUCI, an Environmental Molecular Sciences Institute funded by the National Science Foundation (Grant # CHE-0431312 and -0909227).

Supporting Information Available A table containing a list of chemical reactions associated with i-C3H7ONO photolysis and the reaction of OH with C3H6, including rate constants; Figure S1 compares the IR spectra of the reaction mixture before and after reaction to those of the pure CF3CFO and CF2O. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Vasudev, R.; Zare, R. N.; Dixon, R. N. State-Selected Photodissociation Dynamics: Complete Characterization of the OH Fragment Ejected by the HONO A State. J. Chem. Phys. 1984, 80 (10), 4863–4878. (2) Hennig, S.; Untch, A.; Schinke, R.; Nonella, M.; Huber, J. R. Theoretical Investigation of the Photodissociation Dynamics of HONO: Vibrational Predissociation in the Electronically Excited State S1. Chem. Phys. 1989, 129 (1), 93–107. (3) Cox, R. A.; Derwent, R. G. The Ultra-Violet Absorption Spectrum of Gaseous Nitrous Acid. J. Photochem. 1976, 6 (1), 23–24. 8154

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