Environ. Sci. Technol. 2000, 34, 623-631
Atmospheric Chemistry of 2-Pentanone and 2-Heptanone R O G E R A T K I N S O N , * ,† ERNESTO C. TUAZON, AND SARA M. ASCHMANN Air Pollution Research Center, University of California, Riverside, California 92521
2-Pentanone and 2-heptanone are available for commercial use, which will lead to their possible release into the atmosphere where they are expected to primarily react with the hydroxyl (OH) radical and contribute to the formation of photochemical air pollution in urban and regional areas. We have studied the kinetics and products of the gasphase reactions of the OH radical with 2-pentanone and 2-heptanone at 298 ( 2 K and atmospheric pressure of air. Using a relative rate method, rate constants for the reactions of the OH radical with 2-pentanone and 2-heptanone of (4.56 ( 0.30) × 10-12 and (1.17 ( 0.11) × 10-11 cm3 molecule-1 s-1, respectively, were obtained, where the indicated errors are two least-squares standard deviations and do not include the ∼10-15% uncertainties associated with the rate constant for the reference compound cyclohexane. Reaction products were analyzed by gas chromatography, in situ Fourier transform infrared spectroscopy, and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS). The products identified and quantified were (with their molar yields) as follows: from 2-pentanone, formaldehyde, 1.03 ( 0.10; acetaldehyde, 0.51 ( 0.11; propanal, 0.19 ( 0.03; 2,4pentanedione, 0.12 ( 0.03; and molecular weight 147 organic nitrates, 0.12 ( 0.04; and from 2-heptanone, formaldehyde, 0.38 ( 0.08; acetaldehyde, ∼0.05; propanal, ∼0.05; butanal, 0.07 ( 0.01; pentanal, 0.09 ( 0.01; and molecular weight 175 organic nitrates, 0.18 ( 0.05. API-MS analyses also showed the formation of products of molecular weight 128 and 144 from 2-heptanone, anticipated to be C7dicarbonyl(s) and C7-hydroxydicarbonyl(s), respectively. While 94 ( 13% of the reaction pathways of the 2-pentanone reaction are accounted for, it appears that a substantial fraction (>50%) of the initially formed alkoxy radicals from 2-heptanone undergo isomerization to form products which could not be quantified.
Introduction 2-Pentanone [methyl propyl ketone, CH3C(O)CH2CH2CH3] and 2-heptanone [methyl amyl ketone, CH3C(O)CH2CH2CH2CH2CH3] are available for commercial use, which will lead to their possible release into the atmosphere where they will undergo photolysis and/or chemical reaction (1, 2) and hence contribute to the formation of photochemical air pollution in urban and regional areas (3). On the basis of our current * Corresponding author phone: (909)787-4191; fax: (909)787-5004; e-mail:
[email protected]. † Also at the Department of Environmental Sciences and Department of Chemistry, University of California, Riverside. 10.1021/es9909374 CCC: $19.00 Published on Web 01/13/2000
2000 American Chemical Society
understanding of the atmospheric chemistry of these and other similar ketones (1, 2), it is anticipated that the most important tropospheric transformation process for these two ketones is by reaction with the hydroxyl (OH) radical. While rate constants have been measured for the reactions of the OH radical with 2-pentanone (4-6) and 2-heptanone (5), to date no data have been reported concerning the products and mechanisms of these reactions. Accordingly, we have investigated the products and mechanisms of the gas-phase reactions of 2-pentanone and 2-heptanone with the OH radical. There are some discrepancies between the absolute rate study of Wallington and Kurylo (5) and relative rate studies carried out in this laboratory (4, 6) concerning the rate constants for the reactions of the OH radical with 2- and 3-pentanone and 2-hexanone. We have therefore measured rate constants for the reactions of the OH radical with 2-pentanone and 2-heptanone as part of the present study.
Experimental Methods Kinetic Studies. The experimental methods were similar to those we have used previously (7). Experiments were carried out at 298 ( 2 K and 740 Torr total pressure of purified air (at ∼5% relative humidity) in a 7900-L all-Teflon chamber equipped with two parallel banks of blacklamps for irradiation and a Teflon-coated fan to ensure rapid mixing of reactants during their introduction into the chamber. In the relative rate method used, the relative disappearance rates of the ketones and a reference organic, whose OH radical reaction rate constant is reliably known, were measured in the presence of OH radicals. Provided that the ketones and reference organic were removed solely by reaction with OH radicals, then
ln
{
}
[ketone]to [ketone]t
)
{
}
[reference organic]to k1 ln k2 [reference organic]t
(I)
where [ketone]to and [reference organic]to are the concentrations of the ketones and the reference organic at time to, [ketone]t and [reference organic]t are the corresponding concentrations at time t, and k1 and k2 are the rate constants for reactions 1 and 2, respectively:
OH + ketone f products
(1)
OH + reference organic f products
(2)
Hydroxyl radicals were produced by the photolysis of methyl nitrite (CH3ONO) in air at wavelengths >300 nm (8), and NO was also included in the reactant mixtures to suppress the formation of O3 and, hence, of NO3 radicals (8). The initial concentrations of the reactants (in molecule cm-3 units) were as follows: CH3ONO, ∼2.4 × 1014; NO, ∼2.4 × 1014, and the ketones and reference organic, ∼2.4 × 1013 each. Cyclohexane was used as the reference organic because its OH radical reaction rate constant is similar to those previously reported for 2-pentanone and 2-heptanone (9) and it could be analyzed by gas chromatography using the same sampling and analysis procedures. Irradiations were carried out for 10-45 min at 20% of the maximum light intensity. The concentrations of the ketones and cyclohexane were measured during the reactions by gas chromatography with flame ionization detection (GC-FID). Gas samples of 100 cm3 volume were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent thermal desorption at -225 °C onto a 30-m VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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DB-1701 megabore column held at -20 °C and then temperature programmed to 200 °C at 8 °C min-1. Product Studies. Experiments were carried out at 298 ( 2 K and 740 Torr total pressure of air in three reaction chambers. These consisted of (i) a 5870-L evacuable, Tefloncoated chamber containing an in situ multiple reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FT-IR) spectrometer and with irradiation provided by a 24-kW xenon arc filtered through a 0.25 in. thick Pyrex pane (to remove wavelengths 300 nm (8, 10), and NO was included in the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals (8). Ethyl nitrite was used as the OH radical precursor for the in situ FT-IR analyses of formaldehyde (10) because the photolysis of methyl nitrite leads to the formation of formaldehyde. Teflon Chamber with Analysis by GC-FID and GC-MS. For the experiments carried out in the 7900-L Teflon chamber (at ∼5% relative humidity), the initial reactant concentrations (in molecule cm-3 units) were as follows: CH3ONO, (2.22.3) × 1014; NO, (2.1-2.4) × 1014; and 2-pentanone or 2-heptanone, (2.25-2.46) × 1013. Irradiations were carried out at 20% of the maximum light intensity for 15-45 min, resulting in up to 33% and 44% reaction of the initial 2-pentanone or 2-heptanone, respectively. GC-FID analyses of the ketones and their reaction products were carried out as described above, with the DB-1701 megabore column being initially held at -40 or -60 °C and then temperature programmed to 200 °C at 8 °C min-1. Gas samples were also collected from the chamber onto Tenax-TA solid adsorbent for thermal desorption with analysis by GC-MS after preconcentration by an Entech 7000 dehydration preconcentrator, using a 30-m DB-1701 fused silica capillary column in a Hewlett-Packard (HP) 5890 GC interfaced to a HP 5971A mass selective detector operating in the scanning mode. GCFID response factors for 2-pentanone, 2-heptanone, acetaldehyde, propanal, butanal, pentanal, and 2,4-pentanedione were determined as described previously (11). Teflon Chamber with Analysis by API-MS. In the experiments with API-MS analyses, the chamber contents were sampled through a 25 mm diameter × 75 cm length Pyrex tube at ∼20 L min-1 directly into the API mass spectrometer source. The operation of the API-MS in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes has been described elsewhere (12). Use of the MS/ MS mode with CAD allows the “daughter ion” or “parent ion” spectrum of a given ion peak observed in the MS scanning mode to be obtained (12). The positive ion mode was used in these analyses, with protonated water hydrates [H3O+(H2O)n] being used as the ionizing agent and resulting in the ions that were mass-analyzed being mainly protonated molecular ions ([M + H]+) and their protonated homo- and heterodimers (12). The initial concentrations of CH3ONO, NO, and 2-pentanone or 2-heptanone were ∼4.8 × 1013 molecule cm-3 each, and irradiations were carried out for 10 (2-heptanone) or 15 min (2-pentanone) at 20% of the maximum light intensity. 624
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FIGURE 1. Plot of eq I for the gas-phase reactions of 2-pentanone and 2-heptanone with the OH radical, using cyclohexane as the reference compound. Evacuable Chamber Experiments with FT-IR Analysis. For the experiments carried out in the 5870-L evacuable, Tefloncoated chamber (at
