Multiphase Chemistry of Glyoxal: Revised Kinetics of the Alkyl Radical

Oxygen and the Reaction of Glyoxal with OH, NO3, and SO4– in Aqueous Solution .... Environmental Science & Technology 2015 49 (3), 1237-1244...
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Multiphase Chemistry of Glyoxal: Revised Kinetics of the Alkyl Radical Reaction with Molecular Oxygen and the Reaction of Glyoxal with OH, NO3, and SO4− in Aqueous Solution T. Schaefer, D. van Pinxteren, and H. Herrmann* Leibniz-Institute for Tropospheric Research (TROPOS), Atmospheric Chemistry Department, Permoserstraße 15, 04318 Leipzig, Germany S Supporting Information *

ABSTRACT: The rate constant for the reaction of the hydrated glyoxyl radical (CH(OH)2C(OH)2· with O2 has been determined as k298 K = (1.2 ± 0.3) × 109 L mol−1 s−1 at pH 4.8. This experimental value is considerably higher than a widely used estimated value of about k = 1 × 106 L mol−1 s−1. As the aqueous phase conversion of glyoxal is of wide interest for aqSOA formation, we suggest that the newly determined rate constant should be applied in multiphase models. The formation of the dimerization product tartaric acid has as well been studied. This product is found, however in significant yields only when the oxygen content of the solution is reduced. The formation of dimers from the recombination of alkyl radicals in the atmospheric aqueous phase should hence be treated with great care. Finally, the reactions of the free radicals OH, NO3, and SO4− with glyoxal have been investigated and rate constants of k298 K (OH) = (9.2 ± 0.5) × 108 L mol−1 s−1, k298 K (SO4−) = (2.4 ± 0.2) × 107 L mol−1 s−1 and k298 K (NO3) = (4.5 ± 0.3) × 106 L mol−1 s−1 were obtained.



INTRODUCTION Recently, there has been strong interest in the topic of tropospheric glyoxal multiphase oxidation. Glyoxal is an ubiquitious organic compound being formed from a wide variety of volatile organic compounds VOC oxidation processes in the troposphere, primarily emitted and possibly formed by ocean surface layer chemistry.1−5 The aqueous phase chemistry of glyoxal leads to SOA formation potential in the aqueous phase which might be addressed as aqSOA formation.6−17 Glyoxal is widely found in the environment as evidenced by field measurements in the gas phase, in the aqueous phase in particles and as observed by satellite measurements.17−33 Within cloud of fog droplets and snow, glyoxal is present in concentrations in the range of μM and its concentration might reach up to molar (M) concentrations in aqueous tropospheric aerosol particles.17−19,22,32,34 Since the study of Carlton et al., a drastically small rate constant for the reaction of the glyoxyl alkyl radical with molecular oxygen as the one determined earlier by Buxton et al. was suggested and applied.8,35 Concretely, in the oxidation mechanism of glyoxal by Lim et al. an oxygen addition rate constant k(R + O2) ∼ 106 L mol−1 s−1 based on a photochemistry study of pyruvic acid by Guzman et al. was used.13,14,36 This value is 3 orders of magnitude lower than typically measured and applied rate constants which are found to be in the range of k(R + O2) ∼ 109 L mol−1 s−1 for numerous compounds similar to glyoxal.35,37−43 Clearly, a slower rate constant for the oxygen addition corresponds to a promotion of the formation of dimeric products from the recombination of alkyl-type radicals. Because of this unclear situation, the reaction of the glyoxyl radical with O2 was reinvestigated experimentally by two © 2014 American Chemical Society

independent methods within the present study. The competition kinetics method applied here was also applied to measure the reaction rate constants for the reactions of the alkyl radicals derived from methanol and 2-propanol with dissolved oxygen. Based on the data from the laboratory studies an oxidation scheme consistent with the current findings is suggested. Finally, pH and temperature dependent kinetic investigations of the H abstraction reactions of the atmospherically important OH, NO3, and SO4− radicals with glyoxal were performed and the results are compared with literature data.44



EXPERIMENTAL SECTION The laser flash photolysis is an established method to measure the rate constants of radical reactions in aqueous solution.44−46 The Supporting Information (SI) provides all details about the used reagents and materials, the details about the analytical method of the product studies as well as the experimental details about investigations of the OH, SO4− and NO3 radical reactivity. Investigations of the Alky Radical Reaction with Oxygen. The reactivity of the alky radicals toward molecular oxygen in the aqueous solution was determined using (a) a direct measurement method and (b) a competition kinetic method. In both measurements a modified thermostated laser flash photolysisdifferentially amplified laser longpath absorption (LP-DALLPA) setup was used, which has been Received: December 3, 2014 Accepted: December 5, 2014 Published: December 5, 2014 343

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described in more detail.46 A pulsed multigas excimer laser (COMPex 201, Lambda-Physik, Göttingen, Germany) was applied for the photolysis of the radical precursor compounds in the reaction cell. The measurements were done in a cuboid cell consisting of high-purity SUPRASIL windows having a length of 3.5 cm, a width of 4 cm, and a height of 2 cm, with a resulting volume of 28 cm3. The length of 3.5 cm of the cell corresponds to the photolysis path length and the width of the cell with 4 cm corresponds to the absorption pathway. The temperature was adjusted at T = 298 K. For the direct measurements of the radical species a wavelength λ = 244 nm (double frequency argon ion laser) was used with a White cell mirror configuration was adjusted for 12 passes giving an absorption path length of 48 cm throughout all experiments. The transient species were formed in the oxidation of 1 × 10−3 M glyoxal with OH radicals at pH 2. The latter were generated by photolysis of 2 × 10−4 M H2O2 at λ = 248 nm. The OH radical concentration was determined by the direct observation of the OH radical at λ = 244 nm by using the molar absorption coefficient ε = 530 L mol−1 cm−1 after Herrmann et al.47 In the case of the modified competition kinetic method of Adams and Willson the absorption path length was adjusted to 32 cm and a wavelength λ = 442 nm (HeCd-laser) was used.37 As reference reactant the ferricyanide (K3[Fe(CN)6]) was used, which reacts with alkyl radicals (R·) by electron transfer yielding the nonabsorbing ferrocyanide at λ = 442 nm (see SI Figure SI 1). R · + [Fe(CN)6 ]3 − → R′ + [Fe(CN)6 ]4 −

Article

RESULTS AND DISCUSSIONS Alkyl Radical Reactivity. Reactions of the Hydroxymethyl and Isopropyl Alkyl Radicals with Oxygen. First, for

Figure 1. Measured absorption - time profile for the bleaching reaction of ferricyanide due to the α-alkyl radicals from the glyoxal oxidation at λ = 442 nm.

comparison with established rate constants, the reactivities of the alkyl radicals derived from the parent compounds (I) methanol and (II) 2-propanol were determined by the ferrocyanide competition kinetics method adopted from Adams and Willson (see SI Figure SI 2−11).37 The obtained results are in good agreement with reported values in Table 1 with the rate constants k(R + [Fe(CN)6]3−) = (4.2 ± 0.9) × 109 L mol−1 s−1 and k(R + O2) = (4.2 ± 0.9) × 109 L mol−1 s−1 for the methanol oxidation and the rate constants k(R + [Fe(CN)6]3−) = (5.2 ± 0.3) × 109 L mol−1 s−1 and k(R + O2) = (4.2 ± 0.9) × 109 L mol−1 s−1 for the 2-propanol oxidation. The different experimental conditions indicate that there is no dependency for the rate constants of the alkyl radical reaction with molecular oxygen from the organic precursor concentrations up to 1 M. Guzman et al. provided a rate constant k(R + O2) ≈ 106 L mol−1 s−1 for the pyruvic acid alkyl radical.36 The tertiary radical structure is similar to the isopropyl alkyl radical, which reacts with oxygen a factor of 1000 faster. The different substituents of a carboxylic group or a methyl group could not explain the decreasing reactivity.48 Reaction of the Glyoxyl Alkyl Radical with Oxygen. (i). Ferricyanide Competition Kinetics. In Figure 1 the change of the absorption at λ = 442 nm during the electron transfer reaction of the glyoxyl radicals with ferricyanide at pH 4.8 is shown. The averaged first-order rate constants derived from the curves are linear with ferricyanide concentration (see SI Figure SI 15). From the slope of the plot, the bimolecular rate constant of the glyoxyl radical reaction with ferricyanide was (2.3 ± 0.5) × 109 L mol−1 s−1. At pH 2 the absorption at λ = 442 nm is influenced by the second order contribution, which can be explained by the acid-catalyzed dehydration reaction of the glyoxyl alkyl radical (see SI Figure SI 20).49 The positive absorbance could be explained by the rearrangement of the ferricyanide complex due to the electron transfer and with a small contribution by the photolysis of ferricyanide. The concentration of the transient species, due to the photolysis can be given as 106 s−1 as it was suggested for a different but similar radical by Bothe et al. would provide a smaller rate constant of k(R + O2) ≈ 107 L mol−1 s−1, which would then, however, be in contradiction to the competition kinetics performed in the present study.35,52 Finally, if the HO2 radical would be built up so fast the obtained absorption time−profile would not lead to such a long-lasting absorption (see SI Figure SI 18). A value for the first-order rate constant of the HO 2 elimination is not available from literature, but the kfirst = 190 s−1 for HO2 elimination from the similarly structured RO2radical of 1,2-ethylenglycol is comparable to the value

determined here.53 A first order rate constant of the dehydration reaction of the glyoxyl radical to the glycolic acid alky radical was included into the model mechanism with k = 2.7 × 105 s−1 obtained from the fit of SI Figure SI 17 (see SI Table SI 2 and Figure SI 17). The simulation was done with an absorption coefficient of ε244 nm (R) = 3250 L mol−1 cm−1 for glycolic acid alky radical obtained from the analysis of the absorption time−profile under reduced oxygen concentration and of ε244 nm (RO2) = 500 L mol−1 cm−1 for the corresponding peroxyl radical. The rate constant of the oxygen addition k(R + O2) = 1.8 × 109 L mol−1 s−1 was taken from Adams and Willson.37 If the mentioned estimated molar absorption coefficient would be increased the rate constant of the formation of glyoxal peroxyl radicals will decrease and the corresponding molar absorption coefficient of ε244 nm (RO2) will decrease (see SI Table SI 3). To check the simulations ’sensitivity, it was repeated with a prescribed rate constant of (i) k(R + O2) = 5.0 × 109 L mol−1 s−1 (red dashed line in Figure 3) which results in ε(R) = 2201 L mol−1 cm−1 and ε(RO2) = 917 L mol−1 cm−1 and with (ii) k(R + O2) = 2 × 108 L mol−1 s−1 (blue dashed line in Figure 3) leading to ε(R) = 803 L mol−1 cm−1 and ε(RO2) = 1299 L mol−1 cm−1. As expected, both of these model runs do not match the measurement data very well. 346

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Figure 4. Radical oxidation mechanism in aqueous solution.

Table 3. Kinetic Data and Arrhenius Parameters of the Investigated OH, SO4− and NO3 Reactions with Glyoxal in Aqueous Solution pH 2 [L mol−1 s−1] k298 K k293 K k298 K k298 K k298 K

(OH) (OH)35 (OH)67 (SO4−) (NO3)

(9.4 ± 0.4) × 10

8

(2.2 ± 0.4) × 107

pH 6 [L mol−1 s−1] (9.2 ± 0.5) (1.1 ± 0.1) 6.6 × 107 (2.4 ± 0.2) (4.5 ± 0.3)

pH 9 [L mol−1 s−1]

A [L mol−1 s−1]

EA [kJ mol−1]

× 10 × 109

(1.1 ± 0.1) × 10

(5.8 ± 0.1) × 10 10 ± 1 1.9 × 1011

13 ± 1

× 107 × 106

(2.6 ± 0.1) × 107

(5.4 ± 0.1) × 109 (6.22 ± 0.8) × 1012

13 ± 1 35 ± 10

8

9

The obtained results of the first free fit are in good agreement with reported values from Buxton et al. with a rate constant k(R + O2) = (1.4 ± 0.1) × 109 L mol−1 s−1, a molar absorption coefficient for the alkyl radical ε(R) = 1105 L mol−1 cm−1and for the peroxyl radical ε(RO2) = 941 L mol−1 cm−1 (see Table 1).35 And is in the typically range of the rate constants k(R + O2) ∼ 109 L mol−1 s−1.35,37−43 Product Analysis. The product distribution of the glyoxal oxidation has been determined in two single experiments at pH 4.8 for three different oxygen concentrations: (i) “saturated” (1.25 × 10−3 M), (ii) “normal” (2.6 × 10−4 M), and (iii) “reduced” (5 × 10−9 mol L−1) due to the competition with the HO2 elimination reaction. The main reaction path of the glyoxyl peroxyl radical is the HO2 radical elimination which leads to the hydrated glyoxylic acid 10. This can react via OH radical reaction to the glyoxylic acid alkyl radical 11 with subsequent oxygen addition followed by an HO2 elimination finally yielding oxalic acid 12. It is suggested that the chemical mechanism of Figure 4 should be implemented into tropospheric aqueous phase chemical mechanisms. OH, NO3, and SO4− Radical Reactivity. The H atom abstraction reactions of OH, NO3, or SO4− radicals with glyoxal were measured using the LP−LLPA setup as a function of the pH and T in aqueous solution. The rate constants at T = 298 K and pH 6 and their corresponding Arrhenius parameters from the present work are summarized in Table 3 (see SI Figure SI 21−23 and Table SI 4). The reaction of glyoxal with these radicals appear to be pHindependent. The rate constants and the activation parameters of the OH reaction are in a good agreement with the measured values k293 K (OH) = (1.1 ± 0.1) × 109 M−1 s−1 and EA = 12.6 ± 0.9 kJ mol−1 by Buxton et al. and simulated value k = 1.04 × 109 M−1 s−1 obtained using the SAR method by Doussin and Monod.35,66 Another rate constant of k(OH) = 6.6 × 107 M−1 s−1 appears too slow when compared to the rate constants of reactions of OH with other polyfunctional alcohols by at least 1 order of magnitude.67 There are no kinetic data for comparison for the SO4− and NO3 radical reactions available in the literature. The rate constants obtained here are comparable with those of other mono- and polyfunctional alcohols.67,45



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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 341 2717 7024; fax: +49 341 2717 99 7024; email: [email protected]. Notes

The authors declare no competing financial interest.



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ASSOCIATED CONTENT

S Supporting Information *

Additional data including the reagents and materials, the ferricyanide competition kinetics, the input parameter for the model initialization, the description of the product studies and the measurement results of the free radical reaction toward glyoxal are given in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 348

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dx.doi.org/10.1021/es505860s | Environ. Sci. Technol. 2015, 49, 343−350