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OH-initiated degradation mechanism of 1,4-dioxane in the presence of NOx
10 December 1999
Chemical Physics Letters 314 Ž1999. 465–471 www.elsevier.nlrlocatercplett
OH-initiated degradation mechanism of 1,4-dioxane in the presence of NOx H. Geiger ) , T. Maurer, K.H. Becker Bergische UniÕersitat ¨ – Gesamthochschule Wuppertal, Physikalische Chemier Fachbereich 9, D-42097 Wuppertal, Germany Received 10 August 1999; in final form 22 September 1999
Abstract Based on the results of indoor photoreactor studies, a photochemical mechanism has been developed for the OH-initiated degradation of 1,4-dioxane in the presence of NOx . The reaction scheme constructed has been tested by comparison of computer box model calculations and experimental data taken from indoor photoreactor studies. Experimentally obtained and modelled concentration–time profiles for selected reactants are in excellent agreement. A sensitivity analysis was performed for the reaction system. These calculations highlight the key reactions of the treated chemical model. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The increasing use of oxygenated organic compounds such as ethers, esters and alcohols as fuel additives, alternative fuels or solvents during the last years will lead to an increasing influence of these species on tropospheric chemistry. While the rate coefficients for the OH reactions of many relevant oxygenated VOCs are mostly well established, knowledge of the photooxidant mechanisms of these compounds is relatively poor. The mechanistic studies are often limited to the detection of the primary reaction products. Reaction mechanisms which are derived from these data have usually not been proven by comparison of the measured product concentrations against model simulations of chamber experiments. In other cases, where no suitable experimental results are available, reaction mechanisms are ) Corresponding author. Fax: q49-202-439-2757; e-mail: [email protected]
postulated without any further verification. Both methods are characterised by a high uncertainty when the model is applied, e.g., to chemistry transport modelling. In the present study, the chemical degradation scheme for 1,4-dioxane was constructed considering both experimental data and computer simulations. 1,4-dioxane is currently used as alternative solvent for cellulose acetate, resins, oils and waxes w1x and therefore is expected to become important in tropospheric photochemistry in the near future. The rate coefficient for the reaction of OH radicals with 1,4-dioxane was recently determined by Maurer et al. w2x and also by Porter et al. w3x and Dagaut et al. w4x. Maurer et al. w2x and Platz et al. w5x carried out extensive product studies of the tropospheric degradation of 1,4-dioxane in the laboratory using OH radicals w2x or chlorine atoms w5x for the initiation of the radical chain. Both authors detected ethylene1,2-diformate ŽEDF. as the only reaction product in
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 1 5 - X
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the presence of NOx . Maurer et al. w2x report a yield for EDF of 87–95 mol%. Platz et al. w5x were not able to calibrate the EDF concentrations obtained in their experiments because no reference spectrum was available. The few missing primary reaction products are mainly expected to be organic nitrates. Since the carbon yield for the OH-initiated oxidation of 1,4-dioxane is much less than 100% in the absence of NOx w2x, the present investigations are focused on high NOx conditions such as are found in the urban atmosphere.
2. Experimental data and computer simulation system The experimental data used in this work for the validation of the degradation mechanism of 1,4-dioxane were obtained by Maurer et al. w2x. The experimental set-up used for these experiments has been described in detail elsewhere w6x. Briefly, in a 1080 dm3 quartz reactor, mixtures of 1,4-dioxane, methyl nitrite and NO in air at a total pressure of 760 Torr were irradiated with fluorescence lamps ŽPhilips TL 05r40 W.. Time resolved concentrations of 1,4-dioxane, EDF, NO 2 and ozone were measured by using long path FTIR spectroscopy. For NO, only the initial concentration of each experiment was obtained. In total, data sets of three independent experiments were available for the mechanism validation. All computer simulations were carried out by using the box-model SBOX by Seefeld and Stockwell w7,8x. This FORTRAN program incorporating the Gear algorithm w9x was operated on an SGI Origin 200 workstation running under IRIX 6.5. The program uses the public domain library VODE w10x to integrate the ordinary differential equations.
3. Results and discussion 3.1. Photolysis reactions, inorganic chemistry and OH source In order to describe the photolysis processes in the reaction system investigated in the present work a set of photolysis reactions taken from the RACM
mechanism by Stockwell et al. w11x was used Žreactions 1–11 of the original RACM notation w11x.. The only photolysis frequency, which was experimentally measured under the present conditions, was that of NO 2 for photodissociation into OŽ3 P. and NO w12x. All other photolysis frequencies were not measured and had to be estimated. It can be assumed that the photolysis behaviour of the different species in the photoreactor is almost similar to that in the troposphere when suitable lamps are used. Under this assumption, photolysis frequencies can be calculated for all species relative to JŽNO 2 . for atmospheric conditions by using the algorithm of Madronich et al. w13x. This approximation is justifiable because the radiation strength in the photoreactor is much weaker than in the atmosphere, and the influence of the photolysis processes on the radical budget in the reactor is low for most of the treated species, as was shown in a previous study w14x. In order to describe the inorganic processes in the model, a set of 35 inorganic reactions taken from the RACM mechanism of Stockwell et al. w11x has been added to the chemical model used in the present study Žreactions 24–58 of the original RACM notation w11x.. In the indoor photoreactor experiments of Maurer et al. w2x relevant to this study, OH radicals were formed by the photolysis of methyl nitrite, CH 3 ONO, in the presence of NO. Since it was not possible to accurately measure the initial concentration of CH 3 ONO, the OH precursor in the experiments, the ‘real’ concentration of CH 3 ONO was obtained from a fit of the simulated VOC profiles to the experimental data by variation of the initial CH 3 ONO concentration. Since the CH 3 ONO photolysis should be the only relevant OH source under the given conditions, this approximation can be made. A detailed description of this procedure as well as the elementary reactions considered in the present model for the chemistry of CH 3 ONO is given elsewhere w14x. No significant wall processes for any of the reactants have been observed in the experimental studies of Maurer et al. w2x. Accordingly, no wall reactions were considered in the present reaction scheme. 3.2. Degradation mechanism of 1,4-dioxane The chemical degradation mechanism of 1,4-dioxane proposed in the present Letter is widely based
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
on the studies of Maurer et al. w2x and Platz et al. w5x, who recently investigated the oxidation of 1,4-dioxane in the laboratory. Fig. 1 illustrates the degradation of 1,4-dioxane in the presence of NOx . The first step is an abstraction of one of the eight chemically equivalent H atoms by OH, followed by O 2 addition to the resulting 1,4-dioxyl radical. For the 1,4-dioxylperoxyl radical ŽI., three reaction pathways are possible. The major part will convert NO to NO 2 , yielding the corresponding alkoxyl radical ŽII.. The formation of a peroxyl nitrate by reaction of the peroxyl radical ŽII. with NO 2 proposed by Platz et al. w5x is negligible under the present conditions, since the resulting peroxyl nitrate will not be stable at room temperature. This is confirmed by the fact that Maurer et al. w2x did not observe the formation of peroxyl nitrates in their experiments. Platz et al. w5x showed that the only important reaction of the alkoxyl radical ŽII. is a very fast ring opening leading to the corresponding HCŽO.OŽCH 2 . 2 OCH 2 radical ŽIII., which adds molecular
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oxygen forming another peroxyl radical, HCŽO.OŽCH 2 . 2 OCH 2 O 2 ŽIV.. The major fraction of this peroxyl radical will convert NO to NO 2 leading to HCŽO.OŽCH 2 . 2 OCH 2 O ŽV.. As discussed above, there is no evidence for the formation of a peroxyl nitrate HCŽO.OŽCH 2 . 2 OCH 2 O 2 NO 2 under the present conditions. As a consequence, this possibility was not considered in Fig. 1. Finally, the reaction of the HCŽO.OŽCH 2 . 2OCH 2 O radical ŽV. with molecular oxygen leads to ethylene-1,2-diformate ŽEDF., the only primary reaction product which has been detected in the OH- and Cl-initiated oxidation of 1,4-dioxane in the presence of NOx and O 2 in the laboratory w2,5x. The rate coefficients used for the present computer simulations of the OH-initiated oxidation of 1,4-dioxane in the presence of NOx were taken from literature w2,4,5x. When no literature data were available, rate coefficients were estimated by using data for similar reactions w15,16x. All simulations were carried out for room temperature conditions. The rate
Fig. 1. Tropospheric degradation scheme for the OH-initiated oxidation of 1,4-dioxane in the presence of NOx .
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coefficients used in the model are summarised in Table 1. The experimental data available for the tropospheric degradation of 1,4-dioxane w2,5x give no adequate information about the possible formation of nitrates from the reactions of the intermediate peroxyl radicals ŽC 4 H 7 O 2 .O 2 ŽI. and HCŽO.OŽCH 2 . 2OCH 2 O 2 ŽIV. with NO. Though Maurer et al. w2x did not observe any nitrate bands in the residual FTIR product spectra it can be assumed that – analogous to alkane oxidation – small amounts of nitrates might be formed in the OH-initiated oxidation of 1,4-dioxane. Hence, for the present chemical model, nitrate formation with fractions of 2.5% for each of the overall reactions of the peroxyl radicals ŽI. and ŽIV. with NO ŽR4 and R8. were postulated. The values of 2.5% are reasonable for the nitrate channels of alkylperoxyl radical reactions with NO investigated before w14x. It was shown that consideration of nitrate formation in the model led to a much better agreement of experimental and simulated data Žsee below., indicating that the model is highly sensitive towards reactions of the type RO 2 q NO. In addition, the reaction mechanism was completed by a set of peroxy radical reactions RO 2rRO 2 and RO 2rHO 2 ŽR10–R14.. However, the influence
of these reactions on the model calculations is low, since the NO concentrations in the experimental systems investigated are sufficiently high. The model used for the simulations of the present work consisted of 70 elementary reactions. Fig. 2 shows as an example the comparison of experimental data of Maurer et al. w2x and simulated concentration–time profiles for 1,4-dioxane, ethylene-1,2-diformate, NO 2 and ozone, respectively. Experimental and modelling results are in excellent agreement for all reactants. For the further two experimental sets available, the deviations of the simulation results are similarly small. This indicates that the degradation scheme developed in the present Letter will describe the tropospheric chemistry of 1,4-dioxane in further applications reasonably well for regimes characterised by large NOx mixing ratios, e.g., urban and source-near locations. As mentioned above, the agreement between modelled and experimental concentrations could be increased by integration of small fractions of nitrate formation from the reactions of the peroxyl radicals ŽI. and ŽIV. with NO ŽR4 and R8.. When the rate coefficients of these reactions are set to zero, significant deviations occur for the simulated concentration–time profiles, which is demonstrated in Fig. 3
Table 1 Chemical mechanism for the OH-initiated oxidation of 1,4-dioxane in the presence of NOx used for the computer simulations in the present study No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
OH q 1,4-dioxolane R q O 2 ŽqM. RO 2 q NO RO 2 q NO ŽqM. RO ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 q O 2 ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 O 2 q NO HCŽO.OŽCH 2 . 2 OCH 2 O 2 q NO ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 O q O 2 RO 2 q HO 2 2 RO 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 q RO 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 q HO 2 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 OH q EDF
a
™ R q HŽ O . ™ RO qM ™ RO q NOŽ . ™ RONO qM ™ HCŽŽO..OŽŽCH .. OCH ŽqMŽ . . ™ HC O O CH OCH O qM ™ HCŽŽO..OŽŽCH .. OCH O q NOŽ . ™ HC O O CH OCH ONO qM ™ EDF q HO ™ products ™ products ™ products ™ products ™ products ™ products
In cm3 sy1 Žsecond order. and sy1 Žfirst order.. Averaged rate coefficient from literature data ŽRefs. w2–4x.. c Estimated from similar reactions ŽRefs. w15,16x.. b
2
2
2
2
2 2
2
2
2 2
2
2 2
2
2 2
2
2
2
2
k Ž298 K. a
Ref.
1.1 = 10y1 1 5 = 10y12 1.2 = 10y11 3.3 = 10y13 1 = 10 5 8.8 = 10y12 9.0 = 10y12 2.3 = 10y13 8 = 10y15 1 = 10y11 7.3 = 10y12 7 = 10y12 1 = 10y11 7 = 10y12 4.7 = 10y13
b c
w5x c c
w5x w5x c c c
w5x c c c
w2x
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for 1,4-dioxane, EDF, NO 2 and ozone. Especially the concentrations of ozone and EDF are clearly overestimated by the model if nitrate formation is not considered. From this observation it can be assumed that a small product fraction of the oxidation of 1,4-dioxane in the presence of NOx certainly will consist of organic nitrates. Additionally, the missing 5–10% C of reaction products will also include further reactants with yields below the detection limits of the experiments carried out w2,5x. Besides organic nitrates, another possible reaction product is 3-oxo-d-lactone, which might be formed in small amounts by reaction of the alkoxyl radical ŽII. with molecular oxygen ŽR16.:
Finally, also reaction products of the isomerisation of HCŽO.OŽCH 2 . 2 OCH 2 O radical ŽV., which are expected to be formic acid anhydride, formic acid and formaldehyde w2x, are conceivable. However, Maurer et al. w2x and Platz et al. w5x were not able to detect these compounds in their reaction systems containing NOx , indicating that the branching ratios for the
Fig. 3. Percental deviations of simulated concentration–time profiles of 1,4-dioxane, EDF, NO 2 and ozone to those given in Fig. 2, when the rates of nitrate forming reactions R4 and R8 are set to zero.
additional reaction channels discussed above are very small. 3.3. SensitiÕity analysis In order to identify the most important steps of the present reaction scheme and for further interpretation of the mechanism, a sensitivity analysis was carried out by using the direct decoupled method of Dunker w17x. The sensitivities of selected species to the rate coefficients of the reactions were calculated. Relative sensitivity coefficients, Sri , were calculated using Eq. Ž1. as described by Stockwell et al. w18x: Sri s
Fig. 2. Oxidation of 1,4-dioxane: comparison of experimental w2x and simulated concentration–time profiles for 1,4-dioxane ŽB., EDF ŽI., NO 2 Ž`. and ozone Žv .. The solid lines are the results of computer simulations. Initial conditions: 0.87 ppm 1,4-dioxane, 0.80 ppm NO, 0.04 ppm NO 2 , 1.15 ppm CH 3 ONO, 760 Torr air.
Sai ÝSai
,
Ž 1.
where Sai are time-averaged normalised sensitivity coefficients, obtained by averaging the absolute values of the normalised time dependent sensitivity coefficients over the simulation time period. Fig. 4 shows the relative sensitivities of NO 2 , NO, 1,4-dioxane and EDF to selected rate coefficients for the example run illustrated in Fig. 2. A positive sensitivity indicates that an increase of the rate coefficient will cause an increase of the corresponding reactant concentration, while increasing a
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Fig. 4. Relative sensitivity of NO 2 , NO, 1,4-dioxane and EDF to selected reaction rate coefficients of the 1,4-dioxane degradation scheme. Initial conditions are given in Fig. 2.
rate constant with a negative sensitivity will lead to a smaller concentration of the corresponding species. The plot clearly shows that the reaction system is dominated by the photolysis of methyl nitrite, which is the only significant OH source under the present conditions. Other important steps are the reactions of OH with 1,4-dioxane, NO 2 and NO. Also the sensitivity of the system on nitrate formation in reactions R4 and R8 discussed above can be taken from Fig. 4, though the sensitivity coefficients of these reactions are far smaller than that for methyl nitrite photolysis and the reactions of OH with 1,4-dioxane, NO 2 and NO. Apart from the photodissociation of CH 3 ONO, the system is relatively insensitive to photolysis processes. This can be explained with the spectral characteristics of the fluorescence lamps used in the experiments w2x. The UV fraction of these lamps is small. As a consequence the photolysis frequencies
in the system are lower than under ‘real’ atmospheric conditions.
4. Summary In the present Letter, a tropospheric degradation mechanism for 1,4-dioxane in the presence of NOx was postulated and checked by comparison with suitable laboratory data. Experimental and modelling results are in excellent agreement. Accordingly, these reaction schemes might be used successfully in further applications, e.g., chemistry transport modelling. The results of the sensitivity analysis carried out for the investigated reaction system indicate that photoreactor experiments using methyl nitrite as OH precursor and relatively high concentrations of NOx are characterised by well-defined conditions, which
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
can be modelled with high accuracy. As a consequence, such experiments are highly suitable for mechanism validation procedures for mechanisms under urban conditions.
Acknowledgements Support for this research was provided by the German Bundesministerium fur ¨ Bildung, Wissenschaft, Forschung und Technologie ŽBMBF., project ‘Forderschwerpunkt Tropospharenforschung ¨ ¨ ŽTFS.,’ contract 07TFS30. The authors thank W.R. Stockwell of Reno, NV, USA; and J.B. Milford of Boulder, CO, USA, for the supply of software and very helpful discussions.
References w1x G.W. Ware ŽEd.., Reviews of Environmental Contamination and Toxicology, Vol. 106, Springer, New York, 1988, p. 113. w2x T. Maurer, H. Hass, I. Barnes, K.H. Becker, J. Phys. Chem. A 103 Ž1999. 5032.
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w3x E. Porter, J. Wenger, J. Treacy, H. Sidebottom, A. Mellouki, S. Teton, G. Le Bras, J. Phys. Chem. A 101 Ž1997. 5770. ´ w4x P. Dagaut, R. Liu, T.J. Wallington, M.J. Kurylo, J. Phys. Chem. 94 Ž1990. 1881. w5x J. Platz, J. Sehested, T. Møgelberg, O.J. Nielsen, T.J. Wallington, J. Chem. Soc. Faraday Trans. 93 Ž1997. 2855. w6x I. Barnes, K.H. Becker, N. Mihalopoulos, J. Atmos. Chem. 18 Ž1994. 267. w7x S. Seefeld, Ph.D. Thesis, Swiss Federal Institute of Technology Zurich ŽETH., Switzerland, 1997. w8x S. Seefeld, W.R. Stockwell, Atmos. Environ. 33 Ž1999. 2941. w9x C.W. Gear, Prentice-Hall Series in Automatic Computation, Vol. 17, Prentice-Hall, Englewood Cliffs, NJ, 1971. w10x P.N. Brown, G.D. Byrne, A.C. Hindmarsh, J. Sci. Stat. Comput. 10 Ž1989. 1038. w11x W.R. Stockwell, F. Kirchner, M. Kuhn, S. Seefeld, J. Geophys. Res. 102 Ž1997. 25847. w12x C.G. Sauer, I. Barnes, K.H. Becker, H. Geiger, T.J. Wallington, L.K. Christensen, J. Platz, O.J. Nielsen, J. Phys. Chem. A 103 Ž1999. 5959. w13x S. Madronich, J. Geophys. Res. 92 Ž1987. 9740. w14x H. Geiger, K.H. Becker, Atmos. Environ. 33 Ž1999. 2883. w15x R. Atkinson, J. Phys. Chem. Ref. Data, Monograph 2, 1994, and references therein. w16x W.B. DeMore et al., Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation No. 12, JPL Publication 97-4, Pasadena, CA, 1997. w17x A. Dunker, J. Chem. Phys. 81 Ž1984. 2385. w18x W.R. Stockwell, J.B. Milford, D. Gao, Y.J. Yang, Atmos. Environ. 29 Ž1995. 1599.
Chemical Physics Letters 314 Ž1999. 465–471 www.elsevier.nlrlocatercplett
OH-initiated degradation mechanism of 1,4-dioxane in the presence of NOx H. Geiger ) , T. Maurer, K.H. Becker Bergische UniÕersitat ¨ – Gesamthochschule Wuppertal, Physikalische Chemier Fachbereich 9, D-42097 Wuppertal, Germany Received 10 August 1999; in final form 22 September 1999
Abstract Based on the results of indoor photoreactor studies, a photochemical mechanism has been developed for the OH-initiated degradation of 1,4-dioxane in the presence of NOx . The reaction scheme constructed has been tested by comparison of computer box model calculations and experimental data taken from indoor photoreactor studies. Experimentally obtained and modelled concentration–time profiles for selected reactants are in excellent agreement. A sensitivity analysis was performed for the reaction system. These calculations highlight the key reactions of the treated chemical model. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The increasing use of oxygenated organic compounds such as ethers, esters and alcohols as fuel additives, alternative fuels or solvents during the last years will lead to an increasing influence of these species on tropospheric chemistry. While the rate coefficients for the OH reactions of many relevant oxygenated VOCs are mostly well established, knowledge of the photooxidant mechanisms of these compounds is relatively poor. The mechanistic studies are often limited to the detection of the primary reaction products. Reaction mechanisms which are derived from these data have usually not been proven by comparison of the measured product concentrations against model simulations of chamber experiments. In other cases, where no suitable experimental results are available, reaction mechanisms are ) Corresponding author. Fax: q49-202-439-2757; e-mail: [email protected]
postulated without any further verification. Both methods are characterised by a high uncertainty when the model is applied, e.g., to chemistry transport modelling. In the present study, the chemical degradation scheme for 1,4-dioxane was constructed considering both experimental data and computer simulations. 1,4-dioxane is currently used as alternative solvent for cellulose acetate, resins, oils and waxes w1x and therefore is expected to become important in tropospheric photochemistry in the near future. The rate coefficient for the reaction of OH radicals with 1,4-dioxane was recently determined by Maurer et al. w2x and also by Porter et al. w3x and Dagaut et al. w4x. Maurer et al. w2x and Platz et al. w5x carried out extensive product studies of the tropospheric degradation of 1,4-dioxane in the laboratory using OH radicals w2x or chlorine atoms w5x for the initiation of the radical chain. Both authors detected ethylene1,2-diformate ŽEDF. as the only reaction product in
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 1 5 - X
466
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
the presence of NOx . Maurer et al. w2x report a yield for EDF of 87–95 mol%. Platz et al. w5x were not able to calibrate the EDF concentrations obtained in their experiments because no reference spectrum was available. The few missing primary reaction products are mainly expected to be organic nitrates. Since the carbon yield for the OH-initiated oxidation of 1,4-dioxane is much less than 100% in the absence of NOx w2x, the present investigations are focused on high NOx conditions such as are found in the urban atmosphere.
2. Experimental data and computer simulation system The experimental data used in this work for the validation of the degradation mechanism of 1,4-dioxane were obtained by Maurer et al. w2x. The experimental set-up used for these experiments has been described in detail elsewhere w6x. Briefly, in a 1080 dm3 quartz reactor, mixtures of 1,4-dioxane, methyl nitrite and NO in air at a total pressure of 760 Torr were irradiated with fluorescence lamps ŽPhilips TL 05r40 W.. Time resolved concentrations of 1,4-dioxane, EDF, NO 2 and ozone were measured by using long path FTIR spectroscopy. For NO, only the initial concentration of each experiment was obtained. In total, data sets of three independent experiments were available for the mechanism validation. All computer simulations were carried out by using the box-model SBOX by Seefeld and Stockwell w7,8x. This FORTRAN program incorporating the Gear algorithm w9x was operated on an SGI Origin 200 workstation running under IRIX 6.5. The program uses the public domain library VODE w10x to integrate the ordinary differential equations.
3. Results and discussion 3.1. Photolysis reactions, inorganic chemistry and OH source In order to describe the photolysis processes in the reaction system investigated in the present work a set of photolysis reactions taken from the RACM
mechanism by Stockwell et al. w11x was used Žreactions 1–11 of the original RACM notation w11x.. The only photolysis frequency, which was experimentally measured under the present conditions, was that of NO 2 for photodissociation into OŽ3 P. and NO w12x. All other photolysis frequencies were not measured and had to be estimated. It can be assumed that the photolysis behaviour of the different species in the photoreactor is almost similar to that in the troposphere when suitable lamps are used. Under this assumption, photolysis frequencies can be calculated for all species relative to JŽNO 2 . for atmospheric conditions by using the algorithm of Madronich et al. w13x. This approximation is justifiable because the radiation strength in the photoreactor is much weaker than in the atmosphere, and the influence of the photolysis processes on the radical budget in the reactor is low for most of the treated species, as was shown in a previous study w14x. In order to describe the inorganic processes in the model, a set of 35 inorganic reactions taken from the RACM mechanism of Stockwell et al. w11x has been added to the chemical model used in the present study Žreactions 24–58 of the original RACM notation w11x.. In the indoor photoreactor experiments of Maurer et al. w2x relevant to this study, OH radicals were formed by the photolysis of methyl nitrite, CH 3 ONO, in the presence of NO. Since it was not possible to accurately measure the initial concentration of CH 3 ONO, the OH precursor in the experiments, the ‘real’ concentration of CH 3 ONO was obtained from a fit of the simulated VOC profiles to the experimental data by variation of the initial CH 3 ONO concentration. Since the CH 3 ONO photolysis should be the only relevant OH source under the given conditions, this approximation can be made. A detailed description of this procedure as well as the elementary reactions considered in the present model for the chemistry of CH 3 ONO is given elsewhere w14x. No significant wall processes for any of the reactants have been observed in the experimental studies of Maurer et al. w2x. Accordingly, no wall reactions were considered in the present reaction scheme. 3.2. Degradation mechanism of 1,4-dioxane The chemical degradation mechanism of 1,4-dioxane proposed in the present Letter is widely based
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
on the studies of Maurer et al. w2x and Platz et al. w5x, who recently investigated the oxidation of 1,4-dioxane in the laboratory. Fig. 1 illustrates the degradation of 1,4-dioxane in the presence of NOx . The first step is an abstraction of one of the eight chemically equivalent H atoms by OH, followed by O 2 addition to the resulting 1,4-dioxyl radical. For the 1,4-dioxylperoxyl radical ŽI., three reaction pathways are possible. The major part will convert NO to NO 2 , yielding the corresponding alkoxyl radical ŽII.. The formation of a peroxyl nitrate by reaction of the peroxyl radical ŽII. with NO 2 proposed by Platz et al. w5x is negligible under the present conditions, since the resulting peroxyl nitrate will not be stable at room temperature. This is confirmed by the fact that Maurer et al. w2x did not observe the formation of peroxyl nitrates in their experiments. Platz et al. w5x showed that the only important reaction of the alkoxyl radical ŽII. is a very fast ring opening leading to the corresponding HCŽO.OŽCH 2 . 2 OCH 2 radical ŽIII., which adds molecular
467
oxygen forming another peroxyl radical, HCŽO.OŽCH 2 . 2 OCH 2 O 2 ŽIV.. The major fraction of this peroxyl radical will convert NO to NO 2 leading to HCŽO.OŽCH 2 . 2 OCH 2 O ŽV.. As discussed above, there is no evidence for the formation of a peroxyl nitrate HCŽO.OŽCH 2 . 2 OCH 2 O 2 NO 2 under the present conditions. As a consequence, this possibility was not considered in Fig. 1. Finally, the reaction of the HCŽO.OŽCH 2 . 2OCH 2 O radical ŽV. with molecular oxygen leads to ethylene-1,2-diformate ŽEDF., the only primary reaction product which has been detected in the OH- and Cl-initiated oxidation of 1,4-dioxane in the presence of NOx and O 2 in the laboratory w2,5x. The rate coefficients used for the present computer simulations of the OH-initiated oxidation of 1,4-dioxane in the presence of NOx were taken from literature w2,4,5x. When no literature data were available, rate coefficients were estimated by using data for similar reactions w15,16x. All simulations were carried out for room temperature conditions. The rate
Fig. 1. Tropospheric degradation scheme for the OH-initiated oxidation of 1,4-dioxane in the presence of NOx .
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coefficients used in the model are summarised in Table 1. The experimental data available for the tropospheric degradation of 1,4-dioxane w2,5x give no adequate information about the possible formation of nitrates from the reactions of the intermediate peroxyl radicals ŽC 4 H 7 O 2 .O 2 ŽI. and HCŽO.OŽCH 2 . 2OCH 2 O 2 ŽIV. with NO. Though Maurer et al. w2x did not observe any nitrate bands in the residual FTIR product spectra it can be assumed that – analogous to alkane oxidation – small amounts of nitrates might be formed in the OH-initiated oxidation of 1,4-dioxane. Hence, for the present chemical model, nitrate formation with fractions of 2.5% for each of the overall reactions of the peroxyl radicals ŽI. and ŽIV. with NO ŽR4 and R8. were postulated. The values of 2.5% are reasonable for the nitrate channels of alkylperoxyl radical reactions with NO investigated before w14x. It was shown that consideration of nitrate formation in the model led to a much better agreement of experimental and simulated data Žsee below., indicating that the model is highly sensitive towards reactions of the type RO 2 q NO. In addition, the reaction mechanism was completed by a set of peroxy radical reactions RO 2rRO 2 and RO 2rHO 2 ŽR10–R14.. However, the influence
of these reactions on the model calculations is low, since the NO concentrations in the experimental systems investigated are sufficiently high. The model used for the simulations of the present work consisted of 70 elementary reactions. Fig. 2 shows as an example the comparison of experimental data of Maurer et al. w2x and simulated concentration–time profiles for 1,4-dioxane, ethylene-1,2-diformate, NO 2 and ozone, respectively. Experimental and modelling results are in excellent agreement for all reactants. For the further two experimental sets available, the deviations of the simulation results are similarly small. This indicates that the degradation scheme developed in the present Letter will describe the tropospheric chemistry of 1,4-dioxane in further applications reasonably well for regimes characterised by large NOx mixing ratios, e.g., urban and source-near locations. As mentioned above, the agreement between modelled and experimental concentrations could be increased by integration of small fractions of nitrate formation from the reactions of the peroxyl radicals ŽI. and ŽIV. with NO ŽR4 and R8.. When the rate coefficients of these reactions are set to zero, significant deviations occur for the simulated concentration–time profiles, which is demonstrated in Fig. 3
Table 1 Chemical mechanism for the OH-initiated oxidation of 1,4-dioxane in the presence of NOx used for the computer simulations in the present study No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
OH q 1,4-dioxolane R q O 2 ŽqM. RO 2 q NO RO 2 q NO ŽqM. RO ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 q O 2 ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 O 2 q NO HCŽO.OŽCH 2 . 2 OCH 2 O 2 q NO ŽqM. HCŽO.OŽCH 2 . 2 OCH 2 O q O 2 RO 2 q HO 2 2 RO 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 q RO 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 q HO 2 2 HCŽO.OŽCH 2 . 2 OCH 2 O 2 OH q EDF
a
™ R q HŽ O . ™ RO qM ™ RO q NOŽ . ™ RONO qM ™ HCŽŽO..OŽŽCH .. OCH ŽqMŽ . . ™ HC O O CH OCH O qM ™ HCŽŽO..OŽŽCH .. OCH O q NOŽ . ™ HC O O CH OCH ONO qM ™ EDF q HO ™ products ™ products ™ products ™ products ™ products ™ products
In cm3 sy1 Žsecond order. and sy1 Žfirst order.. Averaged rate coefficient from literature data ŽRefs. w2–4x.. c Estimated from similar reactions ŽRefs. w15,16x.. b
2
2
2
2
2 2
2
2
2 2
2
2 2
2
2 2
2
2
2
2
k Ž298 K. a
Ref.
1.1 = 10y1 1 5 = 10y12 1.2 = 10y11 3.3 = 10y13 1 = 10 5 8.8 = 10y12 9.0 = 10y12 2.3 = 10y13 8 = 10y15 1 = 10y11 7.3 = 10y12 7 = 10y12 1 = 10y11 7 = 10y12 4.7 = 10y13
b c
w5x c c
w5x w5x c c c
w5x c c c
w2x
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
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for 1,4-dioxane, EDF, NO 2 and ozone. Especially the concentrations of ozone and EDF are clearly overestimated by the model if nitrate formation is not considered. From this observation it can be assumed that a small product fraction of the oxidation of 1,4-dioxane in the presence of NOx certainly will consist of organic nitrates. Additionally, the missing 5–10% C of reaction products will also include further reactants with yields below the detection limits of the experiments carried out w2,5x. Besides organic nitrates, another possible reaction product is 3-oxo-d-lactone, which might be formed in small amounts by reaction of the alkoxyl radical ŽII. with molecular oxygen ŽR16.:
Finally, also reaction products of the isomerisation of HCŽO.OŽCH 2 . 2 OCH 2 O radical ŽV., which are expected to be formic acid anhydride, formic acid and formaldehyde w2x, are conceivable. However, Maurer et al. w2x and Platz et al. w5x were not able to detect these compounds in their reaction systems containing NOx , indicating that the branching ratios for the
Fig. 3. Percental deviations of simulated concentration–time profiles of 1,4-dioxane, EDF, NO 2 and ozone to those given in Fig. 2, when the rates of nitrate forming reactions R4 and R8 are set to zero.
additional reaction channels discussed above are very small. 3.3. SensitiÕity analysis In order to identify the most important steps of the present reaction scheme and for further interpretation of the mechanism, a sensitivity analysis was carried out by using the direct decoupled method of Dunker w17x. The sensitivities of selected species to the rate coefficients of the reactions were calculated. Relative sensitivity coefficients, Sri , were calculated using Eq. Ž1. as described by Stockwell et al. w18x: Sri s
Fig. 2. Oxidation of 1,4-dioxane: comparison of experimental w2x and simulated concentration–time profiles for 1,4-dioxane ŽB., EDF ŽI., NO 2 Ž`. and ozone Žv .. The solid lines are the results of computer simulations. Initial conditions: 0.87 ppm 1,4-dioxane, 0.80 ppm NO, 0.04 ppm NO 2 , 1.15 ppm CH 3 ONO, 760 Torr air.
Sai ÝSai
,
Ž 1.
where Sai are time-averaged normalised sensitivity coefficients, obtained by averaging the absolute values of the normalised time dependent sensitivity coefficients over the simulation time period. Fig. 4 shows the relative sensitivities of NO 2 , NO, 1,4-dioxane and EDF to selected rate coefficients for the example run illustrated in Fig. 2. A positive sensitivity indicates that an increase of the rate coefficient will cause an increase of the corresponding reactant concentration, while increasing a
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H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
Fig. 4. Relative sensitivity of NO 2 , NO, 1,4-dioxane and EDF to selected reaction rate coefficients of the 1,4-dioxane degradation scheme. Initial conditions are given in Fig. 2.
rate constant with a negative sensitivity will lead to a smaller concentration of the corresponding species. The plot clearly shows that the reaction system is dominated by the photolysis of methyl nitrite, which is the only significant OH source under the present conditions. Other important steps are the reactions of OH with 1,4-dioxane, NO 2 and NO. Also the sensitivity of the system on nitrate formation in reactions R4 and R8 discussed above can be taken from Fig. 4, though the sensitivity coefficients of these reactions are far smaller than that for methyl nitrite photolysis and the reactions of OH with 1,4-dioxane, NO 2 and NO. Apart from the photodissociation of CH 3 ONO, the system is relatively insensitive to photolysis processes. This can be explained with the spectral characteristics of the fluorescence lamps used in the experiments w2x. The UV fraction of these lamps is small. As a consequence the photolysis frequencies
in the system are lower than under ‘real’ atmospheric conditions.
4. Summary In the present Letter, a tropospheric degradation mechanism for 1,4-dioxane in the presence of NOx was postulated and checked by comparison with suitable laboratory data. Experimental and modelling results are in excellent agreement. Accordingly, these reaction schemes might be used successfully in further applications, e.g., chemistry transport modelling. The results of the sensitivity analysis carried out for the investigated reaction system indicate that photoreactor experiments using methyl nitrite as OH precursor and relatively high concentrations of NOx are characterised by well-defined conditions, which
H. Geiger et al.r Chemical Physics Letters 314 (1999) 465–471
can be modelled with high accuracy. As a consequence, such experiments are highly suitable for mechanism validation procedures for mechanisms under urban conditions.
Acknowledgements Support for this research was provided by the German Bundesministerium fur ¨ Bildung, Wissenschaft, Forschung und Technologie ŽBMBF., project ‘Forderschwerpunkt Tropospharenforschung ¨ ¨ ŽTFS.,’ contract 07TFS30. The authors thank W.R. Stockwell of Reno, NV, USA; and J.B. Milford of Boulder, CO, USA, for the supply of software and very helpful discussions.
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