Rate constants for the atmospheric reactions of alkoxy radicals: An updated estimation method
Introduction
Volatile organic compounds (VOCs) emitted into the atmosphere can generally undergo photolysis and/or chemical reaction with OH radicals, NO3 radicals, Cl atoms, and O3 (Atkinson and Arey, 2003), with the dominant reaction occurring depending on the specific VOC. However, in most cases the reaction involves formation, at least in part, of an alkyl- or substituted alkyl radical (Atkinson and Arey, 2003). In general, these alkyl or substituted alkyl radicals react in the troposphere as shown schematically in Fig. 1, with the key intermediates being organic peroxy (RO2) and alkoxy (RO) radicals. Organic peroxy radicals react with NO, NO2, NO3 radicals, HO2 radicals and organic peroxy radicals in the atmosphere (Atkinson and Arey, 2003; Collins et al., 2005; Atkinson et al., 2006; IUPAC, 2007), with their reactions with NO, organic peroxy radicals and NO3 radicals leading in part to the formation of alkoxy radicals. In the presence of sufficient NO that RO2+NO reactions dominate, the key intermediate species are alkoxy radicals (Fig. 1) and their atmospheric reactions then determine the majority of first-generation products (Devolder, 2003; Atkinson and Arey, 2003; Orlando et al., 2003; Mellouki et al., 2003; Atkinson et al., 2006; IUPAC, 2007).
Under tropospheric conditions, most alkoxy radicals potentially undergo reaction with O2, unimolecular decomposition, and unimolecular isomerization (Devolder, 2003; Atkinson and Arey, 2003), as shown in Fig. 2 for the 2-pentoxy radical. Alkoxy radicals of structure RC(O)OCH(O)R′ formed from esters can also undergo isomerization proceeding through a 5-membered transition state (Tuazon et al., 1998; Christensen et al., 2000).RC(O)OCH(O)R′→RC(O)OH+R′CO.
The database concerning the atmospherically relevant reactions of alkoxy radicals is still rather limited, especially if only absolute rate data are considered. In the sections below the literature data on alkoxy radical reactions with O2, isomerization, and decomposition are summarized, and recommendations made for estimation of rate constants of these reactions. Atkinson (1997) proposed a structure–reactivity relationship (SAR) for estimating the importance of the various alkoxy radical reaction pathways at room temperature, and temperature-dependent expressions were proposed for alkoxy radical decompositions and for reactions with O2, but not for the isomerizations. This SAR has been used in the formulation of the University of Leeds Master Chemical Mechanism, version 3 (Saunders et al., 2003). The original estimation scheme of Atkinson (1997) was slightly revised by Aschmann and Atkinson (1999) to take into account product data from the OH radical-initiated reactions of ethers, and this version was also summarized in the Supporting Information to Aschmann et al. (2001).
There is a continuing need for an estimation method allowing the relative importance of the various alkoxy radical reactions to be reliably calculated for the very large number of larger alkoxy radicals formed during the atmospheric degradations of VOCs, so that these reactions can be included in atmospheric chemistry computer models. At the time of the Atkinson (1997) article, absolute rate constants for ⩾C2 alkoxy radicals were only available for the reactions of ethoxy and 2-propoxy radicals with O2 (Gutman et al., 1982; Balla et al., 1985; Hartmann et al., 1990), and few useful (for development of SARs) relative rate studies had been carried out at room temperature and atmospheric pressure (Carter et al., 1979; Cox et al., 1981; Niki et al., 1981; Atkinson et al., 1995a). Since 1997 a significant number of absolute and relative rate studies of alkoxy radical reactions have been reported, and an update of the Atkinson (1997) estimation method is warranted. However, rate data for alkoxy radicals are still only available for a handful of simple alkoxy radicals, generally the smaller, ⩽C6, alkoxy radicals formed during alkane photooxidations. Unfortunately, these are precisely those alkoxy radicals for which fall-off effects in the decomposition and isomerization reactions are most evident and this leads to added difficulties in deriving estimation methods for the rates of these reaction pathways for the larger alkoxy radicals, which are expected to be at the high-pressure limit under atmospheric conditions.
Section snippets
Reaction with O2
A number of relative rate studies have measured rate constants for alkoxy radical decomposition (kdecomp) or isomerization (kisom) relative to the rate constant for the corresponding O2 reaction () at atmospherically relevant temperatures (Carter et al., 1979; Cox et al., 1981; Niki et al., 1981; Lightfoot et al., 1990; Wallington et al., 1992; Atkinson et al., 1992, Atkinson et al., 1995a; Aschmann et al., 1997; Platz et al., 1999; Orlando et al., 2000a; Libuda et al., 2002; Geiger et al.,
Isomerization
As shown in Fig. 2, if an abstractable hydrogen is available on the 4th carbon atom from that to which the alkoxy “O” is attached the alkoxy radical can undergo isomerization through a 6-member transition state to form a 4-hydroxyalkyl radical. To date, few useful rate data are available concerning the rates of these alkoxy radical isomerizations (Carter et al., 1979; Cox et al., 1981; Niki et al., 1981; Atkinson et al., 1995a; Hein et al., 1999; Geiger et al., 2002; Johnson et al., 2004,
Decomposition
Absolute rate constants for the decomposition of alkoxy radicals, kdecomp, are available only for ethoxy (Caralp et al., 1999; Fittschen et al., 2000), 2-propoxy (Balla et al., 1985; Devolder et al., 1999; Fittschen et al., 2000), 2-butoxy (Hein et al., 1998; Falgayrac et al., 2004), tert-butoxy (Blitz et al., 1999; Fittschen et al., 2000) and 3-pentoxy (Hein et al., 2000) radicals. These reactions are all in the fall-off region at atmospheric pressure and below at the temperatures studied, and
Conclusions and atmospheric implications
Despite the significantly more extensive database presently available for alkoxy radical reactions from both absolute and relative rate studies used in this updated estimation method, the changes in alkoxy radical reaction rates compared to the earlier predictions (Atkinson, 1997; Aschmann and Atkinson, 1999) are, for the most part, fairly minor. Although the temperature dependence of the O2 reaction rate constants is slightly changed, the 298 K rate constants are unchanged. The 298 K rate
Ackowledgements
The author thanks the National Science Foundation (Grant nos. ATM-0234586 and ATM-0650061) for supporting this research. While this research has been supported by the NSF, it has not been reviewed by the NSF and no official endorsement should be inferred. The author also thanks the University of California Agricultural Experiment Station for partial salary support and the two anonymous reviewers for their helpful comments.
References (92)
- et al.
Kinetics of the reactions of isopropoxy radicals with NO, NO2, and O2
Chemical Physics
(1985) - et al.
An FT-IR study of the isomerization of 1-butoxy radicals under atmospheric conditions
Journal of Photochemistry and Photobiology A: Chemistry
(2006) - et al.
The influence of reaction conditions on the photooxidation of diisopropyl ether
Journal of Photochemistry and Photobiology A: Chemistry
(2005) - et al.
Direct kinetic studies of the reactions of 2-butoxy radicals with NO and O2
Chemical Physics Letters
(2000) Atmospheric fate of small alkoxy radicals: recent experimental and theoretical advances
Journal of Photochemistry and Photobiology A: Chemistry
(2003)- et al.
Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure–reactivity relationship: an update
Atmospheric Environment
(1995) - et al.
Products of the gas-phase reactions of the OH radical with n-butyl methyl ether and 2-isopropoxyethanol: reactions of ROC(O•)< radicals
International Journal of Chemical Kinetics
(1999) - et al.
Products of the gas-phase reaction of OH radicals with cyclohexane: reactions of the cyclohexoxy radical
Journal of Physical Chemistry A
(1997) - et al.
Formation of β-hydroxycarbonyls from the OH radical-initiated reactions of selected alkenes
Environmental Science and Technology
(2000) - et al.
Kinetic and product studies of the reactions of selected glycol ethers with OH radicals
Environmental Science and Technology
(2001)
Products of reaction of OH radicals with α-pinene
Journal of Geophysical Research
Kinetics and mechanism of the gas-phase reactions of the hydroxyl radical with organic compounds under atmospheric conditions
Chemical Reviews
A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds
International Journal of Chemical Kinetics
Gas-phase tropospheric chemistry of organic compounds
Journal of Physical and Chemical Reference Data Monograph
Atmospheric reactions of alkoxy and β-hydroxyalkoxy radicals
International Journal of Chemical Kinetics
Atmospheric oxidation
Atmospheric degradation of volatile organic compounds
Chemical Reviews
Products of the gas-phase OH radical-initiated reactions of 4-methyl-2-pentanone and 2,6-dimethyl-4-heptanone
International Journal of Chemical Kinetics
Formation of OH radicals in the gas phase reactions of O3 with a series of terpenes
Journal of Geophysical Research
Reactions of alkoxy radicals in the atmosphere
Faraday Discussions
Products of the gas-phase reactions of a series of 1-alkenes and 1-methylcyclohexene with the OH radical in the presence of NO
Environmental Science and Technology
Evaluated kinetic and photochemical data for atmospheric chemistry: volume II—gas phase reactions of organic species
Atmospheric Chemistry and Physics
Photochemical smog. Rate parameter estimates and computer simulations
Journal of Physical Chemistry
The gas-phase decomposition of alkoxy radicals
International Journal of Chemical Kinetics
Arrhenius parameters for the decomposition of the t-butoxy radical
International Journal of Chemical Kinetics
Decomposition of the t-butoxy radical—I. Studies over the temperature range 402–443 K
International Journal of Chemical Kinetics
The gas-phase pyrolysis of alkyl nitrites VI. t-Amyl nitrite
International Journal of Chemical Kinetics
Decomposition of the t-butoxy radical: II. Studies over the temperature range 303–393 K
International Journal of Chemical Kinetics
Kinetics and products of the reactions of selected diols with the OH radical
International Journal of Chemical Kinetics
Direct studies on the decomposition of the tert-butoxy radical and its reaction with NO
Physical Chemistry Chemical Physics
The thermal unimolecular decomposition rate constants of ethoxy radicals
Physical Chemistry Chemical Physics
Computer modeling of smog chamber data: progress in validation of a detailed mechanism for the photooxidation of propene and n-butane in photochemical smog
International Journal of Chemical Kinetics
A temperature-dependent relative-rate study of the OH initiated oxidation of n-butane: the kinetics of the reactions of the 1- and 2-butoxy radicals
Physical Chemistry Chemical Physics
Arrhenius parameters for the alkoxy radical decomposition reactions
International Journal of Chemical Kinetics
Atmospheric oxidation mechanism of methyl acetate
Journal of Physical Chemistry A
Mechanism of atmospheric photooxidation of organic compounds. Reactions of alkoxy radicals in oxidation of n-butane and simple ketones
Environmental Science and Technology
Direct kinetic studies of reactions of 3-pentoxy radicals with NO and O2
Journal of Physical Chemistry A
Complete falloff curves for the unimolecular decomposition of i-propoxy radicals between 330 and 408 K
Physical Chemistry Chemical Physics
Isomerization of OH-isoprene adducts and hydroxyalkoxy isoprene radicals
Journal of Physical Chemistry A
The photo-oxidation of diethyl ether in smog chamber experiments simulating tropospheric conditions: product studies and proposed mechanism
International Journal of Chemical Kinetics
Rate constants for the decomposition of 2-butoxy radicals and their reaction with NO and O2
Physical Chemistry Chemical Physics
Isomerization and decomposition reactions of primary alkoxy radicals derived from oxygenated solvents
Journal of Physical Chemistry A
Rate constants for the reactions of C2H5O, i-C3H7O, and n-C3H7O with NO and O2 as a function of temperature
International Journal of Chemical Kinetics
The β C–C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals
Physical Chemistry Chemical Physics
Chemical mechanism development: laboratory studies and model applications
Journal of Atmospheric Chemistry
Cited by (155)
Atmospheric degradation of 2-Isopropoxyethanol: Reactions with Cl, OH and NO<inf>3</inf>
2024, Atmospheric EnvironmentDynamic evaluation of modeled ozone concentrations in Germany with four chemistry transport models
2024, Science of the Total EnvironmentPreventing biogenic secondary organic aerosols formation in India
2022, Atmospheric EnvironmentA neglected pathway for the accretion products formation in the atmosphere
2022, Science of the Total Environment
- 1
Also at: Department of Environmental Sciences and Department of Chemistry, University of California, Riverside, CA 92521, USA.