Rate Coefficients and Mechanisms of Atmospheric Oxidation of the Esters

Esters are emitted directly into the atmosphere from both natural and anthropogenic sources and are produced during the atmospheric oxidation of ethers. Methyl acetate and ethyl acetate have found widespread use as solvents. Vegetable oils and animal fats are esters. Transesterification of vegetable oils and animal fats with methanol gives fatty acid methyl esters (FAMEs) which are used in biodiesel. Many esters have pleasant odors and are present in essential oils, fruits, and pheromones, and are often added to fragrances and consumer products to provide a pleasant odor. Table VII-A-1 provides a list of common esters and their odors. It is surprising to note that despite their ubiquitous nature, volatility, and fragrance, it is only very recently that quantitative measurements of esters in ambient air have been reported (Niedojadlo et al., 2007; Legreid et al., 2007). The atmospheric oxidation of saturated esters is largely initiated by OH radical attack. Reaction with O3 and NO3 radicals contributes to the atmospheric oxidation of unsaturated esters. As discussed in chapter IX, UV absorption by esters is only important for wavelengths below approximately 240 nm and, hence, photolysis is not a significant tropospheric loss mechanism. When compared to the ethers from which they can be derived, the esters are substantially less reactive towards OH radicals. The ester functionality —C(O)O— in R1C(O)OR2 deactivates the alkyl groups to which it is attached with the deactivation being most pronounced for the R1 group attached to the carbonyl group. The atmospheric oxidation mechanisms of the esters are reviewed in the present chapter. The reaction of OH with methyl formate has been studied by Wallington et al. (1988b) and Le Calvé et al. (1997a) over the temperature range 233–372 K. Data are summarized in table VII-B-1 and are plotted in figure VII-B-1. The room temperature determination of k(OH + CH3OCHO) by Wallington et al. is in agreement with that by Le Calvé et al. (1997) within the experimental uncertainties. Significant curvature is evident in the Arrhenius plot in figure VII-B-1.

The Oxygenates: Their Properties, Sources, and Use as Alternative Fuels

Partially oxidized organic compounds, i.e., those containing carbon, hydrogen, and oxygen atoms, and optionally other heteroatoms, are often referred to as “oxygenates” because they contain O-atoms as well as a C-atom skeleton. These enter the atmosphere as emissions from various industrial and transportation-related operations, evaporation and release from home usage of certain products, and release from vegetation. They are also formed in the atmosphere as oxidation products of all hydrocarbon emissions that enter the atmosphere from mobile and stationary sources as well as natural emissions from plants and animals. The common oxygenates consist of the alcohols (ROH), ethers (ROR), aldehydes (RCHO), ketones [RC(O)R], esters [RC(O)OR], and acids [RC(O)OH] together with N-atom-containing oxygenates and other less abundant classes of oxygen-containing organic compounds. The use of alternative fuels is increasing and is anticipated to continue to grow in the future. Many of these alternative fuels are oxygenates: methanol, ethanol, butanol, fatty acid methyl esters, and other biofuels. Thus, the scientific community is interested in identifying the important sources and sinks for these compounds. As with the hydrocarbons, the oxygenates serve as fuel for the reactions that generate ozone and other air pollutants within the troposphere. In illustration, consider the influence of the very common and important oxygenate, formaldehyde (CH2O).

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the Ketones

Ketones are emitted directly to the atmosphere, and their sources were discussed in detail in chapter I. In the U.K. acetone and butanone comprise about 7% and 5%, respectively, of the total anthropogenic emissions of oxygenated compounds, and 1.6% and 1.1%, respectively, of the total anthropogenic emissions of nonmethane volatile organic compounds. Ketone emissions from solvents (both industrial and personal) are substantial; emissions from both gasoline- and diesel-fueled vehicles also contribute. Ketones are also formed extensively in the atmosphere in the oxidation of other compounds. Acetone, for example is formed in the OH-initiated oxidation of propane, iso-butane, iso-pentane, and neopentane and from a number of higher hydrocarbons. It is also formed in the oxidation of terpenes. The distribution, sources, and sinks of acetone in the atmosphere have been analyzed by Simpson et al. (1994). Methyl vinyl ketone is an important first generation product in the OH-initiated oxidation of isoprene. In this chapter, we discuss the rate coefficients and the mechanisms of oxidation of ketones. The classes covered include alkanones, hydroxyketones, diketones, unsaturated ketones, ketenes, cyclic ketones, ketones derived from biogenic compounds, and halogen-substituted ketones. Photolysis is a major atmospheric process for many ketones, and will be discussed in chapter IX. The major bimolecular reactions removing ketones from the atmosphere are with OH. Although less important than the OH reactions, reactions with Cl have been studied quite extensively. Other than for unsaturated ketones, reactions with NO3 and O3 are unimportant in tropospheric chemistry and have been studied little. The carbonyl group deactivates the α-position with respect to reaction with OH, but activates the β-position, and possibly more distant sites as well. The net result is that the overall rate coefficient of an alkanone generally exceeds that of the equivalent alkane. The temperature dependences of the rate coefficients can be quite complex, with acetone and possibly butanone showing a minimum in the rate coefficient at ∼250 K, while the higher alkanones show negative temperature dependences across the more limited temperature ranges that have been investigated. The most likely explanation of this behavior is the formation of a pre-reaction, hydrogen-bonded complex.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the Alcohols

The presence of alcohols in the atmosphere is attributable to natural and anthropogenic sources (e.g., Graedel, 1978). They are released into the atmosphere as a result of their use as fuels (e.g., methanol, ethanol), fuel additives (e.g., ethanol), solvents (e.g., ethanol, propanol) and as starting materials or intermediates for organic synthesis in a large number of industries (e.g., isobutanol, isopentanol, hexylene glycol). Vegetation also provides a significant source of alcohols to the atmosphere (Kesselmeier and Straudt, 1999; Fall, 2003). A number of saturated and unsaturated alcohols have been identified to be biogenically emitted (e.g., methanol, ethanol, methylbutenol, linalool, Z-hex-3-en-1-ol “leaf alcohol”). High ambient concentrations of certain alcohols have been measured in some areas, up to 68 ppb for ethanol in Porto Alegre, Brazil (Grosjean et al., 1998a) and up to 3 ppb for 2-methyl-3-buten-2-ol in the Colorado mountains (Goldan et al., 1993). Amore complete description of the emission sources and ambient concentrations of alcohols is given in chapter I. Reaction with OH radicals is the dominant atmospheric loss process for the saturated alcohols while reactions with NO3, O3, and photolysis are negligible.

Mechanisms of Photodecomposition of the Sunlight-Absorbing Oxygenates

In this chapter we consider the chemical changes induced by absorption of sunlight by oxygenated organic compounds. In contrast to the hydrocarbons, sunlight absorption by the oxygenated products of hydrocarbon oxidation shows a shift in absorption to the longer wavelengths such that their photodecompositions can become important atmospheric processes. This is illustrated in the absorption data given for methane and its oxidation products in figure IX-A-1. As a H3C—H bond in methane is replaced with a H3C—OH bond (alcohols), H3C—OCH3 bond (ethers), or H3C—O2H bond (hydroperoxides), light absorption involving excitation of nonbonding O-atom electrons to an antibonding sigma orbital (n → σ* transitions) becomes possible. These “forbidden” absorptions are smaller than the allowed σ → σ*transitions in the CH4 and the other alkanes, but the absorption bands are shifted to longer wavelengths progressing from CH4 to CH3OH, CH3OCH3, and CH3OOH as less energy is required to excite the nonbonding electron to the σ* orbital.

The Influence of Oxygenates on the Atmospheric Chemistry of Urban, Rural, and Global Environments

One cannot overestimate the importance of oxygenated organic compounds in atmospheric chemistry. As discussed in the previous chapters of this book and elsewhere (e.g., Wayne, 1991; Seinfeld and Pandis, 1998; Brasseur et al., 1999; Finlayson-Pitts and Pitts, 2000; Calvert et al., 2000, 2002, 2008) the atmosphere is an oxidizing environment and all organic compounds emitted into the atmosphere are converted into oxygenated organic compounds. The first-generation products are oxidized further. As an example, the oxidation of ethane gives CH3CHO, C2H5OH, and C2H5OOH as first-generation products and CH3OH, CH3OOH, CH2O, and HC(O)OH as second-generation products. An understanding of the chemistry of oxygenated organic compounds is central to unraveling the complex processes in the atmosphere. In this chapter we discuss the representation of oxygenates in atmospheric models, their participation in secondary organic aerosol formation, contribution to HOx chemistry in the upper troposphere, role in the transport of pollutants, and use as proxies for volatile organic compound (VOC) emissions. A major application of the chemical kinetics and mechanisms of VOC oxidation is the development of an understanding of the chemistry occurring in the troposphere and the use of that understanding to predict and develop strategies which help to mitigate adverse changes in air quality and climate change. Such applications depend on the development of models that assess chemical impacts; chemical mechanisms lie at the heart of such models. The mechanisms can be very detailed, often termed explicit, in models where the aim is to understand the chemistry occurring in a small volume of air, for example, in an analysis of processes determining radical concentrations in field measurements. Such a mechanistic approach can also be used, with increased computer resources, when a trajectory approach is used to assess the coupled impacts of atmospheric transport and chemistry. An Eulerian approach to modeling both regional and global processes presents greater problems, because the chemical rate equations have to be solved for each species at each spatial grid point in the model; this severely limits the number of chemical species that can be incorporated realistically in the model.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the N-Atom-Containing Oxygenates

The many different nitrogen-containing oxygenated volatile organic compounds that are present in the troposphere play important roles in the chemistry of our atmosphere. They can be emitted directly into the atmosphere, such as in the case of amides that are widely used as organic solvents, starting materials, or intermediates in different industries (e.g., synthetic polymers, manufacture of dyes, and synthesis of pesticides). Amides are formed in situ as intermediate products in the degradation of amines (e.g., see Tuazon et al., 1994; Finlayson-Pitts and Pitts, 2000). Nitrogen-containing oxygenated organic compounds are formed in the atmosphere also via reactions of alkoxy (RO) and alkyl peroxy radicals (RO2) with NO or NO2 leading to alkyl nitrates, alkyl nitrites, and peroxy acetyl nitrates. However, primary sources of these organic species have also been suggested such as diesel and other engines and biomass burning (e.g., see Simpson et al., 2002). Alkyl nitrates (RONO2) have been detected in both the urban and the remote troposphere (e.g., see Roberts, 1990; Walega et al., 1992; Atlas et al., 1992; Ridley et al., 1997; and Stroud et al., 2001; see also section I-D). Nitrates are formed as minor products in the reaction of peroxy radicals with NO. The nitrate yield increases with the size of peroxy radicals and can be as high as 20–30% for large (>C6) radicals (Calvert et al., 2008). Peroxyacyl nitrates (RC(O)O2NO2) are important constituents of urban air pollution. They have been identified in ambient air (e.g., see Bertman and Roberts, 1991; Williams et al., 1997, 2000; Nouaime et al., 1998; Hansel and Wisthaler, 2000; also see section I-D). They are formed from photochemical reactions via RC(O)O2 + NO2. A major role of these compounds is their capacity to act as a reservoir for NOx that can be transported from polluted urban to remote regions that are poor NOx regions and where their presence can increase NOx levels (Roberts, 1990). As with other volatile organic compounds (VOCs), once released to the atmosphere, nitrogen-containing organic compounds are expected to undergo degradation primarily via reaction with hydroxyl and nitrate radicals, reaction with ozone, and photolysis. Thermal decomposition is an important loss process for the peroxyacyl nitrates.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the Organic Acids

Organic acids, particularly formic and acetic acid, are ubiquitous components of the troposphere (Chebbi and Carlier, 1996); see table I-D-1. However, the atmospheric budget of these species is at present poorly constrained, and global models often underestimate their abundance (von Kuhlmann et al., 2003). The presence of organic acids in the atmosphere can be attributed to two distinct mechanisms: direct emission from anthropogenic and natural sources; and in situ production via gas-phase or condensed-phase chemistry. Direct emissions result from biomass burning (e.g., Christian et al., 2007), from motor vehicle use (Kawamura et al., 2000) and other anthropogenic activities (see chapter I), and from biogenic sources (e.g., Seco et al., 2007). Production in the gas phase can occur via the reactions of acylperoxy radicals with HO2: . . . CH3C(O)O2 + HO2 → CH3C(O)OOH + O2 . . . . . . CH3C(O)O2 + HO2 → CH3C(O)O + OH + O2 . . . . . . CH3C(O)O2 + HO2 → CH3C(O)OH + O3 . . . or via the ozonolysis of unsaturated species (Orzechowska and Paulson, 2005a, b). Additional in situ acid production (particularly with multi-functional species and diacids) likely occurs in the condensed phase as well, via the oxidation of carbonyl and other oxygen-containing and multi-functional organics (e.g., Ervens et al., 2004). In general, the organic acid moiety, —C(O)OH, is rather unreactive in the gas phase. This is in large part due to the strength of the O—H bond, ∼460 kJ mole−1 versus 400–420 kJ mole−1 for typical C—H bonds (Sander et al., 2006). The organic acid moiety also acts to inhibit somewhat the reactivity of neighboring sites (Kwok and Atkinson, 1995), further decreasing the reactivity of small saturated acids. UV spectra for unsubstituted acids are located at relatively short wavelengths, [e.g., λmax< 210 nm for acetic acid, Orlando and Tyndall (2003); see figure IX-A-1], so tropospheric photolysis is of negligible importance. Thus, the gas-phase lifetime for small saturated organic acids (e.g., formic and acetic acid) can be quite long, about 1 month.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the Aldehydes

Aldehydes are emitted from a variety of anthropogenic sources associated with natural gas and petroleum combustion (for examples, see tables I-C-2 and I-C-3). Winer et al. (1992) have discussed direct emissions of aldehydes from biogenic sources. They are also important intermediates in the oxidation of directly emitted organic compounds. For example, formaldehyde, CH2O formed in the reaction of CH3O with O2 . . . CH3O + O2 → CH2O + HO2 . . . CH3O is formed in the oxidation of methane, and a number of other compounds. There are also many other sources of CH2O; for example, the Leeds University’s Master Chemical Mechanism (MCM) lists a total of ∼ 140 CH2O precursors: http://mcm.leeds.ac.uk/MCM/. Aldehydes with saturated hydrocarbon chains (termed alkanals or acyclic aldehydes) react mainly with OH during the day and with NO3 at night. The aldehydic C—H bond is weaker than those in the hydrocarbon chain; and, certainly for the shorter carbon chain species, abstraction by both OH and NO3 occurs primarily at the aldehydic center to form an acyl radical which reacts rapidly with O2 to form an acylperoxy radical, e.g., . . . CH3CHO + OH → CH3CO + H2O . . . . . . CH3CO + O2 → CH3C(O)O2 . . . An important reaction of the acylperoxy radical is with NO2 to form an acylperoxy nitrate. In the example shown, the oxidation of acetaldehyde gives acetyl peroxy radicals which can react with NO2 to form peroxyacetyl nitrate, CH3C(O)O2NO2, generally known as PAN: . . . CH3C(O)O2 + NO2 → CH3C(O)O2NO2 . . . Peroxyacyl nitrates dissociate quite quickly at 298 K, to regenerate peroxyacyl radicals. For example, PAN has a lifetime of about 50 min. The lifetime increases rapidly at the lower temperatures experienced at higher altitudes and is several months at the temperatures (∼ 250 K) of the upper troposphere. This long lifetime provides a mechanism for the transport of NOx from polluted areas to less polluted areas, by transfer of peroxyacyl nitrates from the boundary layer to the free troposphere; subsequent subsidence can return them to the boundary layer where they dissociate at the higher temperatures encountered there. The atmospheric reactions of the nitrates are discussed in detail in chapters VIII and IX.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the Ethers

The presence of ethers in the atmosphere is almost entirely the result of direct emissions from anthropogenic sources (e.g., Arif et al., 1997; Intergovernmental Panel on Climate Change, 2001; Johnson and Andino, 2001; http://en.wikipedia.org/wiki/Ethers; and references therein). These sources can be quite varied and species dependent; for example, many ethers are commonly used as industrial solvents; many are formed as combustion intermediates and in the burning of biomass; various branched ethers (e.g., methyl tert-butyl ether) are (or have been) used as fuel additives to increase octane number and reduce CO emissions; dimethyl ether has being proposed as an alternative diesel fuel; many fluorinated species have been manufactured, evaluated and used as possible chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) replacement compounds; some halogenated species are used as inhalation anaesthetics or as chlorofluorocarbon replacements; and polybrominated diphenyl ethers (PBDEs) are used as flame retardants. There are no major routes to ether formation in the atmosphere itself. The atmospheric behavior of the simple aliphatic ethers, to a first approximation, mirrors that of the alkanes. Reaction with OH is the dominant removal pathway, and occurs via abstraction of an H-atom. In general, however, the ethers are more reactive than the alkanes, as the ether linkage leads to a weakening of the neighboring C—H bonds, and thus imparts an “activation effect” on the OH reaction. The major oxidation steps occurring subsequent to abstraction are also similar to those occurring in atmospheric alkane chemistry, involving the formation of a peroxy radical and an alkoxy radical prior to end product formation; However, less is know about the quantitative details of these processes than is the case for the alkanes. In general, unimolecular decomposition of the alkoxy radicals [e.g., CH3CH2OCH(O•)CH3 in the above reaction sequence] is more rapid than for an alkane-derived radical of similar structure. The major end product of ether oxidation is quite often an ester, the analog to the carbonyl compounds (aldehydes and ketones) generated from alkane oxidation.