Troposphere ketones

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

As discussed in Chapter 6.J, acetone photochemistry is of interest because this ketone is distributed globally, has both biogenic and anthropogenic sources, and has been proposed to be a significant source of free radicals in the upper troposphere. The absorption cross sections of acetone (as well as other aldehydes and ketones) are temperature dependent at the longer wavelenths, which is important for application to the colder upper troposphere. Figure 4.29, for example, shows the absorption cross sections of acetone at 298 and 261 K, respectively (Hynes et al., 1992 see also Gierczak et al., 1998). 

The family of photo-oxidants includes tropospheric ozone, O3 (the bad ozone), ketones, aldehydes and nitrated oxidants, such as peroxy-acetylnitrate (PAN) and peroxybenzoylnitrate (PBN). The modeling of photo-oxidants is more complicated than that of acid deposition (NRC 1991). Here, the primary precursor is NOx, which as mentioned before, is emitted as a result of fossil fuel combustion. A part of NOx is the N02 molecule, which splits (photodissociates)... 

Products of these reactions, aldehydes and ketones, undergo photodissociations under tropospheric sunlight. Aldehydes absorb actinic UV-A radiation and methanal absorption extends out to approximately 370 nm, whereas the heavier aldehydes absorb only to approximately 345 nm. Methanal has two photodissociation paths ... 

Figure 2 Schematic showing principle oxidation processes in the troposphere in NO -rich air (after Prinn, 1994). In NOj.-poor air (e.g., remote marine air), recychng of HO2 to OH is achieved hy reactions of O3 with HO2 or hy conversion of 2HO2 to H2O2 followed hy photodissociation of H2O2. In a more complete schematic, nonmethane hydrocarbons (RH) would also react with OH to form acids, aldehydes and ketones in...

This class of organic compounds is exemplified by acetone and its higher homologues. As for the aldehydes, photolysis and reaction with the OH radical are the major atmospheric loss processes (Atkinson, 1989). The limited experimental data available indicate that, with the exception of acetone (see Figure 5.11), photolysis is probably of minor importance. Reaction with the OH radical is then the major tropospheric loss process. For example, for methyl ethyl ketone the OH radical can attack any of the three carbon atoms that contain hydrogen atoms ... 

Once released into die atmosphere, the most rapid mechanism to attenuate most of the solvents in Table 17.1.1 appears to be by photo-oxidation by hydroxyl radicals in the troposphere. Based on the estimates by Howard et al., it appeared that nine of the solvents can be characterized by an atmospheric residence half-life of 10 days or less (Figure 17.1.3). The photo-oxidation of solvents yields products. For example, die reaction of OH radicals with n-hexane can yield aldehydes, ketones, and nitrates. ... 

All carbonyl oxides proved to be highly photolabile, and on photolysis yield dioxiranes 3 or split off oxygen atoms to produce ketones. Oxygen atoms are also formed thermally from vibrationally excited 1. Thus, if the large exothermicity of the ozonolysis reaction is taken into account, 1 might be a source of O atoms and OH radicals in the troposphere. The role of dioxiranes has not yet been discussed in context with atmospheric chemistry, although the formation of these species in contrast to the isomeric carbonyl oxides - in ozone/alkene reactions has been unequivocally demonstrated [13]. 

Determination of traces of aldehydes and ketones in the troposphere via solid phase derivatisation with DNSH,... 

Absorption Spectrum and Cross Sections of HCHO For carbonyl compounds such as aldehydes and ketones, absorption bands due to the electronic transition called n — r transition, in which the isolated pair of lone-pair electrons on the O atom of carbonyl group (—C = O) is excited to the excited Jt orbital of the double bond, appears around 300 nm. Since this transition is a forbidden transition, the absorption cross sections are not very large ( 10 cm molecule ) in general. However, since the absorption bands extend to near 350 nm where solar actinic flux grows, their photolyses are very important in the troposphere. 

Aldehydes and ketones are the dominant oxygenates found within the lower troposphere. As discussed in chapters IV, V, and IX, aldehydes and ketones are reactive towards OH radicals and readily undergo photodecomposition in sunlight. Table I-D-1 lists representative measurements of the ambient concentrations of several oxygenates. Data are shown for regions classified as Urban, Rural, and Remote. 

Both photodecomposition (see chapter IX) and reaction with OH are important for the removal of 2-pentanone in the troposphere. The lifetime with respect to reaction with OH is given in table V-B-19 and the oxidation products of the higher ketones are discussed in section V-B-18. 

The lifetimes of the alkanones with respect to reaction with OH for [OH] = 10 molecule cm" are summarized in table V-B-19. The limited data related to the photodissociation lifetimes of the ketones indicate that these are comparable to the lifetimes shown for reaction with OH in this table. Thus lifetimes for photodissociation in the lower troposphere with an overhead Sun are estimated as follows acetone, 12 to 16 days butanone, about 2.5 days 2-pentanone, 4.3 to 24 days 1-hexanone, 1.4 to 8.3 days. See chapter IX and table IX-M-1. 

Hydroxyacetone is formed in the tropospheric oxidation of isoprene. It is a second-generation intermediate, formed from methacrolein, just as methylglyoxal is a second-generation product formed from methyl vinyl ketone. 

The major loss mechanism for methyl vinyl ketone is its reaction with OH. The lifetime is estimated at 5.5 h for a daytime [OH] = 2.5 x 10 moleculecm . Lifetimes for photodissociation of methyl vinyl ketone are relatively long, 2.5 days in the lower troposphere for an overhead Sun see section IX-F-10 and table IX-M-1. 

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