Troposphere aldehydes

Aldehydes are important constituents of atmospheric chemistry. We have already seen the role played by formaldehyde in the chemistry of the background troposphere. Aldehydes are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with NO3 radicals. Reaction with N03 radicals is of relatively minor importance as a consumption process for aldehydes, thus the major loss processes involve photolysis and reaction with OH radicals. 

Aldehydes are important constituents of atmospheric chemistry. We have already seen the role played by formaldehyde in the chemistry of the background troposphere. Aldehydes are formed in the atmosphere from the photochemical degradation of other organic com-... 

Low-molecular mass carbonyls are among the most abundant and ubiquitous volatile organic compounds in the atmosphere. They are produced from industrial activity and incomplete combustion of fossil fuels and biomass. Many aldehydes are also emitted indoors (plastic, foam insulation, lacquers, etc.). As a source of free radicals, aldehydes play an important role in the ozone formation, in urban smog events, as well as in the photochemistry of the unpolluted troposphere. Aldehydes are recognized irritants of the eye and respiratory tract, and often, carcinogenic and mutagenic characteristics are also attributed to them. 

Lloyd AC. 1979. Tropospheric chemistry of aldehydes. Washington, DC National Bureau of Standards. Special Publication No. 557. 

This region of the spectrum around 300 nm is a crucial one for tropospheric photochemistiy in both clean and polluted atmospheres. As we have indicated earlier, it is here that species such as ozone and aldehydes photolyze to produce atoms and free radicals critical to the chemistry of the troposphere. 

To illustrate the application of Eqs. (OO) and (PP), let us calculate the rate of photolysis of acetaldehyde. Aldehydes such as CH3CHO play an important role in tropospheric chemistry because they photodissociate to produce free radicals. In the case of acetaldehyde, there are four possible sets of products ... 

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 H02-aldehyde reaction is in parentheses because, as we shall see later, it is a reversible reaction that is sufficiently fast in the reverse direction under typical tropospheric conditions that no overall reaction, in effect, occurs. 

That is, it abstracts from saturated hydrocarbons and aldehydes and adds to unsaturated hydrocarbons. As discussed in Chapter 6, the reactions with aromatic hydrocarbons are generally too slow to be important in the troposphere the exceptions are particular compounds such as the cresols where the reaction is rapid. 

For complex formation between aldehydes and S(IV) to be important in the troposphere, the aldehydes not only must have high solubility but also be present in air at significant concentrations and form stable adducts with S(IV) at a sufficiently fast rate that it can occur during the lifetime of a typical cloud or fog event. Table 8.4 gives the rate constants /c,4 and kt5 for formation of the S(IV) complexes as well as the stability constants Ku and apparent stability constant K p, defined as... 

The aldehyde HC(0)CFC12 formed in the oxidation of HCFC-141b is expected to photolyze in the troposphere, forming in part CFHC12 (Wallington et al., 1994a), which is itself oxidized in the troposphere by reaction with OH. By analogy to the photolysis of... 

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)... 

Peroxy radicals are also formed in the troposphere through the photolysis of aldehydes (10, 11) and through nitrate radical (N03) reactions (12-14). The hydrogen atom and formyl radical that are formed then react with molecular oxygen (02) (reactions 11 and 12) under tropospheric conditions. 

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 ... 

Dhar and Ram (50) found formaldehyde in rain water and estimated a tropospheric mixing ratio of 0.7 ppb, while Cauer (36) measured a mean value of 0.4 ppb. Lodge and Pate (160) obtained an average value of 1.1 ppb for the total aliphatic aldehydes in surface air in the tropics. Levy (152) proposed the formation of formaldehyde via the tropospheric oxidation of methane and calculated (155) an upper limit of 1 ppb for the mixing ratio, with an altitude profile for a summer midlatitude decreasing from 0.6 ppb at the ground to less than 0.01 ppb in the upper troposphere, where methane oxidation is very slow (154). 

Carbon monoxide is oxidized in the troposphere ((133) and (134)). With a high concentration of nitric oxide in the troposphere, reactions (135) and (136) take place. This sequence is a formation of ozone catalyzed by nitric oxide. If the nitric oxide concentration is too low, the perhydryl radicals decompose ozone to form hydroxyl radicals (136). Ozone and peroxyacylnitrates PAN are the major toxins of smog. Peroxyacylnitrates are formed from aldehydes in a reaction catalyzed by nitric oxide. 

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...

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

Aldehydes are emitted directly into the atmosphere from a variety of natural and anthropogenic sources and are also formed in situ from the atmospheric degradation of volatile organic compounds (VOCs). The atmospheric fate of aldehydes is controlled by photolysis and reaction with hydroxyl (OH) or nitrate (NO3) radicals and, in the case of unsaturated compounds, reaction with ozone (Atkinson, 1994). The photolysis of aldehydes is of particular importance because it is a source of free radicals in the troposphere, and thus may significantly influence the oxidizing capacity of the lower atmosphere (Finlayson-Pitts and Pitts, 1986). 

The subsequent atmospheric fate of these aromatic aldehydes is controlled by reaction with OH and NO3 radicals and photolysis by sunlight. Kinetic studies of the gas-phase reactions of the tolualdehydes and dimethlybenzaldehydes have been recently performed (Thiault et al, 2002 and Clifford, 2004) and preliminary studies of the photolysis of the tolualdehydes have also been reported (Volkamer et al., 2000 and Thiault et al., 2001). In this work the photolysis of the 3 tolualdehydes and 6 dimethylbenzaldehydes (DMBAs) were investigated during July 2003. The measured photolysis rate coefficients have been used to calculate tropospheric lifetimes and provide an assessment of the relative importance of photolysis as an atmospheric loss process for the compounds. 

The photolysis rate coefficients obtained in this work can be used to calculate the tropospheric lifetimes of the aromatic aldehydes with respect to photolysis. The lifetimes listed in Table 4 were calculated from the values of 7(aldehyde)/y(N02) obtained at... 

For the future, detailed measurements of NO, HONO and aldehydes in diesel exhaust for typical motor conditions should be considered. The use of such data as input parameters for the calculation of ozone formation potentials (e.g. incremental reactivities by the method of Carter et al. 1995) can contribute to a better estimation of the influence of diesel exhaust on tropospheric ozone formation. 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

This class of organic emissions 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, photolysis is probably of minor importance. Reaction with the OH radical is then the major tropospheric loss process. For example, for... 

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