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.