Mechanisms of Photodecomposition of the Sunlight-Absorbing Oxygenates

Abstract

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.

A computer-based solution to the oxidation kinetics of fluorinated and oxygenated volatile organic compounds

“…The fit to the Arrhenius–Kooij equation yields k (298.15 K) = (5.1 ± 2.2) × 10 −15 cm 3 molecule −1 s −1 . The only available experimental 10,85 rate coefficient is k (298.15 K) = (1.1 ± 0.6) × 10 −14 cm 3 molecule −1 s −1 , which leads to ρ ≈ 2.2. Note that at the date that this manuscript was being written, the rate coefficient given in the online database 85 was incorrect, since it was citing the value present in ref.…”

“…The correct value for the recommended rate constant is given in ref. 10. The k OH (calc) points calculated in the 230 ≤ T /K ≤ 320 temperature interval are shown in Fig.…”

“…8 Relative to their non-oxygenated fluorinated counterparts, the oxygen functional groups promote chemical activation by hydrogen-atom abstraction at distant sites from the substituent group, and thus increase the atmospheric reactivity by OH radicals. 9–11 There are a variety of applications 8 for fluorinated alcohols, aldehydes, esters, ethers and ketones in industry and they can potentially replace HFCs, as mandated by the Kigali Amendment to the Montreal Protocol.…”

Kinetics, Products, and Mechanisms Study of the Atmospheric Degradation of (E)-4-Methoxy-3-buten-2-one with Hydroxyl Radicals

“…Carbonyl compounds such as methyl formate, methyl glyoxal, and 2-hydroxy-2-methoxyacetaldehyde are observed as products that are expected to further react with OH radicals and Cl atoms. On the other hand, the photolysis and dry or wet deposition may become important removal processes of these carbonylic compounds …”

“…Methyl glyoxal can also be absorbed by aqueous aerosols and cloud droplets since it is highly soluble in water . Besides, methyl glyoxal can undergo further photodecomposition and oxidation to give mainly acetyl radicals, , which can take part in the formation of PAN that acts as a key reservoir for nitrogen oxides NO x in the troposphere. , …”