Photo-oxidation of methane in the presence of NO and NO2

Abstract

The photolysis of CH₄+ HONO + NO + NO₂ mixtures, diluted in N₂+ O₂ at atmospheric pressure and room temperature has been studied to provide information on the mechanism of the photo-oxidation of methane under conditions relevant to the lower atmosphere. Following the attack of OH produced by photodissociation of HONO, CH₄ is converted almost quantitatively to formaldehyde with the accompanying oxidation of 2 molecules of NO to NO₂. These observations can be interpreted in terms of the following simple chain mechanism OH + CH₄ [graphic omitted] H₂O + CH₃(12), CH₃+ O₂(+M) [graphic omitted] CH₃O₂(+M)(13), CH₃O₂+NO [graphic omitted] CH₃O + NO₂(14), CH₃O₂+ NO [graphic omitted] HCHO + HO₂(16), HO₂+ NO [graphic omitted] OH + NO₂. (5). Small amounts of methyl nitrite and methyl nitrate were observed as products, and could be attributed to the recombination of CH₃O with NO and NO₂ respectively. The conversion of CH₃O₂ to CH₃O via reaction (14) was not greatly influenced by the ratio [NO₂]/[NO], indicating that reaction (15) is unimportant. CH₃O₂+ NO₂= products (15). A kinetic analysis of the data gave k₁₅/k₁₄≃ 0.05 and k₃/k₁₂= 904 ± 94, where k₃ is the rate constant for the reaction of OH with HONO. The mutual combination reaction CH₃O₂+ CH₃O₂= products (19) was not competitive with reaction (14) under the experimental conditions. This allows a lower limit estimate of 1.2 × 10^–12 cm³ molecule^–1 s^–1 for k₁₄, at 298 K, based on a recent direct determination of k₁₉.