Oxidation Mechanism for Methane

In addition to limiting the radiative forcing potential of CH4, its oxidation in the atmo sphere is very important, and contributes to the production of radicals, thereby affecting the oxidizing capacity. Under clean air conditions, CH4 oxidation leads to the formation of hydrogen peroxide (H2O2) and methyl hydroperoxide (CH3O2H). Equation 11.2 describes the formation of methyl radicals (CH3), which instantly add to oxygen (O2) to form a methyl peroxy radical (CH3O2) in Eq. 11.3:

CH3O2 can react with HO2 radicals (Eq. 11.4) to form CH3O2H, or can react with other peroxy radicals (RO2) (Eq. 11.5) to form the methoxyl radical (CH3O), which is a source of HO2 (Eq. 11.6) and hence of H2O2 (Eq. 11.7):

CH3O2+ HO2 ^ CH3O2H + O2 (11.4) CH3O2 + RO2 ^ CH3O + RO + O2 (11.5)

Both H2O2 and CH3O2H are susceptible to wet and dry depositions and H2O2 is particularly important as an oxidant in aqueous media. Formaldehyde (HCHO) formed in Eq. 11.6 can be photolysed in the troposphere:

Equation 11.8a shows molecular products H2 and CO and is in fact a major source of H2 in the atmosphere. Equation 11.8b yields the radical products H and HCO that ultimately form HO2 radicals and is therefore a source of HOX (OH and HO2) in the atmosphere.

Under polluted conditions, CH3O2 radicals react with NO to yield CH3O and NO2. NO2 is rapidly photolysed to yield NO and O atoms, which in turn rapidly form ozone (O3):

NO2 + hu ^ NO + O (11.10) O + O2 + M ^ O3 + M (11.11)

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