Methane, tropospheric reaction with hydroxyl

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

Methane is the most abundant hydrocarbon in the atmosphere. Table 2.9 summarizes the global sources of CH4, which are estimated at 535 Tg(CH4) yr (range 410 to 660) (IPCC, 1995). Of the estimated global annual emissions, 160 Tg(CH4) yr is attributed to natural sources, with the most prominent contribution being emissions from wetlands. Of the estimated 375 Tg(CH4) yr from anthropogenic sources, 100 Tg(CH4) yr comes from fossil fuel combustion, and the remainder from biospheric sources. Methane is removed from the atmosphere through reaction with hydroxyl radicals (OH) in the troposphere, estimated at 445 Tg(CH4) yr, and by reaction in the stratosphere, estimated at 40 Tg(CH4) yr. Microbial uptake in soils contributes an estimated 30 Tg(CH4) yr removal rate. The im-... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

Hydroxyl is unreactive toward the main atmospheric constituents N2, 02, H20, and C02, but reacts readily with many trace gases. Table 4-2 lists trace gases, their approximate mixing ratios, and rate coefficients for reaction with OH for three tropospheric conditions. The reactions with methane and... 

In this model, for every methane molecule which reacts, the sequence leads to 4 ozone and 2 hydroxyl radicals, extra. Formation of ozone in the lower troposphere is therefore catalysed by photochemical oxidation of organic molecules, but it does require comparatively high levels of NO (mixing ratio > 5 — 10 x 10 ) to be present. If it goes to completion, OH can react further with CO to make CO2 thus completing the oxidation of methane (Scheme 5.2). At low NO levels, the net reaction is the destruction of ozone via the reaction with CO [Scheme 5.2b]. 

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