Hydrogen, 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. 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

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. 

Carbonyl halides containing a hydrogen atom are likely to undergo reaction in the troposphere with hydroxyl radicals, which dominate the day-time chemistry. Reaction 37 of carbonyl halides CHXO (where X = F or Cl) may occur by two mechanisms (i) direct hydrogen abstraction or (ii) radical addition to carbonyl. 

The alkyl radical lies at the centre of the oxidation scheme shown in Fig. 2.1 and the first two sections discuss its method of generation both in the initiation and chain propagation processes. The former has already been discussed in some detail in Chapter 1 and is not amenable to study by direct techniques. It is included here to produce a more complete picture of the overall methodology needed to quantify a reaction mechanism and to emphasize that direct techniques cannot provide all of the answers. The hydroxyl radical (OH) is the main chain carrier and the bulk of Section 2.3, is devoted to the measurement of rate constants for its hydrogen atom abstraction reactions with hydrocarbons. Much of the data and the methodology described in this section derive from studies of tropospheric chemistry, where the oxidative chain, at least in its early stages, shows a close relationship to the higher temperature processes which are central to this book. Indeed, it is fair to say that many of the developments in... 

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