NO_x plays an important role in the oxidation of hydrocarbons in the lower atmosphere. We shall take the case of the oxidation of methane, CH₄, as an example of this process. Methane is a major trace gas in the troposphere, with an average mixing ratio of approximately 1500 ppb.

CH₄ reacts with an hydroxyl radical, removing hydrogen to form a methyl radical and water.

CH4 + OH

→

CH3 + H2O

(1)

The methyl radical reacts with O₂ to form a methylperoxy radical.

CH3 + O2

→

CH3O2

(2)

In a continental atmosphere during the day, the methylperoxy radical will probably react with the nitric oxide released by combustion processes to form a methoxy radical, CH₃O.

CH3O2 + NO

→

CH3O + NO2

(3)

The methoxy radical in turn reacts with O₂ to form formaldehyde, HCHO and a hydroperoxy radical, HO₂. HO₂ produced in this way can also be a significant factor in a catalytic cycle that produces O₃ from the photolysis of NO₂ (see section 1.1.3.3, 'O₃ creation by NO₂').

CH3O + O2

→

HCHO + HO2

(4)

Note also that the formaldehyde molecule itself may be oxidised by OH in a similar sequence of reactions to the oxidation of methane, the end result being the formation of CO or CO₂ and H₂O).

The hydroperoxy radical from reaction (4) oxidises NO to NO₂ and regenerates an OH radical.

HO2 + NO

→

OH + NO2

(5)

Note the following features of this process:

• The reaction sequence (1) to (5) constitutes a chemical chain reaction with OH as the chain carrier: OH is consumed in reaction (1) and regenerated in reaction (5). The OH radical can thus be regarded as a catalyst, only a small amount being needed to oxidise much larger amounts of CH₄.
• In the same reaction chain, two molecules of NO are also oxidised to NO₂, reactions (3) and (5).
• Volatile organic compounds (VOCs) such as hydrocarbons are all oxidised by similar mechanisms.

The following diagram summarizes the cycle: