The nitrate radical is formed by the reaction of N02 with 03:

As discussed in Chapter 4, N03 only exists in sufficient concentrations to play a role in nighttime chemistry, due to its strong absorption of light in the visible and subsequent photodissociation.

Any reactions that produce H or HCO in the troposphere act as H02 sources:

Thus, formaldehyde photolysis is a major source of H02 during the day:

The photolysis of higher aldehydes, RCHO, also forms HCO (see Chapter 4) and hence H02:

However, as discussed in Chapter 4, the absorption spectrum of higher aldehydes cuts off at shorter wavelengths than formaldehyde. This, combined with higher quantum yields for radical production in the 290- to 340-nm range and the fact that HCHO produces 2H02 essentially immediately upon dissociation, makes the photolysis of aldehydes larger than formaldehyde less important at equal concentrations of the aldehydes.

The reactions of some alkoxy radicals generated in the VOC oxidation sequence, for example those of CH30 and C2H50 radicals discussed in more detail later, with 02 also generate HOz:

The alkoxy radical originates in the oxidation of a VOC to an alkyl radical by any of the oxidants described here or, alternatively, in the thermal decomposition of species such as PAN which gives the CH3 radical. In short, HOz is a natural consequence of the oxidation of organics.

Finally, the decomposition of peroxynitric acid, H02N02, which is strongly temperature dependent, generates H02 directly:

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