When released into the atmosphere, NO is rapidly oxidised to NO2 through processes

involving ozone (O3) and, in highly polluted areas radicals, such as peroxy-radicals

(RO2@) and the hydroperoxy radical (HO2

.) according to the following reactions:

NO + O3 Þ NO2 + O2

Oxidation by radicals is very fast and may cause high concentrations of nitrogen

dioxide at roadsides (Allegrini & Febo, 1995), especially if high concentrations of

HNO2 provide OH radicals to trigger the oxidation process. In the presence of

sunlight, NO2 is photolytically decomposed back to NO with the subsequent

formation of O3:

O + O2 Þ O3

This leads to the photo-stationary state where the rate of oxidation of NO is balanced

by its reformation by the photolytic dissociation of NO2. This balance is disturbed by

the presence of VOC and other compounds which may be oxidised to peroxy radicals,

and net ozone production may occur. At night the oxidation of NO by O3 proceeds to

completion, i.e. until either NO or O3 is totally depleted.

 

 

 

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