The above reactions form NO2 photolytic cycle. However, if only these reactions are involved then, NO2 concentration in the atmosphere would remain constant. But, volatile organic compounds (VOCs) that include unburned hydrocarbons and their volatile derivatives also react with NO and O2 to form NO2 . The reactions between HC and NO do not necessarily involve ozone and provide another route to form NO2 and thus, the concentration of ozone and NO2 in the urban air rises. The most reactive VOCs in atmosphere are olefins i.e., the hydrocarbons with C=C bond. The general reaction between hydrocarbons (RH) and NO may be written as
The overall global reaction is
Main processes in photochemical smog formation are shown in Fig. 1.6.
| Figure1.6 |
Main processes in photochemical smog formation (adapted from http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm) |
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