Consider the steps (1) NO2 ->[hnu] NO + O, (2) O + O2 -> O3, (3) O3 + NO -> NO2 + O2. In an atmosphere containing only NO, NO2, O2 and sunlight (no hydrocarbons), ozone does not build up to high levels because

step (3) regenerates NO2 and consumes the ozone formed in step (2)

NO is converted irreversibly into N2 in sunlight

ozone reacts with O2 to give O atoms again

step (2) cannot occur without hydrocarbons

step (3) regenerates NO2 and consumes the ozone formed in step (2)

JEE Advanced levelSingle correct optionDifficulty 3 of 3ev035

Consider the steps (1) $NO X_{2} h ν NO + O$ , (2) $O + O X_{2} O X_{3}$ , (3) $O X_{3} + NO NO X_{2} + O X_{2}$ . In an atmosphere containing only $NO$ , $NO X_{2}$ , $O X_{2}$ and sunlight (no hydrocarbons), ozone does not build up to high levels because

Adding (1), (2) and (3):

NO X_{2} + O + O X_{2} + NO + O X_{3} NO + O + O X_{3} + NO X_{2} + O X_{2}

, i.e. no net change. Ozone formed is used up by

NO

, giving a null cycle. Hydrocarbons are needed to break this cycle.