Q.Ozone at ground level is described as a secondary pollutant. Write the short cycle of reactions, starting from the photolysis of $\text{NO}_2$, that leads to the build-up of ozone in polluted city air.

Ground-level ozone is a textbook secondary pollutant: no source emits ozone gas directly in any significant amount, yet it builds up steadily in polluted, sunlit air. The cycle begins with nitrogen dioxide, itself a primary/secondary pollutant present from vehicle exhaust. Sunlight of a suitable wavelength provides enough energy to break the $\text{N-O}$ bond in $\text{NO}_2$, splitting it into nitric oxide and a highly reactive free oxygen atom: $\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$. This free oxygen atom is far too reactive to persist on its own, and combines almost immediately with ordinary molecular oxygen in the surrounding air: $\text{O} + \text{O}_2 \rightarrow \text{O}_3$. The net result is that sunlight, acting on $\text{NO}_2$ that traffic has already put into the air, steadily converts atmospheric $\text{O}_2$ into ozone throughout the day.

Start from $\text{NO}_2$, apply sunlight to release a free oxygen atom, then combine that atom with ordinary $\text{O}_2$ -- keep the two steps separate rather than trying to write one combined equation.

Confusing this ground-level, harmful ozone formation with the beneficial ozone layer in the stratosphere, which is formed and protected by a different (UV-driven) mechanism.