Dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature.

At high altitudes when lightning strikes, they combine to form oxides of nitrogen. $\mathrm{NO}_{2}$ is oxidised to nitrate ion, $\mathrm{NO}_{3}^{-}$which is washed into soil, where it serves as a fertilizer.

In an automobile engine, (at high temperature) when fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide (NO) and nitrogen dioxide $\left(\mathrm{NO}_{2}\right)$ as given below :

$\mathrm{N}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \stackrel{1405 \mathrm{~K}}{\longrightarrow} 2 \mathrm{NO}_{(\mathrm{g})}$

$\mathrm{NO}$ reacts instantly with oxygen to give $\mathrm{NO}_{2}$.

$2 \mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{NO}_{2(\mathrm{~g})}$

Rate of production of $\mathrm{NO}_{2}$ is faster when nitric oxide reacts with ozone in the stratosphere. $\mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{3(\mathrm{~g})} \rightarrow \mathrm{NO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}$

The irritant red haze in the traffic and congested places is due to oxides of nitrogen.

Higher concentrations of $\mathrm{NO}_{2}$ damage the leaves of plants and retard the rate of photosynthesis.

$\mathrm{NO}_{2}$ is a lung irritant that can lead to an acute respiratory disease in children. It is toxic to

living tissues also. Nitrogen dioxide is also harmful to various textile fibres and metals.