On the basis of interaction of these substances with the environment they can be classified as primary or secondary pollutants. Substances which remain in the same form as they are after being added to the environment are called primary pollutants. Certain pollutants are formed by chemical reactions with the primary pollutants these are called secondary pollutants. When ${\mathrm{N}\mathrm{O}}_2$ and ${\mathrm{S}\mathrm{O}}_2$ are released from different sources they undergo chemical reactions and form secondary pollutants. They get dissolved in atmospheric water vapor to form sulphuric acid and nitric acid. These come down on earth with rain water this is called acid rain. The $\mathrm{p}\mathrm{H}$ of acid rain is lesser than $5.6$ . In acid rain the ratio of ${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4$ and ${\mathrm{H}\mathrm{N}\mathrm{O}}_3$ is $7 : 3$ . Due to acid rain acidity of soil and water increases and it is also responsible for the damage of historical monuments like Taj Mahal and Red fort.