Rain water has a $\mathrm{pH}$ of 5.6. Because rain water reacts with $\mathrm{CO}_{2}$ present in atmosphere and formed $\mathrm{H}^{+}$ions.

$\mathrm{H}_{2} \mathrm{O}_{(l)}+\mathrm{CO}_{2(g)} \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3(\text { aq })}$

$\mathrm{H}_{2} \mathrm{CO}_{3(\text { aq })} \rightleftharpoons \mathrm{H}^{+}{ }_{(\text {aq })}+\mathrm{HCO}_{3(\text { aq })}^{-}$

When the $\mathrm{pH}$ of the rain water drops below $5.6$, it is called acid rain. Acid rain refers to the ways in which acid from the atmosphere is deposited on the earth's surface. Oxides of nitrogen and sulphur which are acidic in nature can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.

Oxides of sulphur and nitrogen formed and mix in the atmosphere. Which is responsible for the acid rain. Burning of fossil fuels which contain sulphur and nitrogenous matter such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produce sulphur dioxide and nitrogen oxides.

$\mathrm{SO}_{2}$ and $\mathrm{NO}_{2}$ after oxidation and reaction with water are major contributors to acid rain, because polluted air usually contains particulate matter that catalyse the oxidation.

$2 \mathrm{SO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}+2 \mathrm{H}_{2} \mathrm{O}_{(l)} \rightarrow 2 \mathrm{H}_{2} \mathrm{SO}_{4(\mathrm{aq})}$

$4 \mathrm{NO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}+2 \mathrm{H}_{2} \mathrm{O}_{(l)} \rightarrow 4 \mathrm{HNO}_{3(\mathrm{aq})}$

Thus, ammonium salts are also formed and can be seen as an atmospheric haze aerosol of fine particles.

Aerosol particles of oxides or ammonium salts in rain drops result in wet deposition. $\mathrm{SO}_{2}$ is also absorbed directly on both solid and liquid ground surfaces and is thus deposited as drydeposition