Most acid precipitation results from the combination of with water in the atmosphere, forming strong acids that fall with rain or snow. see concept 3.

Most acid precipitation results from the combination of sulfur and nitrogen oxides with water in the atmosphere, forming strong acids that fall with rain or snow.

Acid rain has low pH because of the elevated levels of hydrogen ions. It can cause damage on plants, animals and infrastructure.  

Forming of acid rain:  

NO2 + OH• → HNO3

SO3 (g) + H2O (l) → H2SO4 (aq)