The smoke that comes from a fire or the fumes that come out of a car exhaust don't just contain the sooty grey particles that you can see - they also contains lots of invisible gases.
Some of these gases (especially nitrogen oxides and sulphur dioxide) react with the tiny droplets of water in clouds to form sulphuric and nitric acids. The rain from these clouds then falls as very weak acid - which is why it is known as "acid rain".
Acid rain has an adverse effect on forest, freshwater and soil, killing off insects and aquatic life forms as well as causing damage to building and having possible impact on human health.
It is thought that acid rain can cause tree to grow more slowly or even to die. As it falls on a forest it trickles through the leaves of the trees and run down into the soil below. Some of it find its way into streams and then on into the river and lakes, thus effecting its creatures.
On lakes and river water, acid rain has a diverse effect. Both the lower Ph and higher aluminum concentration in the surface water that occur as a result of the acid rain can cause damage to fish and other aquatic animals. At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish. As lakes become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species.
Soil biology can also be seriously damaged by acid rain. Some tropical microbes can quickly consume acids but other microbes are unable to tolerate low pHs and are killed. The hydronium ions of acid rain also mobilize toxins and leach away essential nutrients and minerals of the soil.
The effects of acid rain can last for generations, as the effects of pH level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life.
This is because the sulfuric acid in the rain chemically reacts with the calcium compounds in the stones (limestone, sandstone, marble and granite) to create gypsum, which then flakes off.
This is also commonly seen on old gravestones where the acid rain can cause the inscription to become completely illegible. Acid rain also causes an increased rate of oxidation for iron. Visibility is also reduced by sulfate and nitrate in the atmosphere.