Increasing acidity in natural water and soil has become a global concern. Acidification and climate change are interrelated as the sources responsible for acidification and greenhouse gases are the same.
Unpolluted rain water normally is always slightly acidic, because CO2 in atmosphere dissolves in it forming carbonic acid (H2CO3).The pH of unpolluted rain water is about 5.5-5.7 but due to presence of SO2 and NO2 gases as pollutants in the atmosphere, the pH of rain water is further lowered, often as low as 2.4 and this type of precipitation is generally referred to as acid rain.
Acid rain means the presence of excessive acids in rain water. Acid rain is a blend of H2SO4, HNO3, and HCl.
The contribution of various gases is as follows:
H2SO4 = 60-70%
HNO3 = 30-40%
HCl = 10%
These oxides travel long distances in the atmosphere and undergo several physical and chemical transformations to produce more hazardous substances.
Every source of energy that we use be it coal, wood or petroleum products has sulphur and nitrogen. These elements when burnt in atmospheric oxygen are converted into their respective oxides SO2 and NOx which are highly soluble in water.
In the atmosphere the following reactions take place
In case of sulphur
S + O2 → SO2
2SO2 + O2 → 2SO3
SO3 in humid atmosphere forms droplets of H2SO4.
In case of nitrogen following reactions take place
NO + O → NO2 + O2
NO2 + O3→ NO3 + O2
NO2 + NO3 → N2O5
N2O5 in humid conditions form nitric acid
N2O5 + H2O → 2HNO3
Effects of Acid Rain:
- Acid rain causes extensive damage to buildings and structural materials such as marble, limestone and slate. Taj Mahal has suffered lot due to SO2 and H2SO4 acid fumes or other pollutants from Mathura Refinery.
- Acid rain contaminates potable ground water with toxic compounds present in it. These toxic compounds enter into human body and effect human respiratory, nervous and digestive system.
- It produces smog which is a safety hazard.
- It increases the acidity of lakes and rivers which kills fish, algae and the aquatic system gets collapsed.