Acid rain is one of the atmospheric events due to the increased pollution load of gaseous and particulate pollutants. Normal rain water is always slightly acidic because CO2 present in the atmosphere gets dissolved in it forming carbonic acid, H2CO3. Because of the presence of SO2 and NO2 molecules as contaminants in the atmosphere, the rainwater pH is reduced higher, often to as low as 2.4 and this form of lower pH accumulation is called acid rain.