Acid rain is a broad term that refers to a mixture of wet and dry acidic deposition from the atmosphere containing higher than normal amounts of HNO3 and H2SO4. Primarily, the emissions of SO2 and NOx resulting from fossil fuel combustion, mainly from automobiles and power plants, are responsible. Acid rain occurs when these gases react in the atmosphere with H2O, O2 and other chemicals to form various acidic compounds and precipitate along with rain, fog, snow and dew. The pH value is often as low as 4, which is highly detrimental to plants, aquatic animals and the infrastructure. The deposition need not be always wet. In tropical countries, it is in dry form, carried by the airborne particulate matter (PM). SO2 and NOx released from power plants and other sources are blown by the prevailing winds sometimes over hundreds of kilometres. Figure 1.1 taken from EPA publication illustrates the concept of acid rain very lucidly.