What causes Acid Rain?

Acid rain is caused by sulfur dioxide (SO2) and nitrogen oxide (NOx).  Sulfur dioxide and nitrogen oxides enter the atmosphere through natural sources, however the man-made sources is what causes the problem. Natural sources of sulfur dioxide include oceans and volcanic eruptions. Natural sources of nitrogen oxide include lightening, volcanic eruptions, and microbial activity. Man-made pollutants of sulfur dioxide stem from the burning of coal/petroleum and the smelting of iron/other metallic ores. Man made pollutants of nitrogen oxide stem from power stations, vehicle exhausts, and industrial emission.


“When the pollutants are vented into the atmosphere by tall smoke stakes, molecules of SO2 and NOx are caught up in prevailing winds, where they interact in the presence of sunlight with vapors to form sulfuric acid and nitric acid mists. These acids remain in vapor state under the prevalent high temperatures conditions. When the temperature falls, condensation takes the form of aerosol droplets, which owing to the presence of unburnt carbon particles will be black, acidic and carbonaceous in nature” (Singh et al 2006).

Nitrogen oxides are released in the atmosphere during combustion when when atmospheric nitrogen and oxygen combine. Nitrogen oxide is also formed when the oxygen from fuels combine with atmospheric oxygen. Sulfur oxides are formed when an object containing sulfur is burned and sulfur combines with atmospheric oxygen.  Most sulfur oxide comes from the combustion of coal, oil, as well as smelting. Half of  nitrogen oxides generated is from cars.

Source: Acid rain and its ecological consequences
The Oxford Companion to Global Change
The Hutchinson Unabridged Encyclepedia
International Journal of Environmental Studies