There are coal, crude oils, oil shales, tar sands and natural gases such as butane, ethane, methane which occur naturally from the decomposition of plant and sea and land organisms over millions of years. These natural resources contain stored energy from the sun which is released upon combustion. These fuels also release various types of pollutants such as sulfur dioxide, carbon dioxide and nitrogen dioxide when burned.
[Living in the Environment; Miller, G. Tyler; Wadsworth Publishing Company; Belmont, California; pages 476-486; 1992.] [Atmospheric Change; Graedel, T. E.; W. H. Freeman and Company; New York; page 430; 1993.]
Source: Atmospheric Chemistry Glossary