Sulfur dioxide is a common byproduct of the incomplete combustion of fossil fuels. It is created when sulfur atoms present in the fuel are released into the atmosphere, joining with a pair of freed oxygen atoms. Sulfur dioxide also occurs as a byproduct of volcanic eruptions.
The simplest reaction that creates sulfur dioxide is the burning of sulfur in the presence of oxygen. This causes a single sulfur atom to combine with a molecule of oxygen to create sulfur dioxide. It is also a exothermic process, producing intense heat that can be harnessed to generate power. Hydrogen sulfide can also be burned, producing sulfur dioxide and water as a byproduct. These methods are often used to produce sulfur dioxide for industrial purposes, such as the creation of sulfuric acid.
Most atmospheric sulfur dioxide comes from the burning of fossil fuels. Coal and oil is often contaminated with sulfur during its creation, and burning these fuels releases sulfur dioxide and a host of other gases into the atmosphere.
In nature, sulfur dioxide most often comes from the burning of sulfide ores such as pyrite and cinnabar. These ores are common in the Earth's crust, and when magma wells up from the mantle, it may incorporate and melt these ores. This dissolves sulfur throughout the magma pocket, and if the magma reaches the surface, the resulting volcanic eruption can release sulfur dioxide into the atmosphere.