Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. This is one of the causes for concern over the environmental impact of the use of these fuels as power sources.

Application:

Precursor to sulfuric acid
Sulfur dioxide is an intermediate in the production of sulfuric acid, being converted to sulfur trioxide, and then to oleum, which is made into sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. The method of converting sulfur dioxide to sulfuric acid is called the contact process. Several billion kilograms are produced annually for this purpose.

As a preservative
Sulfur dioxide is sometimes used as a preservative for dried apricots and other dried fruits owing to its antimicrobial properties, and it is sometimes called E**0 when used in this way. As a preservative, it maintains the appearance of the fruit and prevents rotting. Its presence gives the fruit a distinctive chemical taste.

In winemaking
Sulfur dioxide is an important compound in winemaking, and is designated as parts per million in wine, E number: E**0. It is present even in so-called unsulphurated wine at concentrations of up to *0 milligrams per litre. It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation. It also helps to keep volatile acidity at desirable levels. Sulfur dioxide is responsible for the words "contains sulfites" found on wine labels. Wines with SO2 concentrations below *0 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of SO2 allowed in wine is **0 ppm in US, in the EU is **0 ppm for red wines and **0 ppm for white and ros wines. In low concentrations SO2 is mostly undetectable in wine, but at over *0ppm, SO2 becomes evident in the nose and taste of wine.

SO2 is also a very important element in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery, a mixture of SO2, water, and citric acid is commonly used to clean and sanitize equipment.

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