Sulfur is a chemical element that is yellow in color and has a strong smell. It is found in nature and is often used in various products, like fertilizers and medicines. Full definition
While it is an emission from vehicles the most common producer of sulfur dioxide in the air is from volcano eruptions. (cheapcarinsurance.net)
However, while field measurements of sulfur dioxide emissions are increasing, they still remain too sparse to piece together a cohesive global picture. (sciencedaily.com)
And the same approach that brought a decline in sulfur dioxide emissions can have an equally dramatic and permanent effect on carbon emissions. (dotearth.blogs.nytimes.com)