For the pale yellow nonmetallic element found especially in volcanic deposits, sulfur is the usual spelling in American English. Sulphur is generally the preferred spelling in nonscientific texts from outside North America, but sulfur is gaining ground in scientific writing throughout the English-speaking world.
The spelling distinction extends to derivative words such as sulfuric/sulphuric, sulfate/sulphate, and sulfide/sulphide.
The word, from the French soufre, entered English around the end of the 14th century. Both modern spellings have been in use for many centuries, but sulphur prevailed by a wide margin until the Americans adopted sulfur around the start of the 20th century.
It developed a cap-and-trade system in the 1990s that successfully reduced sulfur emissions that cause acid rain. [Scientific American]
In the afternoon, however, she mixed nitric acid and sugar rather than the intended sulfuric acid and sugar. [LA Times]
The oil companies say that removing additional sulfur from fuel will increase refinery costs. [New York Times]
Outside the U.S.
Fortunately, they have a trick up their sleeves to help them do so: sulphur-powered bacteria living in their cells. [New Scientist]
Acid rain was caused by the emission of sulphur dioxide from power stations, which became sulphuric acid in the atmosphere. [Independent]
A smell of dust and sulphur hung in the area. [Scotland on Sunday]