Most certainly carbon dioxide. Though here I think you make the grave mistake of lulling those unfamiliar with Chinese into the false sense of security, with the belief that attaching a radical to a character will produce another character with a related meaning (from my experience, such logical formations are in the minority. ^^;;;).

Japanese uses 二酸化炭素, and 炭 is charcoal, carbon -- like many other languages, there's no differentiation.