In The Fellowship of the Ring Frodo's first experience with the Nazgûl is described.
When [the Nazgûl] reached the tree and was level with Frodo the horse stopped. The riding figure sat quite still with its head bowed, as if listening. From inside the hood came a noise as of someone sniffing to catch an elusive scent; the head turned from side to side of the road.
The Fellowship of the Ring - chapter 3 - Three Is Company
In the Jackson movies the Nazgûl also try to ferret out the ring by sniffing and smelling for it, first in the scene where Frodo, Sam, Merry, and Pippen encounter a Nazgûl on the road after they fall down the hill from Farmer Maggot's fields, and then, when Frodo and Sam and Gollum are crossing the Dead Marshes, a Nazgûl flies overhead on a fell beast sniffing for the ring.
So my question is what element of the One Ring could the Nazgûl actually smell? Is it the metal the ring is forged from? Could Sauron's evil and malice and hatred have an actual scent, even if only to the Nazgûl? Why do the Nazgûl sniff and smell for the One Ring? I'd prefer a canon-based answer if possible, especially from any of Tolkien's books including the supplemental volumes.