I heard that 'petrichor', which is defined as a pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather, is the only noun in English that means a specific scent. Is this true?
There is a related term, geosmin. So, petrichor is not the only such word.
In any sentence in which you use the word petrichor, you could substitute linen, rose, pine, citrus, or any number of other words which identify a scent. I leave it to the reader to decide whether this means that linen is a noun which identifies a scent or that petrichor is an adjective. But either way it's clear that petrichor is not in a category by itself.