Here's why:
* Collective nouns refer to a group of people, animals, or things. Examples include: "flock", "herd", "team", "jury".
* "Whiff" refers to a small amount of something, especially a smell. It's a singular noun.
Instead of "a whiff of fragrance", you might use a collective noun like "bouquet" to describe a collection of fragrances:
* "A bouquet of fragrances filled the air."