In Ray Bradbury's novel, Fahrenheit 451, to my way of thinking, all of the things that Montag notices, especially in terms of smell, are symbolic of his freedom leaving a society that was so sterilized, and entering a new world where he may think and notice things freely and at his leisure.
One of the story's themes is change and transformation. This is the story of how Montag starts to change (with Clarisse's help) and the transformation that follows, fully realized when he enters the water of the river.
The things that Montag notices at the end may not be such a surprise if we look back at the beginning of the story, when Montag first meets Clarisse. She is the catalyst for many new things that start to happen to Montag. The first time they meet, he smells fruit.
They walked in the warm-cool blowing night on the silvered pavement and there was the faintest breath of fresh apricots and strawberries in the air, and he looked around and realized this was quite impossible, so late in the year.
Clarisse may be symbolic of all the things he and his society have been conditioned to ignore. There is another time when Montag is speaking to Clarisse and she describes the the grass in the morning:
There's dew on the grass in the morning.
Montag is confused for he cannot remember if he knew this already or not, but Clarisse has a way of making him notice things and question things. In time, it's as if Clarisse becomes a part of him. Then Clarisse is killed by an automobile, and one might ask if it was intentional: she was a wild canon, asking questions that no one else asked. However, even when she is gone, Montag continues to notice things and ask questions.
At the end of the book, when Montag escapes the Mechanical Hound by running into the river, he is at last free, and here again he notices things that he did not notice on dry land. The smell of carnations from a neighbor's yard seems like something that would always have been there, but went unnoticed by Montag until he crossed into the river. The weed brushing against his hand, smelling like licorice almost seems like a flashback to a discussion with Clarisse when she asked if he had children, and why not?
Once Montag crosses into the water, he is, symbolically, reborn. As he walks along, he smells other things:
...a dry river smelling of hot cloves...there was a smell like a cut potato...a smell like pickles from a bottle...a smell like parsley on the table at home...[and] a faint yellow odor like mustard from a jar.
For Montag, this is a new world—and that is what he and others like him will be building, as the bombs drop on the city, destroying everything he has ever known...or perhaps more accurately, all the things he has never known, or has forgotten.