A `smell’ can be pleasant or unpleasant.
For example you can always complain about the smell of someone’s dirty socks.”
“And I can comment on the smell of your breath!”
But `smell’ need not always be associated with something bad. You can talk about the smell of freshly brewed coffee.” “Or the smell of rain!”
`Aroma’, on the other hand, is usually associated with something pleasant and stimulating.”
“So one doesn’t normally use the word to talk about someone’s armpit?”
“Not unless you find the smell that comes from one’s armpit very pleasant.”
“I certainly don’t. But I can talk about the aroma of early morning coffee.”
“Yes, you can. The word `aroma’ is normally associated with food.