Here's why:
* Concrete nouns refer to things that can be perceived by the senses (sight, touch, smell, taste, sound).
* Aroma is a smell, which is an abstract sensation. You can't physically hold or touch an aroma.
Examples of concrete nouns:
* Table
* Flower
* Book
* Rain
Examples of abstract nouns:
* Love
* Happiness
* Justice
* Aroma
Therefore, "aroma" is an abstract noun.