Full Idea
Tropes seem to be abstract entities because, unlike concrete entities, they are ontologically dependent; ..there are no 'free' tropes, and they must always be bundled with other appropriate tropes to exist.
Gist of Idea
Tropes seem to be abstract entities, because they can't exist alone, but must come in bundles
Source
E.J. Lowe (A Survey of Metaphysics [2002], p.367)
Book Reference
Lowe,E.J.: 'A Survey of Metaphysics' [OUP 2002], p.367
A Reaction
Only a Platonist would think that a universal property could 'exist alone'. I presume Aristotle thought universals were real, though bound up with substances.