posted by Regina .
which is a vague pronoun and why? this increasing pollution is a big problem or this is a big problem
A vague pronoun means that it's not clear to what or whom it refers.
I assume this is your pair of sentences:
"25a. Air pollution normally increases along with population growth. This increasing pollution is a big problem for many cities.
25b. Air pollution normally increases along with population growth. This is a big problem for many cities."
In 25a, "this" could refer to population growth which is clearly not the intention.
25b clarifies this confusion.
In a senior moment, I reversed these two sentences.
25a does not have a vague pronoun.