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.

The pronoun "this" can be considered vague when it is used without a clear antecedent or context. In the sentence "This increasing pollution is a big problem," the pronoun "this" is unclear because it does not specifically refer to any previously mentioned noun or idea.

To understand which noun "this" refers to, you would need to look for the noun or idea that it is replacing. In this case, it would be helpful to identify the specific pollution being referred to. For example, the sentence could be rewritten as "The increasing pollution caused by industrial waste is a big problem." In this revised sentence, "this" is no longer vague because it is clear what pollution is being referred to.

On the other hand, in the sentence "This is a big problem," the pronoun "this" is also vague because it lacks a specific antecedent. To remove the vagueness, you could rephrase the sentence to include a clearer noun or idea. For example, "The lack of clean drinking water is a big problem."

Overall, it is important to provide proper context and specific reference when using pronouns to avoid ambiguity and ensure clarity in communication.