<p>Most major air pollutants cannot be seen, although large amounts (of them) (concentrated in) cities (are visible) (as) smog.</p>
<p>In this question, the only error I could possibly think of was 'of them.' I thought it should be changed to 'of those.' Can someone explain to me why it's fine to use them instead of those?</p>
<p>Thanks!</p>
<p>if you write “of those” it will be wrong
it has to be “of those air pollutants”
or you can avoid the hassle and write of them</p>
<p>“Those” can be either a pronoun or an adjective. “Them” is always used as a pronoun. In this sentence, “large amounts of ____” would need to be filled in with a noun of some kind, since the blank is the location of the object of the preposition “of.”</p>
<p>You should check a grammar source for more information about usage of those vs. them.</p>