Do you think the authors use of the term airpocalypse is a fitting way to discribe the air pollution

Yes, and here's why (PS I'm getting technical here so listen carefully!):
You're definition for apocalypse is something that involves extreme damage or destruction, right? Well, that means air-pocalyspe would mean air that causes extreme damage or destruction, correct? Air pollution absolutely DESTROYS our air, so I believe this term is fitting for describing air pollution. I also think that this is a bit of an opinion question, so you can use this as a guide if you don't want to use this direct answer!

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).