Each database or search tool has a different set of items contained within it, and you will want to consider what words the author may have used (e.g. air pollution, or air quality?) Use quotation marks around a specific phrase to search for exactly "state implementation plan" documents, as an example. Once you have your search results on screen, look for ways to limit the year or source type in your search results to clear away unwanted items.
You may also want to broaden your search to include items UNLV doesn't own - we can usually get you a copy. Look for options to "Add results beyond your library" or "UNLV find text". Try these options and see if you find useful items, and request the ones you like through interlibrary loan (http://unlv.illiad.oclc.org/illiad/logon.html) . In some cases, the full text of certain items may take a couple of days to reach you through an email request to an agency, or through an interlibrary loan request. Don't delay (the interlibrary loan service is free to you as a UNLV student), and ask a librarian for help if you can't find the full text of something!