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.
Often you can use Boolean search commands to modify your search.
For instance, "state implementation plan" AND "Maricopa County" would narrow your search to materials with both of those exact phrases, and likely not include other area implementation plan results.
Using OR is a way to search for synonyms simultaneously for instance: spectacles OR eyeglasses
Using NOT (or in google search tools, the minus sign - ) excludes results that include the keywords that follow. For instance, clean water NOT Flint would exclude the many recent information sources about Flint, Michigan, and allow you to more easily see other areas and issues related to that topic.
Once you have your search results on screen, if the number of results is overwhelming, look for ways to limit the year or source type in your search results to clear away additional 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 "Expand" "Add results beyond your library" "get full text" or "UNLV find text". Try these options and see if you find useful items, and request the ones you like through interlibrary loan (http://guides.library.unlv.edu/interlibraryloan/ ) . 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!
The below video also contains ideas about how to evaluate the information you have located, and what you may still need.