The EPA's Applicability Determination Index (ADI) database contains hundreds of letters and memoranda on the applicability and monitoring requests made by source owners/operators under the New Source Performance Standards (40 CFR Part 60) and the National Emission Standards for Hazardous Air Pollutants (40 CFR Parts 61 and 63) programs. 

The database also contains "regulatory interpretations" which are written responses that apply to the broad range of NSPS and NESHAP regulatory requirements as they pertain to a whole source category and applicability determinations issued pursuant to the chlorofluorocarbons (CFC) regulations, in 40 CFR Part 82.

The database is available at http://www.epa.gov/oecaerth/monitoring/programs/caa/adi.html.