Chief - AP42 (emission factors) on-line. Check out TTN while here.
EPA Region V - best of the regional EPA sites.
40 CFR - federal air regulations.
MACT Updates - track HAP regulation development and implementation for your industry.
SIC vs NAICS Tables - determine the proper North America Industrial Classification System code for your facility based on your current SIC.
Sun Altitude and Azimuth - determine whether or not opacity data was collected with the correct sun location.
Magnetic Declination - to correctly interpret the above sun location data, you will need to determine the magnetic declination (difference between magnetic north and true north).
State and Local Sites
Meck. Co. DEP
Air and Waste Management Assoc. - good source of peer reviewed air quality articles.
AAAS - good source of peer reviewed articles covering all aspects of science.
ECO IQ - good source of regulatory agency links not found on this page.
MSHA - good source of information on respirable dust.
OSHA - we all need to be safety conscious.
Scientific America - good technical articles on all aspects of science.
Apex Instruments - good source for stack sampling equipment.
More to Come !!!