My knowlege is pretty basic, so forgive me if I'm telling you what you already know. Also, doublecheck anything I writ ein here because it is off the top of my head.
I work in power plant regulation in California, but not really in Air Quality regulations, although part of my job is making sure the plant maintains certain systems related to air quality (ie SCR, CEMS).
As you probalby know, the EPA controls the permitting of plants and emissions limits through title V with various AQMD enforcing the regulations.
In my area it is the SCAQMD.
http://www.aqmd.gov/rules/reg/reg30_tofc.html
THere are pretty strict regulations around here,particularly on NOX emissions. Almost all the plants have SCR systems, plus often other controls - steam or water injection, flue gas recirculation, etc. THere are also a lot of other limits - SOX, particulate, CO, etc. I don't remember them all. Plus all the new GHG rules, and the ones the governor is proposing and they are now fighting over in the budget. I think a lot of these limitis are done by rolling average. Anyway, I think these are public documents so you can find the plants limits on the web if you look.
All the plants continuously monitor these emissions through CEMS (continuous emmissions monitoring sytems). Periodically they must be tested for accuracy through what are called RATA tests. In addition they perform some other periodic tests. For example, since the SCR systems use Ammonia to catalyze NOX to harmless byproducts, they must periodically test the SCR for "ammonia slip" - that is how much ammonia slips out into the atmosphere.
The plants seldom bump up against their limits, but often they are constrained in operation in other ways. For example, some plants cannot run at low loads because they have a biger problem meeting their limits at these loads.
And if I'm not mistaken, there are the same sorts of credits you can buy sell and trade to get around these limits. At least I think so.
