40 C.F.R. § 60.71a
As used in this subpart, all terms not defined herein shall have the meaning given them in the Act and in subpart A of this part.
Affirmative defense means, in the context of an enforcement proceeding, a response or defense put forward by a defendant, regarding which the defendant has the burden of proof, and the merits of which are independently and objectively evaluated in a judicial or administrative proceeding.
Monitoring system malfunction means a sudden, infrequent, not reasonably preventable failure of the monitoring system to provide valid data. Monitoring system failures that are caused in part by poor maintenance or careless operation are not malfunctions. You are required to implement monitoring system repairs in response to monitoring system malfunctions or out-of-control periods, and to return the monitoring system to operation as expeditiously as practicable.
Nitric acid production unit means any facility producing weak nitric acid by either the pressure or atmospheric pressure process.
Operating day means a 24-hour period beginning at 12:00 a.m. during which the nitric acid production unit operated at any time during this period.
Weak nitric acid means acid which is 30 to 70 percent in strength.
40 C.F.R. §60.71a