N.Y. Comp. Codes R. & Regs. tit. 6, § 209.3 – Opacity of emissions.

(a) No person will cause or allow visible emissions to the outdoor atmosphere from any potroom group at a primary aluminum reduction plant or any furnace at an anode bake plant, for which an application for a permit to construct was received by the commissioner before October 24, 1974, having an average opacity (six minutes) of 20 percent or greater, except uncombined water.
(b) Upon written application by the owner or operator of a source subject to subdivision (a) of this section, the commissioner, at his discretion, may grant an equivalent opacity standard which exceeds the applicable opacity standard if the source owner can demonstrate through acceptable tests for such source that he is in compliance with all applicable emission requirements other than the opacity standard and that the source and any associated emission control equipment is being operated and maintained in a manner acceptable to the commissioner. An equivalent opacity standard for an emission source will only be granted where reasonably available control technology, as determined by the commissioner, has been utilized. In such case, the source owner or operator will not cause or allow emissions to exceed the equivalent opacity.
(c) No person will cause or allow emissions to the outdoor atmosphere from any potroom group at a primary aluminum reduction plant or any furnace at an anode bake plant for which an application for a permit to construct was received by the commissioner after October 23, 1974, having the following six-minute average opacity, except uncombined water:
1. (1) 10 percent or greater from any potroom group at a primary aluminum reduction plant; and
2. (2) 20 percent or greater from any furnace at an anode bake plant.
(d) Section 212.7(a) of this Title will apply to any other confined process at a primary aluminum reduction plant or anode bake plant for which a permit to construct was received by the commissioner before October 24, 1974.