33 CFR §157.166

If the tank vessel having a COW system under § 157.10a(a)(2) or § 157.10c(b)(2) transfers cargo at a port in the United States that is in an area designated in 40 CFR part 81 as an area that does not meet the national primary ambient air quality ozone standard under 40 CFR part 50, issued under the Clean Air Act, as amended (42 U.S.C. 1857), the master of the vessel shall ensure that when cargo tanks are ballasted in that port the hydrocarbon vapors in each tank are contained by a means under § 157.132.