40 C.F.R. § 60.2255
How must I monitor opacity for air curtain incinerators?
(a) Use Method 9 of appendix A of this part to determine compliance with the opacity limitation.
(b) Conduct an initial test for opacity as specified in § 60.8.
(c) After the initial test for opacity, conduct annual tests no more than 12 calendar months following the date of your previous test.