(1) No person shall cause or contribute to concentrations of particulate matter such that the scattering coefficient of particulate matter in the ambient air exceeds the following standard:

(a) annual average: 3 x 10^-5 per meter, annual average, not to be exceeded.

(2) The provisions of (1) are applicable only in Class I areas as are designated under the Montana Clean Air Act rules, Prevention of Significant Deterioration of Air Quality, (ARM Title 17, chapter 8, subchapter 8) on the effective date of this rule. Areas redesignated Class I subsequent to the effective date of this rule shall be subject to the provisions of (1) only upon a finding by the department that visibility is an important attribute of such area.

(3) Measurement method: For determining compliance with this rule, visibility shall be measured by the integrating nephelometer method, as more fully described in "Methods of Air Sampling and Analysis, Second Edition" (1977) Method No. 11203-03-76 T, as modified by the addition of a heated sample inlet line and green spectral sensitivity; or by an approved equivalent method.