Smoke, Dust or other Air Contaminant. Smoke, dust or other air contaminant shall not be discharged into the atmosphere for a period or periods aggregating more than three minutes in any hour, which is dark or darker in shade than that designated as No. 2 in the Ringelman Chart, as published by the United States Bureau of Mines, or which is of such opacity as to obscure an observer's view to a degree equal to or greater than does smoke, designated as No. 2 on the Ringelman Chart.