Smoke. No emission of smoke from any source shall be permitted to exceed a greater density than that density described as No. 1 on the Ringelmann Chart. However, smoke may be emitted which is equal to but not darker than No. 2 on the Ringelmann Chart, for not more than four minutes in any thirty-minute period. For the purpose of grading the density of smoke, the Ringelmann chart, as published by the U.S. Bureau of Mines, shall be standard.