concentrations of such pollutant attributable to the increase in emissions from stationary sources which have converted from the use of petroleum products, or natural gas, or both, by reason of an order which is in effect under the provisions of sections 792(a) and (b) of title 15 (or any subsequent legislation which supersedes such provisions) over the emissions from such sources before the effective date of such order.
1
So in original. The period probably should be a comma.