The attainment program embodied in the state implementation plan may, with respect to proposed new or modified major stationary sources within nonattainment areas which would cause or contribute to a violation of a national ambient air quality standard, provide an allowance for growth while providing reasonable further progress toward attainment, and new sources may be allowed that do not result, individually or in the aggregate, in emissions that exceed the allowance. Particulate matter not of a size or substance to adversely affect public health or welfare shall not be considered in determining whether any applicable growth allowance has been consumed. Modifications to an existing facility within a source may be allowed which are accompanied by emission reduction offsets within the same source sufficient to provide no net increase in emissions of an air pollutant from the source. If an applicable growth allowance has been consumed, the attainment program shall permit sources to be constructed only if emission reduction offsets providing a greater than one-for-one emission reduction are obtained from existing sources sufficient to provide reasonable further progress toward attaining the applicable national ambient air quality standards by the attainment date prescribed under Part D (Nonattainment Program) of the federal act. Any emission offsets required for sulfur dioxide, particulates, and carbon monoxide shall provide a positive net air quality benefit in the area affected by the proposed source.