text: The NSR program is by no means the primary regulatory tool to address air pollution from existing sources. The Clean Air Act provides for several other public health-driven and visibility-related control efforts: for example, the National Ambient Air Quality Standards Program implemented through enforceable State Implementation Plans, the NOx SIP Call, the Acid Rain Program, the Regional Haze Program, etc. ... he national cap on SO2 emissions established under the Acid Rain Program applies to all existing electricity generating units, without regard to the date of construction or whether a given source has been modified.6