The Environmental Protection Agency (EPA) is proposing to revise its prior action that fully approved a state implementation plan (SIP) revision submitted by the Commonwealth of Pennsylvania, through the Pennsylvania Department of Environmental Protection (PADEP), to EPA on October 11, 2017, and supplemented on February 5, 2020. The SIP revision provided a plan for attainment of the 2010 sulfur dioxide (SO<INF>2</INF>) primary national ambient air quality standard (NAAQS) in the Indiana, Pennsylvania SO<INF>2</INF> nonattainment area (hereafter referred to as the ``Indiana, PA NAA'' or ``Indiana Area''). The attainment plan submission included a base year emissions inventory, an analysis of the reasonably available control technology (RACT) and reasonably available control measure (RACM) requirements, enforceable emission limitations and control measures, a reasonable further progress (RFP) plan, a modeling demonstration of SO<INF>2</INF> attainment, and contingency measures for the Indiana Area. EPA is proposing to revise its prior action to partially approve and partially disapprove the SIP. This action is being taken under the Clean Air Act (CAA).