The EPA is taking this action to propose a Greenhouse Gas (GHG) Significant Emission Rate (SER) under the Prevention of Significant Deterioration (PSD) air permitting program and propose certain revisions to the provisions of the Prevention of Significant Deterioration (PSD) and Title V Greenhouse Gas (GHG) Tailoring Rule. The proposed GHG SER would establish an appropriate threshold level below which Best Available Control Technology (BACT) is not required for a source's GHG emissions. The Tailoring Rule revisions will allow us to revise certain GHG permitting regulatory provisions, which include the PSD GHG Plantwide Applicability Limits (PALs), and will also implement a recent Court of Appeals for the District of Columbia decision that ordered, among other things, that the Tailoring Rule regulations under review be vacated to the extent they require a stationary source to obtain a title V permit solely because the source emits or has the potential to emit GHG above the applicable thresholds.