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This action proposes corrections and updates to source test methods, performance specifications, and testing regulations for air emission sources under 40 CFR parts 51, 60, and 63. The revisions include corrections to testing provisions that contain inaccuracies and typographical errors, updates to outdated test methods, and the addition of alternative testing procedures the agency has deemed acceptable to use. For example, in Method 204, the enclosure area ratio must be less than 0.05, but the method currently inadvertently indicates less than 10.05. In Performance Specification 12B, the paired sorbent trap agreement currently indicates less than or equal to 20 percent relative deviation if the average concentration is less than or equal to 1.0 ug/m3; the proposed revision would indicate less than or equal to 20 percent relative deviation or less than or equal to 0.2 ug/dscm absolute difference for mercury concentrations less than or equal to 1.0 ug/dscm. This action is developed every few years to keep rules up-to-date and to ensure that compliance testing and monitoring are done correctly.