      While EPA expects the yearly emissions in each state, on average, to equal the level of the budgets, EPA also estimated the air quality impacts of 5, 10, 15, and 20 percent emission variability using the air quality assessment tool, which is presented in section 4 of the "Power Sector Variability Final Rule TSD."  That analysis shows that year-to-year fluctuations of up to 20 percent in SO2 emissions from upwind states linked to a given downwind receptor do not undermine the ability of the Transport Rule programs to resolve nonattainment or maintenance concerns at that receptor.  The analysis presented in the TSD focuses on SO2 emissions and was designed to examine the sensitivity of downwind air quality to upwind EGU emission levels.  The share of total SO2 emitted by EGUs is significantly larger than the share of total NOX emitted by EGUs.  For example, in the states for which EPA modeled base case contributions of these pollutants, EGUs accounted for 74 percent of total SO2, 14 percent of total annual NOX, and 15 percent of total ozone-season NOX emissions.  Therefore, when varying EGU emissions only, downwind air quality would be most sensitive to upwind variations in SO2, because relative variations in EGU SO2 emissions have a greater impact on total SO2 emissions than the same relative variation in EGU NOX emissions would have on total NOX emissions affecting downwind air quality.  Because the Transport Rule only affects upwind emissions from EGU sources, downwind air quality would be more sensitive to variability in upwind state SO2 emissions under this rule than variability in upwind state NOx emissions under this rule (given that the rule affects a smaller scope of total NOX emissions compared to the scope affected of total SO2 emissions).  Thus, EPA chose to analyze the "worst-case" potential downwind air quality impacts from year-to-year variability above upwind state SO2 budgets, and EPA therefore believes that its findings from this analysis are valid for ascertaining the potential downwind air quality impacts from variation at those levels in both SO2 and NOX under the Transport Rule programs.  

