Technical Support Document (TSD)
                         for the final Transport Rule
                      Docket ID No. EPA-HQ-OAR-2009-0491

                            Power Sector Variability
                                Final Rule TSD

                     U.S. Environmental Protection Agency
                          Office of Air and Radiation
                                   June 2011
                           Power Sector Variability

      This Technical Support Document (TSD) provides information in support of section VI.E, "Approach to Power Sector Emissions Variability," of the preamble to the final Transport Rule.  This TSD is organized as follows:

      1. Introduction
      2. Estimating variability in power system operations and emissions
      3. Numerical simulation of 1 and 3-year variability
      4. Results of an analysis using the air quality assessment tool

      1. Introduction.
      Section VI of the preamble to the final Transport Rule discusses EPA's approach to define "significant contribution" and "interference with maintenance" with respect to the 1997 8-hour ozone and annual fine particle (PM2.5) National Ambient Air Quality Standards (NAAQS) and the 2006 24-hour PM2.5 NAAQS.  As discussed in preamble section VI, EPA has identified the emissions that must be reduced by each state to address the state's significant contribution and interference with maintenance.  To facilitate implementation of the requirement that these emissions be eliminated, EPA also developed SO2, annual NOX, and ozone-season NOX state budgets based on its projections of state-by-state power sector emissions in an average year reflecting these emission reductions.
      However, because of inherent variability in year-to-year baseline emissions  -  resulting from the inherent variability in power system operations  -  state-level emissions may vary somewhat from year to year even after all the significant contribution and interference with maintenance that EPA has identified in the Transport Rule has been eliminated.  Therefore, and for the reasons discussed in preamble sections VI.E and VI.F, EPA has determined that it is appropriate to develop variability limits for each state budget.  These limits are used to identify the range of emissions that EPA believes may occur in each state following the elimination of all emissions identified by EPA as significantly contributing to nonattainment or interfering with maintenance in one or more other states.
      Preamble sections VI.A through VI.D discuss EPA's approach to quantify for each upwind state the emissions that significantly contribute to nonattainment or interfere with maintenance downwind for the existing ozone and PM2.5 NAAQS and to determine state emissions budgets for SO2, annual NOX, and ozone season NOX.  Preamble sections VI.E and VI.F discuss the inherent variability in electric power system operations and EPA's approach to determine appropriate variability limits on emissions for each state covered by the final Transport Rule to account for this variability.  As explained in preamble section VI.E, EPA calculated variability limits for each state emissions budget.  The final Transport Rule applies those variability limits in addition to the state budgets (which are based on expected average conditions) to determine the "assurance level" for each state in each compliance period.  The Agency believes that because baseline power system operations (and therefore emissions) are variable at the state level, emissions after the elimination of all significant contribution and interference with maintenance are also variable and thus it is appropriate to take this variability into account while assuring that each state makes necessary reductions.
      In the final Transport Rule, EPA derived "1-year" variability limits from an assessment of historical year-to-year variability in heat input, which was originally described in the "Power Sector Variability" TSD from the proposed Transport Rule and is re-analyzed in this TSD for all states included in the final Transport Rule, as well as for the additional states EPA is proposing, through a supplemental proposal, to include in the Transport Rule ozone program.  For the reasons discussed in section VI.E of the preamble, although EPA proposed to implement both 1-year variability limits and 3-year average variability limits, the final Transport Rule decided to finalize only the 1-year limits.  Section 3 of this TSD presents an analysis of 1 and 3-year variability that supports EPA's decision to finalize only the 1-year variability limits in the final rule.
      Section 4 of this TSD describes an analysis using the air quality assessment tool (AQAT) to estimate the resulting air quality effects in a given year assuming several potential degrees of variability in year-to-year emissions from the power sector.

      2. Estimating variability in power system operations and emissions.
      This section describes the method that the Agency used to estimate the year-to-year ("1-year") variability in power system operations and thereby annual SO2, annual NOX and ozone season NOX emissions.  As discussed above, the goal of this assessment is to determine the amount of variability in state-level emissions that is due to inherent variability in power system operations.  To quantify this expected variability, EPA used variation in historic heat input.  EPA considered the use of variation in historic emissions but determined this to be inappropriate because it reflected factors such as the installation of new emissions controls and changes in the operation of existing controls.  These factors are not directly related to variability in power system operations.  In fact, these factors are activities that EPA is attempting to regulate with the implementation of the state budgets themselves.  EPA believes that all controls  (existing or newly installed) should be operated with consistency under the rule's programs.  As such, EPA is assuming a constant relationship between heat input variability and emissions variability for the purposes of this historic variability analysis (i.e., variation in heat input is a proxy for variation in emissions holding the effectiveness of emissions control in that state constant).  This section in the TSD provides information on:
   * The historical data set EPA established on a state-by-state basis of yearly heat input values applicable to each of the pollutants regulated in the final Transport Rule (SO2, annual NOx, and ozone season NOx).  EPA used these historical heat input values to estimate the inherent variability in emissions due to power system operation.
   * The method EPA utilized to estimate the year-to-year variability in the heat input values.  The year-to-year variability in heat input was estimated on a state-by-state basis.
   * The approach EPA used to link inherent heat input variability with projected pollutant emission levels to estimate the resulting variation expected in future pollutant emissions on a year-to-year basis.

   (a) Establishing a historical data set for use in estimating inherent variability in emissions.
       The objective of this section is to describe the inherent year-to-year (1-year) variability in emissions by characterizing the year-to-year variance in total annual heat input for each state in the Transport Rule.  EPA is concerned with variation in total emissions from year to year (or the variation in total emissions from one ozone season to the next), as compliance with the Transport Rule programs must be demonstrated for total tonnage emitted during a single year or ozone season.  For this assessment, EPA used total yearly heat input values equaling the sum of heat input from all units operating in each state during a particular year.
      EPA estimated the expected variation in power sector emissions for a yearly time period based on the "standard deviation" of yearly power sector heat input (HI) assessed over a 7-year time frame (2002 through 2008).  EPA believes such data would capture inherent variation in year-to-year emissions due to factors such as variation in power demand, timing of maintenance activities, and unexpected shutdown of units.  These factors are strongly correlated with heat input and variation in electric generation.
      EPA chose the time period 2002 through 2008 for the analysis of variation in heat input because it features nearly comprehensive reporting of heat input across many states and EGUs while also minimizing unusual (i.e., statistically misleading for long-run averaging) upswings or downswings in state-level heat input.  EPA selected 2002 as the first baseline year because prior years (2000 and 2001) experienced large, uneven changes in annual heat input from fossil units for some states due, in part, to structural changes in the composition of the power sector fleet and a significant increase in newly reported heat input due to the commencement of the second phase of the Acid Rain Program (ARP).  Consequently, incorporating data from years prior to 2002 in the analysis would lead to the inclusion of a year where there is large change in heat input that is not representative of typical year-to-year variability in power system operation, thereby leading to overestimates of variability.  EPA determined that including years marked by significant changes in the number of units reporting heat input data or years experiencing significant shifts in state-level EGU fleet composition (i.e., sharp regional swings from coal to gas generation) would skew the variability estimates.  EPA maintained 2008 as the final year for the assessment for a similar reason:  The economic downturn in 2009 resulted in an atypical dropoff in electric demand and, thus, in heat input, that would overstate long-run year-to-year variability in heat input.
      For each year of the 7-year time period, EPA estimated total power sector heat input on a state-by-state basis using the sum of reported heat input for all units within each of three general classes of EGUs:  (1) coal-fired units; (2) combined cycle turbines; and (3) a combination of oil- and gas-fired boilers and simple-cycle combustion turbines.  Total annual heat input values (in million MMBtu) for the coal-fired EGUs and for all EGUs can be found in Table 1 and Table 2, respectively.  Ozone-season heat input values for all EGUs can be found in Table 3.  EPA assessed the inherent year-to-year variability in annual and ozone season NOx emissions using the total yearly heat inputs summed across all EGUs (Tables 2 and 3, respectively), because heat input from each of these sources is related to significant NOx emissions.  The Agency assessed year-to-year variability in annual SO2 emissions using the yearly heat inputs from the coal-fired EGUs (Table 1) which account for a substantial majority of the SO2 emissions.  SO2 emission rates are considerably higher at most coal-fired units compared to other types of generators; as a result, heat input variation at coal-fired units has a disproportionate impact on a state's SO2 emissions.  For this reason, in this assessment, we analyzed historic variability in heat input at coal-fired units as a better proxy for SO2 emissions variability.
      EGUs built prior to 2002 that reported heat input and emissions data between 2002 and 2008 and new EGUs built in 2002 or later were included for this analysis.  EGUs that were built prior to 2002 but that did not start reporting until after 2002 were excluded from the assessment, in order to prevent their newly reported heat input from falsely registering as "additional" heat input gained in that state in that year.
Table 1: Annual Heat Input* (million MMBtu) from Coal-Fired Units for Each Year.
                                     State
                                                                           2002
                                                                           2003
                                                                           2004
                                                                           2005
                                                                           2006
                                                                           2007
                                                                           2008
                                                                     Average HI
                                      AL
                                                                          779.0
                                                                          822.6
                                                                          784.5
                                                                          828.3
                                                                          813.0
                                                                          818.8
                                                                          768.9
                                                                          802.1
                                      GA
                                                                          781.2
                                                                          787.4
                                                                          808.8
                                                                          871.6
                                                                          872.5
                                                                          910.6
                                                                          861.1
                                                                          841.9
                                      IA
                                                                          393.1
                                                                          389.6
                                                                          386.7
                                                                          384.5
                                                                          378.9
                                                                          415.9
                                                                          431.2
                                                                          397.1
                                      IL
                                                                          930.9
                                                                          961.9
                                                                        1,027.1
                                                                        1,002.1
                                                                        1,002.1
                                                                        1,030.5
                                                                        1,032.9
                                                                          998.2
                                      IN
                                                                        1,231.4
                                                                        1,236.3
                                                                        1,266.0
                                                                        1,281.2
                                                                        1,269.9
                                                                        1,256.9
                                                                        1,243.6
                                                                        1,255.0
                                      KS
                                                                          427.5
                                                                          427.5
                                                                          411.9
                                                                          405.2
                                                                          377.6
                                                                          405.4
                                                                          373.5
                                                                          404.1
                                      KY
                                                                          946.5
                                                                          931.1
                                                                          938.1
                                                                          970.3
                                                                          987.0
                                                                          977.9
                                                                          978.5
                                                                          961.3
                                      MD
                                                                          276.9
                                                                          278.0
                                                                          269.9
                                                                          273.7
                                                                          274.7
                                                                          277.1
                                                                          256.0
                                                                          272.3
                                      MI
                                                                          700.1
                                                                          706.9
                                                                          709.7
                                                                          721.1
                                                                          699.7
                                                                          736.0
                                                                          714.4
                                                                          712.5
                                      MN
                                                                          389.2
                                                                          411.0
                                                                          386.7
                                                                          380.1
                                                                          370.3
                                                                          362.9
                                                                          349.8
                                                                          378.6
                                      MO
                                                                          724.5
                                                                          769.6
                                                                          770.8
                                                                          783.6
                                                                          783.5
                                                                          762.0
                                                                          732.8
                                                                          761.0
                                      NC
                                                                          693.3
                                                                          689.1
                                                                          696.5
                                                                          719.7
                                                                          699.1
                                                                          726.4
                                                                          713.2
                                                                          705.3
                                      NE
                                                                          227.0
                                                                          238.3
                                                                          231.2
                                                                          238.3
                                                                          234.0
                                                                          222.4
                                                                          233.8
                                                                          232.1
                                      NJ
                                                                           80.0
                                                                           75.3
                                                                           79.6
                                                                           92.2
                                                                           81.9
                                                                           77.8
                                                                           62.8
                                                                           78.5
                                      NY
                                                                          234.1
                                                                          235.9
                                                                          226.7
                                                                          213.4
                                                                          211.2
                                                                          211.0
                                                                          181.0
                                                                          216.2
                                      OH
                                                                        1,290.2
                                                                        1,316.3
                                                                        1,247.3
                                                                        1,315.1
                                                                        1,294.0
                                                                        1,319.7
                                                                        1,290.0
                                                                        1,296.1
                                      PA
                                                                        1,026.9
                                                                        1,050.1
                                                                        1,078.6
                                                                        1,090.2
                                                                        1,090.5
                                                                        1,105.2
                                                                        1,065.1
                                                                        1,072.4
                                      SC
                                                                          381.5
                                                                          384.7
                                                                          401.4
                                                                          407.7
                                                                          410.4
                                                                          422.6
                                                                          417.1
                                                                          403.7
                                      TN
                                                                          624.7
                                                                          574.9
                                                                          589.4
                                                                          586.3
                                                                          606.9
                                                                          617.2
                                                                          578.0
                                                                          596.8
                                      TX
                                                                        1,529.9
                                                                        1,572.9
                                                                        1,580.3
                                                                        1,581.3
                                                                        1,546.5
                                                                        1,590.7
                                                                        1,565.7
                                                                        1,566.7
                                      VA
                                                                          340.5
                                                                          312.5
                                                                          296.7
                                                                          298.4
                                                                          288.0
                                                                          298.9
                                                                          264.2
                                                                          299.9
                                      WI
                                                                          457.1
                                                                          475.9
                                                                          482.9
                                                                          473.2
                                                                          451.2
                                                                          447.5
                                                                          450.8
                                                                          462.6
                                      WV
                                                                          898.8
                                                                          898.5
                                                                          851.0
                                                                          861.2
                                                                          852.4
                                                                          883.3
                                                                          851.6
                                                                          871.0
*Source:  EPA, March 2010. All relevant ARP units in the Transport Rule region.  These data are available at http://www.epa.gov/airmarkets/ through Data and Maps.Table 2: Total Annual Heat Input* (million MMBtu) from All Three** EGU Categories for Units Greater Than or Equal to 25 MW for Each Year.
                                     State
                                     2002
                                     2003
                                     2004
                                     2005
                                     2006
                                     2007
                                     2008
                                  Average HI
                                      AL
                                                                          901.0
                                                                          916.0
                                                                          904.6
                                                                          936.6
                                                                          964.1
                                                                        1,000.0
                                                                          938.9
                                                                          937.3
                                      GA
                                                                          841.9
                                                                          822.4
                                                                          853.4
                                                                          948.0
                                                                          968.5
                                                                        1,033.0
                                                                          958.2
                                                                          917.9
                                      IA
                                                                          393.2
                                                                          390.3
                                                                          391.2
                                                                          401.3
                                                                          396.3
                                                                          438.0
                                                                          446.5
                                                                          408.1
                                      IL
                                                                          976.1
                                                                          982.7
                                                                        1,046.2
                                                                        1,053.8
                                                                        1,038.2
                                                                        1,086.2
                                                                        1,059.9
                                                                        1,034.7
                                      IN
                                                                        1,256.8
                                                                        1,257.5
                                                                        1,283.6
                                                                        1,315.1
                                                                        1,300.4
                                                                        1,301.2
                                                                        1,282.2
                                                                        1,285.3
                                      KS
                                                                          429.2
                                                                          429.4
                                                                          412.5
                                                                          406.8
                                                                          381.2
                                                                          410.0
                                                                          378.6
                                                                          406.8
                                      KY
                                                                          962.4
                                                                          935.1
                                                                          942.4
                                                                          988.3
                                                                          999.0
                                                                          997.2
                                                                          987.8
                                                                          973.2
                                      MD
                                                                          288.2
                                                                          290.6
                                                                          280.8
                                                                          287.8
                                                                          282.6
                                                                          286.0
                                                                          263.7
                                                                          282.8
                                      MI
                                                                          726.9
                                                                          725.1
                                                                          727.3
                                                                          760.5
                                                                          730.7
                                                                          776.5
                                                                          743.8
                                                                          741.5
                                      MN
                                                                          399.0
                                                                          423.1
                                                                          396.4
                                                                          401.8
                                                                          391.7
                                                                          394.6
                                                                          371.6
                                                                          396.9
                                      MO
                                                                          752.1
                                                                          788.7
                                                                          792.2
                                                                          813.6
                                                                          812.5
                                                                          800.6
                                                                          770.3
                                                                          790.0
                                      NC
                                                                          718.8
                                                                          706.3
                                                                          718.4
                                                                          747.2
                                                                          727.1
                                                                          767.7
                                                                          749.7
                                                                          733.6
                                      NE
                                                                          228.4
                                                                          240.5
                                                                          233.5
                                                                          245.3
                                                                          240.7
                                                                          231.9
                                                                          240.3
                                                                          237.2
                                      NJ
                                                                          139.3
                                                                          132.9
                                                                          151.5
                                                                          154.9
                                                                          158.8
                                                                          172.7
                                                                          172.8
                                                                          154.7
                                      NY
                                                                          288.3
                                                                          297.3
                                                                          309.9
                                                                          322.9
                                                                          380.6
                                                                          393.1
                                                                          366.6
                                                                          337.0
                                      OH
                                                                        1,309.6
                                                                        1,330.3
                                                                        1,258.3
                                                                        1,340.0
                                                                        1,313.6
                                                                        1,354.5
                                                                        1,310.4
                                                                        1,316.7
                                      PA
                                                                        1,060.2
                                                                        1,090.0
                                                                        1,155.1
                                                                        1,172.4
                                                                        1,185.4
                                                                        1,240.9
                                                                        1,200.0
                                                                        1,157.7
                                      SC
                                                                          418.2
                                                                          402.7
                                                                          442.1
                                                                          454.9
                                                                          461.5
                                                                          474.0
                                                                          464.4
                                                                          445.4
                                      TN
                                                                          635.7
                                                                          580.5
                                                                          592.1
                                                                          592.6
                                                                          613.2
                                                                          623.8
                                                                          582.3
                                                                          602.9
                                      TX
                                                                        2,247.3
                                                                        2,378.4
                                                                        2,540.8
                                                                        2,650.2
                                                                        2,658.4
                                                                        2,752.9
                                                                        2,698.6
                                                                        2,561.0
                                      VA
                                                                          356.6
                                                                          333.6
                                                                          329.5
                                                                          339.5
                                                                          324.0
                                                                          354.8
                                                                          312.3
                                                                          335.7
                                      WI
                                                                          475.3
                                                                          494.1
                                                                          498.4
                                                                          528.5
                                                                          492.3
                                                                          497.4
                                                                          490.0
                                                                          496.6
                                      WV
                                                                          900.5
                                                                          899.9
                                                                          852.0
                                                                          863.3
                                                                          855.8
                                                                          887.1
                                                                          853.3
                                                                          873.1
*Source:  EPA, March 2010. All relevant ARP units in the Transport Rule region.  These data are available at http://www.epa.gov/airmarkets/ through Data and Maps.
** Coal-fired units; combined cycle turbines; and a combination of oil- and gas-fired boilers and simple-cycle combustion turbines.

Table 3: Total Annual Heat Input* (million MMBtu) from All Three** EGU Categories for Units Greater Than or Equal to 25 MW for Ozone Season.

                                     State
                                     2002
                                     2003
                                     2004
                                     2005
                                     2006
                                     2007
                                     2008
                                  Average HI
                                      AL
                                                                          427.4
                                                                          428.8
                                                                          423.6
                                                                          427.2
                                                                          456.9
                                                                          469.9
                                                                          432.5
                                                                          438.1
                                      AR
                                                                          124.0
                                                                          152.5
                                                                          149.2
                                                                          142.0
                                                                          173.6
                                                                          170.4
                                                                          150.5
                                                                          151.7
                                      FL
                                                                          548.6
                                                                          554.9
                                                                          563.3
                                                                          615.1
                                                                          639.0
                                                                          656.3
                                                                          646.0
                                                                          603.3
                                      GA
                                                                          403.8
                                                                          382.9
                                                                          407.1
                                                                          461.8
                                                                          469.5
                                                                          489.6
                                                                          449.9
                                                                          437.8
                                      IA
                                                                          170.8
                                                                          161.8
                                                                          161.8
                                                                          183.5
                                                                          168.9
                                                                          189.8
                                                                          182.0
                                                                          174.1
                                      IL
                                                                          454.9
                                                                          430.2
                                                                          449.6
                                                                          478.0
                                                                          460.8
                                                                          474.6
                                                                          452.6
                                                                          457.2
                                      IN
                                                                          557.2
                                                                          545.4
                                                                          540.7
                                                                          578.4
                                                                          566.4
                                                                          559.8
                                                                          550.5
                                                                          556.9
                                      KS
                                                                          183.2
                                                                          180.9
                                                                          176.8
                                                                          180.6
                                                                          172.2
                                                                          177.1
                                                                          160.7
                                                                          175.9
                                      KY
                                                                          433.3
                                                                          400.1
                                                                          401.0
                                                                          434.4
                                                                          435.3
                                                                          436.1
                                                                          417.8
                                                                          422.6
                                      LA
                                                                           94.7
                                                                          121.7
                                                                          144.9
                                                                          149.9
                                                                          161.8
                                                                          150.7
                                                                          152.2
                                                                          139.4
                                      MD
                                                                          134.8
                                                                          118.8
                                                                          121.8
                                                                          131.5
                                                                          123.3
                                                                          126.8
                                                                          117.1
                                                                          124.9
                                      MI
                                                                          331.0
                                                                          312.1
                                                                          304.6
                                                                          339.9
                                                                          328.5
                                                                          344.5
                                                                          321.1
                                                                          326.0
                                      MO
                                                                          337.7
                                                                          347.6
                                                                          348.0
                                                                          354.2
                                                                          362.8
                                                                          357.7
                                                                          341.7
                                                                          349.9
                                      MS
                                                                          117.8
                                                                          116.2
                                                                          129.5
                                                                          137.3
                                                                          147.7
                                                                          155.9
                                                                          143.2
                                                                          135.4
                                      NC
                                                                          332.9
                                                                          308.5
                                                                          314.0
                                                                          341.5
                                                                          340.4
                                                                          351.5
                                                                          344.8
                                                                          333.4
                                      NJ
                                                                           68.7
                                                                           59.9
                                                                           73.9
                                                                           76.1
                                                                           79.9
                                                                           86.1
                                                                           86.6
                                                                           75.9
                                      NY
                                                                          126.1
                                                                          121.4
                                                                          136.3
                                                                          152.5
                                                                          174.7
                                                                          170.5
                                                                          158.9
                                                                          148.6
                                      OH
                                                                          583.7
                                                                          562.8
                                                                          538.4
                                                                          574.4
                                                                          562.0
                                                                          596.7
                                                                          553.3
                                                                          567.3
                                      OK
                                                                          204.5
                                                                          223.4
                                                                          220.0
                                                                          248.4
                                                                          244.6
                                                                          241.1
                                                                          247.9
                                                                          232.8
                                      PA
                                                                          465.4
                                                                          468.7
                                                                          494.6
                                                                          529.3
                                                                          530.6
                                                                          550.8
                                                                          515.5
                                                                          507.8
                                      SC
                                                                          203.1
                                                                          178.4
                                                                          204.8
                                                                          214.1
                                                                          215.8
                                                                          222.7
                                                                          222.0
                                                                          208.7
                                      TN
                                                                          291.2
                                                                          244.3
                                                                          256.2
                                                                          270.1
                                                                          273.2
                                                                          278.9
                                                                          261.2
                                                                          267.9
                                      TX
                                                                        1,031.9
                                                                        1,089.9
                                                                        1,170.0
                                                                        1,229.1
                                                                        1,255.4
                                                                        1,262.8
                                                                        1,252.7
                                                                        1,184.6
                                      VA
                                                                          160.0
                                                                          146.9
                                                                          150.9
                                                                          154.3
                                                                          151.5
                                                                          167.9
                                                                          144.5
                                                                          153.7
                                      WI
                                                                          218.5
                                                                          211.1
                                                                          207.4
                                                                          240.0
                                                                          223.0
                                                                          219.8
                                                                          208.5
                                                                          218.3
                                      WV
                                                                          379.5
                                                                          384.5
                                                                          361.0
                                                                          375.4
                                                                          373.1
                                                                          390.3
                                                                          366.9
                                                                          375.8
*Source:  EPA, March 2010; All relevant ARP units in the Transport Rule region.  These data are available at http://www.epa.gov/airmarkets/ through Data and Maps.
** Coal-fired units; combined cycle turbines; and a combination of oil- and gas-fired boilers and simple-cycle combustion turbines.

      (b) Estimating the state-by-state variability in heat input using historical heat input data sets.
      This subsection describes the method EPA used to estimate the variability in heat input for each state covered by the final Transport Rule.  It repeats some of the relevant data and calculations presented in the proposed Transport Rule TSD "Power Sector Variability" with updates reflecting the states covered by the final Transport Rule.
      In the final Transport Rule, EPA used the method presented in the proposed Transport Rule and assessed the year-to-year variability over the 7-year time period of the yearly total heat input values or ozone season heat input values by deriving a standard deviation for each state while "screening out" overall growth or decline in heat input over that time period.  This method is described in detail in this and the following subsections and was selected for two reasons: First, it held constant any longer trend in growth or decline in a state's heat input over the baseline period analyzed.  Second, a statistical approach (i.e., using the standard deviation) is less sensitive to data anomalies present in finite data sets; in other words, it effectively screens out extraneous information to characterize long-run variability.
      For each state, it was important to first adjust the data set to screen out trends (growth or decline) in heat input over the 7-year time period, so that the data would then reflect year-to-year variability independent of trends in electricity demand.  After controlling for growth in heat input over time, the year-to-year variation in heat input was assessed, as the differences between the actual yearly heat input values and the "yearly" average heat input values estimated according to the trend.  To account for trends, a simple least-squares linear regression equation was fit to the heat input data as a function of time.  In other words, this step of the analysis attempts to define the amount of variability in the data set for each state that can be attributed to a trend over time, which EPA is attempting to hold constant in this analysis to determine year-to-year variability independent of longer-run trends in heat input (e.g., rising electricity demand).
      This process fits a straight line to the data points using an equation of the form (y = mx+b).  In this equation, "y" is the estimated heat input (million MMBtu) for a particular year "x", m is the slope of the line (with units of million MMBtu/year), and b is the "y-intercept" (the heat input value when the line is extrapolated to x = 0).  The value of r[2], the square of the Pearson product moment correlation coefficient, describes the proportion of variance in "y" that is accounted for by knowing "x".  It indicates how significant a trend (growth or decline) is present in the data.  Large r[2] values (r[2] values are maximized at 1) indicate that the least squares fit picks up a strong trend (upward or downward) in the data which should be removed by the procedure described below.  This is important for states where there was substantial growth or decline in heat input over the time period (i.e., for states that have slopes for the regression line that are substantially different than zero).  Small r[2] values, on the other hand, indicate that the least squares fit did not detect a strong trend (upward or downward) in the heat input data.  Consequently, the procedure described below has less of a corrective role to play, since there is no strong trend that needs to be controlled for (held constant) when characterizing the data set's year-to-year variability.
      For each state, the slopes of the regression equations, y-intercepts, and r[2] values can be found in Tables 4, 5, and 6.  Using the slope and y-intercept from the regression equation for each state, yearly heat input values were estimated for each state for each year between 2002 and 2008 (Tables 4, 5, and 6).  Year-to-year variation was assessed for each year or for each ozone season as the difference between the actual heat input (from Tables 1-3) and the heat input estimated using the regression equation (Tables 4-6).

Table 4: Heat Input (million MMBtu) from Coal-Fired Units for Each Year Estimated Using the Regression Equation.

                                     State
                Slope of Linear Regression (million MMBtu/year)
                                   Intercept
                                  r[2] Value
                                     2002
                                     2003
                                     2004
                                     2005
                                     2006
                                     2007
                                     2008
                                      AL
                                                                          -0.34
                                                                          1,492
                                                                           0.00
                                                                          803.2
                                                                          802.8
                                                                          802.5
                                                                          802.1
                                                                          801.8
                                                                          801.5
                                                                          801.1
                                      GA
                                                                          19.64
                                                                        -38,527
                                                                           0.74
                                                                          783.0
                                                                          802.6
                                                                          822.3
                                                                          841.9
                                                                          861.5
                                                                          881.2
                                                                          900.8
                                      IA
                                                                           5.69
                                                                        -11,002
                                                                           0.41
                                                                          380.1
                                                                          385.8
                                                                          391.4
                                                                          397.1
                                                                          402.8
                                                                          408.5
                                                                          414.2
                                      IL
                                                                          14.94
                                                                        -28,960
                                                                           0.70
                                                                          953.4
                                                                          968.3
                                                                          983.3
                                                                          998.2
                                                                        1,013.2
                                                                        1,028.1
                                                                        1,043.0
                                      IN
                                                                           2.92
                                                                         -4,592
                                                                           0.12
                                                                        1,246.3
                                                                        1,249.2
                                                                        1,252.1
                                                                        1,255.0
                                                                        1,257.9
                                                                        1,260.9
                                                                        1,263.8
                                      KS
                                                                          -8.59
                                                                         17,633
                                                                           0.74
                                                                          429.9
                                                                          421.3
                                                                          412.7
                                                                          404.1
                                                                          395.5
                                                                          386.9
                                                                          378.3
                                      KY
                                                                           8.52
                                                                        -16,113
                                                                           0.68
                                                                          935.8
                                                                          944.3
                                                                          952.8
                                                                          961.3
                                                                          969.9
                                                                          978.4
                                                                          986.9
                                      MD
                                                                          -2.13
                                                                          4,545
                                                                           0.36
                                                                          278.7
                                                                          276.6
                                                                          274.5
                                                                          272.3
                                                                          270.2
                                                                          268.1
                                                                          265.9
                                      MI
                                                                           3.26
                                                                         -5,825
                                                                           0.30
                                                                          702.8
                                                                          706.0
                                                                          709.3
                                                                          712.5
                                                                          715.8
                                                                          719.1
                                                                          722.3
                                      MN
                                                                          -8.25
                                                                         16,915
                                                                           0.80
                                                                          403.3
                                                                          395.1
                                                                          386.8
                                                                          378.6
                                                                          370.3
                                                                          362.1
                                                                          353.8
                                      MO
                                                                           0.80
                                                                           -836
                                                                           0.01
                                                                          758.6
                                                                          759.4
                                                                          760.2
                                                                          761.0
                                                                          761.8
                                                                          762.6
                                                                          763.4
                                      NC
                                                                           4.90
                                                                         -9,111
                                                                           0.54
                                                                          690.6
                                                                          695.5
                                                                          700.4
                                                                          705.3
                                                                          710.2
                                                                          715.1
                                                                          720.0
                                      NE
                                                                          -0.32
                                                                            869
                                                                           0.01
                                                                          233.1
                                                                          232.8
                                                                          232.5
                                                                          232.1
                                                                          231.8
                                                                          231.5
                                                                          231.2
                                      NJ
                                                                          -1.58
                                                                          3,241
                                                                           0.15
                                                                           83.2
                                                                           81.7
                                                                           80.1
                                                                           78.5
                                                                           76.9
                                                                           75.4
                                                                           73.8
                                      NY
                                                                          -8.01
                                                                         16,282
                                                                           0.85
                                                                          240.2
                                                                          232.2
                                                                          224.2
                                                                          216.2
                                                                          208.2
                                                                          200.2
                                                                          192.2
                                      OH
                                                                           1.88
                                                                         -2,480
                                                                           0.03
                                                                        1,290.4
                                                                        1,292.3
                                                                        1,294.2
                                                                        1,296.1
                                                                        1,298.0
                                                                        1,299.8
                                                                        1,301.7
                                      PA
                                                                           8.46
                                                                        -15,880
                                                                           0.46
                                                                        1,047.0
                                                                        1,055.5
                                                                        1,063.9
                                                                        1,072.4
                                                                        1,080.8
                                                                        1,089.3
                                                                        1,097.7
                                      SC
                                                                           6.84
                                                                        -13,312
                                                                           0.90
                                                                          383.1
                                                                          390.0
                                                                          396.8
                                                                          403.7
                                                                          410.5
                                                                          417.3
                                                                          424.2
                                      TN
                                                                          -1.36
                                                                          3,314
                                                                           0.02
                                                                          600.8
                                                                          599.5
                                                                          598.1
                                                                          596.8
                                                                          595.4
                                                                          594.1
                                                                          592.7
                                      TX
                                                                           3.89
                                                                         -6,231
                                                                           0.15
                                                                        1,555.1
                                                                        1,559.0
                                                                        1,562.9
                                                                        1,566.7
                                                                        1,570.6
                                                                        1,574.5
                                                                        1,578.4
                                      VA
                                                                          -9.45
                                                                         19,252
                                                                           0.77
                                                                          328.2
                                                                          318.8
                                                                          309.3
                                                                          299.9
                                                                          290.4
                                                                          281.0
                                                                          271.5
                                      WI
                                                                          -3.84
                                                                          8,161
                                                                           0.34
                                                                          474.2
                                                                          470.3
                                                                          466.5
                                                                          462.6
                                                                          458.8
                                                                          455.0
                                                                          451.1
                                      WV
                                                                          -6.09
                                                                         13,082
                                                                           0.36
                                                                          889.2
                                                                          883.2
                                                                          877.1
                                                                          871.0
                                                                          864.9
                                                                          858.8
                                                                          852.7

Table 5: Total Heat Input (million MMBtu) from All Three* EGU Categories for Each Year Estimated Using the Regression Equation.

                                     State
                Slope of Linear Regression (million MMBtu/year)
                                   Intercept
                                  r[2] Value
                                     2002
                                     2003
                                     2004
                                     2005
                                     2006
                                     2007
                                     2008
                                      AL
                                                                          12.18
                                                                        -23,490
                                                                           0.55
                                                                          900.8
                                                                          913.0
                                                                          925.1
                                                                          937.3
                                                                          949.5
                                                                          961.7
                                                                          973.9
                                      GA
                                                                          31.61
                                                                        -62,466
                                                                           0.75
                                                                          823.1
                                                                          854.7
                                                                          886.3
                                                                          917.9
                                                                          949.5
                                                                          981.1
                                                                        1,012.8
                                      IA
                                                                           9.30
                                                                        -18,233
                                                                           0.72
                                                                          380.2
                                                                          389.5
                                                                          398.8
                                                                          408.1
                                                                          417.4
                                                                          426.7
                                                                          436.0
                                      IL
                                                                          16.09
                                                                        -31,231
                                                                           0.73
                                                                          986.5
                                                                        1,002.5
                                                                        1,018.6
                                                                        1,034.7
                                                                        1,050.8
                                                                        1,066.9
                                                                        1,083.0
                                      IN
                                                                           6.44
                                                                        -11,635
                                                                           0.39
                                                                        1,265.9
                                                                        1,272.4
                                                                        1,278.8
                                                                        1,285.3
                                                                        1,291.7
                                                                        1,298.2
                                                                        1,304.6
                                      KS
                                                                          -7.92
                                                                         16,295
                                                                           0.70
                                                                          430.6
                                                                          422.7
                                                                          414.8
                                                                          406.8
                                                                          398.9
                                                                          391.0
                                                                          383.1
                                      KY
                                                                           9.17
                                                                        -17,418
                                                                           0.56
                                                                          945.7
                                                                          954.8
                                                                          964.0
                                                                          973.2
                                                                          982.3
                                                                          991.5
                                                                        1,000.7
                                      MD
                                                                          -2.89
                                                                          6,080
                                                                           0.47
                                                                          291.5
                                                                          288.6
                                                                          285.7
                                                                          282.8
                                                                          279.9
                                                                          277.0
                                                                          274.1
                                      MI
                                                                           5.60
                                                                        -10,480
                                                                           0.37
                                                                          724.7
                                                                          730.3
                                                                          735.9
                                                                          741.5
                                                                          747.1
                                                                          752.7
                                                                          758.3
                                      MN
                                                                          -5.14
                                                                         10,702
                                                                           0.53
                                                                          412.3
                                                                          407.2
                                                                          402.0
                                                                          396.9
                                                                          391.7
                                                                          386.6
                                                                          381.5
                                      MO
                                                                           3.53
                                                                         -6,280
                                                                           0.12
                                                                          779.4
                                                                          782.9
                                                                          786.5
                                                                          790.0
                                                                          793.5
                                                                          797.0
                                                                          800.6
                                      NC
                                                                           8.00
                                                                        -15,302
                                                                           0.63
                                                                          709.6
                                                                          717.6
                                                                          725.6
                                                                          733.6
                                                                          741.6
                                                                          749.6
                                                                          757.6
                                      NE
                                                                           0.92
                                                                         -1,612
                                                                           0.11
                                                                          234.4
                                                                          235.4
                                                                          236.3
                                                                          237.2
                                                                          238.1
                                                                          239.1
                                                                          240.0
                                      NJ
                                                                           6.69
                                                                        -13,250
                                                                           0.90
                                                                          134.6
                                                                          141.3
                                                                          148.0
                                                                          154.7
                                                                          161.4
                                                                          168.1
                                                                          174.8
                                      NY
                                                                          17.76
                                                                        -35,264
                                                                           0.82
                                                                          283.7
                                                                          301.4
                                                                          319.2
                                                                          337.0
                                                                          354.7
                                                                          372.5
                                                                          390.2
                                      OH
                                                                           3.79
                                                                         -6,281
                                                                           0.07
                                                                        1,305.3
                                                                        1,309.1
                                                                        1,312.9
                                                                        1,316.7
                                                                        1,320.5
                                                                        1,324.3
                                                                        1,328.1
                                      PA
                                                                          26.84
                                                                        -52,648
                                                                           0.85
                                                                        1,077.2
                                                                        1,104.0
                                                                        1,130.9
                                                                        1,157.7
                                                                        1,184.5
                                                                        1,211.4
                                                                        1,238.2
                                      SC
                                                                          10.75
                                                                        -21,104
                                                                           0.79
                                                                          413.2
                                                                          423.9
                                                                          434.6
                                                                          445.4
                                                                          456.1
                                                                          466.9
                                                                          477.6
                                      TN
                                                                          -1.88
                                                                          4,376
                                                                           0.04
                                                                          608.5
                                                                          606.6
                                                                          604.8
                                                                          602.9
                                                                          601.0
                                                                          599.1
                                                                          597.2
                                      TX
                                                                          79.30
                                                                       -156,429
                                                                           0.86
                                                                        2,323.1
                                                                        2,402.4
                                                                        2,481.7
                                                                        2,561.0
                                                                        2,640.3
                                                                        2,719.6
                                                                        2,798.8
                                      VA
                                                                          -3.42
                                                                          7,203
                                                                           0.21
                                                                          346.0
                                                                          342.6
                                                                          339.2
                                                                          335.7
                                                                          332.3
                                                                          328.9
                                                                          325.5
                                      WI
                                                                           1.60
                                                                         -2,711
                                                                           0.05
                                                                          491.8
                                                                          493.4
                                                                          495.0
                                                                          496.6
                                                                          498.2
                                                                          499.8
                                                                          501.4
                                      WV
                                                                          -5.83
                                                                         12,568
                                                                           0.33
                                                                          890.6
                                                                          884.8
                                                                          879.0
                                                                          873.1
                                                                          867.3
                                                                          861.5
                                                                          855.6

* Coal-fired units; combined cycle turbines; and a combination of oil- and gas-fired boilers and simple-cycle combustion turbines.
Table 6: Heat Input (million MMBtu) from All Three* EGU Categories for Ozone Season Estimated Using the Regression Equation.

                                     State
                Slope of Linear Regression (million MMBtu/year)
                                   Intercept
                                  r[2] Value
                                     2002
                                     2003
                                     2004
                                     2005
                                     2006
                                     2007
                                     2008
                                      AL
                                                                           4.68
                                                                         -8,936
                                                                           0.32
                                                                          424.0
                                                                          428.7
                                                                          433.4
                                                                          438.1
                                                                          442.7
                                                                          447.4
                                                                          452.1
                                      AR
                                                                           4.99
                                                                         -9,850
                                                                           0.41
                                                                          136.8
                                                                          141.8
                                                                          146.8
                                                                          151.7
                                                                          156.7
                                                                          161.7
                                                                          166.7
                                      FL
                                                                          20.38
                                                                        -40,256
                                                                           0.90
                                                                          542.2
                                                                          562.5
                                                                          582.9
                                                                          603.3
                                                                          623.7
                                                                          644.1
                                                                          664.4
                                      GA
                                                                          14.80
                                                                        -29,231
                                                                           0.64
                                                                          393.4
                                                                          408.2
                                                                          423.0
                                                                          437.8
                                                                          452.6
                                                                          467.4
                                                                          482.2
                                      IA
                                                                           3.44
                                                                         -6,732
                                                                           0.45
                                                                          163.7
                                                                          167.2
                                                                          170.6
                                                                          174.1
                                                                          177.5
                                                                          181.0
                                                                          184.4
                                      IL
                                                                           3.33
                                                                         -6,214
                                                                           0.20
                                                                          447.3
                                                                          450.6
                                                                          453.9
                                                                          457.2
                                                                          460.6
                                                                          463.9
                                                                          467.2
                                      IN
                                                                           1.23
                                                                         -1,911
                                                                           0.04
                                                                          553.2
                                                                          554.4
                                                                          555.7
                                                                          556.9
                                                                          558.1
                                                                          559.4
                                                                          560.6
                                      KS
                                                                          -2.84
                                                                          5,880
                                                                           0.65
                                                                          184.5
                                                                          181.6
                                                                          178.8
                                                                          175.9
                                                                          173.1
                                                                          170.3
                                                                          167.4
                                      KY
                                                                           2.15
                                                                         -3,883
                                                                           0.08
                                                                          416.1
                                                                          418.3
                                                                          420.4
                                                                          422.6
                                                                          424.7
                                                                          426.9
                                                                          429.0
                                      LA
                                                                           8.84
                                                                        -17,590
                                                                           0.67
                                                                          112.9
                                                                          121.7
                                                                          130.6
                                                                          139.4
                                                                          148.3
                                                                          157.1
                                                                          166.0
                                      MD
                                                                          -1.27
                                                                          2,671
                                                                           0.18
                                                                          128.7
                                                                          127.4
                                                                          126.1
                                                                          124.9
                                                                          123.6
                                                                          122.3
                                                                          121.1
                                      MI
                                                                           2.10
                                                                         -3,891
                                                                           0.10
                                                                          319.7
                                                                          321.8
                                                                          323.9
                                                                          326.0
                                                                          328.1
                                                                          330.2
                                                                          332.3
                                      MO
                                                                           1.68
                                                                         -3,026
                                                                           0.17
                                                                          344.9
                                                                          346.6
                                                                          348.3
                                                                          349.9
                                                                          351.6
                                                                          353.3
                                                                          355.0
                                      MS
                                                                           6.21
                                                                        -12,325
                                                                           0.80
                                                                          116.7
                                                                          122.9
                                                                          129.1
                                                                          135.4
                                                                          141.6
                                                                          147.8
                                                                          154.0
                                      NC
                                                                           5.29
                                                                        -10,265
                                                                           0.50
                                                                          317.5
                                                                          322.8
                                                                          328.1
                                                                          333.4
                                                                          338.7
                                                                          343.9
                                                                          349.2
                                      NJ
                                                                           4.01
                                                                         -7,956
                                                                           0.82
                                                                           63.9
                                                                           67.9
                                                                           71.9
                                                                           75.9
                                                                           79.9
                                                                           83.9
                                                                           87.9
                                      NY
                                                                           8.39
                                                                        -16,679
                                                                           0.74
                                                                          123.4
                                                                          131.8
                                                                          140.2
                                                                          148.6
                                                                          157.0
                                                                          165.4
                                                                          173.8
                                      OH
                                                                           0.01
                                                                            552
                                                                           0.00
                                                                          567.3
                                                                          567.3
                                                                          567.3
                                                                          567.3
                                                                          567.3
                                                                          567.3
                                                                          567.3
                                      OK
                                                                           6.80
                                                                        -13,402
                                                                           0.75
                                                                          212.4
                                                                          219.2
                                                                          226.0
                                                                          232.8
                                                                          239.6
                                                                          246.4
                                                                          253.2
                                      PA
                                                                          12.52
                                                                        -24,588
                                                                           0.69
                                                                          470.3
                                                                          482.8
                                                                          495.3
                                                                          507.8
                                                                          520.4
                                                                          532.9
                                                                          545.4
                                      SC
                                                                           5.58
                                                                        -10,974
                                                                           0.62
                                                                          192.0
                                                                          197.5
                                                                          203.1
                                                                          208.7
                                                                          214.3
                                                                          219.8
                                                                          225.4
                                      TN
                                                                          -0.12
                                                                            517
                                                                           0.00
                                                                          268.2
                                                                          268.1
                                                                          268.0
                                                                          267.9
                                                                          267.7
                                                                          267.6
                                                                          267.5
                                      TX
                                                                          39.05
                                                                        -77,113
                                                                           0.85
                                                                        1,067.4
                                                                        1,106.5
                                                                        1,145.5