Memorandum
August 8, 2017
To: 		Docket EPA-R09-OAR-2017-0377
From: 		Ashley Graham, U.S. EPA Region 9, Air Planning Office
Subject: 	Technical note regarding emissions inventories in Arizona's San Manuel SO2 2[nd] 10-year Maintenance Plan
EPA performed additional analyses and data review to examine the relative impact of inconsistencies in population and land area values on emissions inventories presented in Arizona's "San Manuel Sulfur Dioxide Maintenance Plan Renewal, 1971 Sulfur Dioxide National Ambient Air Quality Standards". EPA's analysis found that the inconsistencies in Arizona's submittal have only a minor impact, or in some cases no impact, on the emissions estimates derived. In cases where discrepancies did have a minor impact on emissions estimates, the EPA found that they led to an overestimate in SO2 emissions and thus do not alter Arizona's findings that emissions through 2028 are projected to be well below the 2005 attainment inventory emissions and that emissions estimates demonstrate continued attainment of the 1971 SO2 NAAQS in the San Manuel maintenance area.
Attachment: Technical Note Regarding Arizona's San Manuel SO2 2[nd] Maintenance Plan













Technical Note Regarding Arizona's San Manuel SO2 2[nd] Maintenance Plan 
Arizona's "San Manuel Sulfur Dioxide Maintenance Plan Renewal, 1971 Sulfur Dioxide National Ambient Air Quality Standards" demonstrates maintenance of the 1971 primary SO2 NAAQS, in part, by showing that current and projected future emissions of SO2 do not exceed the level of the attainment inventory. The plan compares emissions inventories representing current emissions for 2014 and projected emissions for three years extending through the end of the second maintenance period (2019, 2023, and 2028) to a 2005 attainment year inventory for sources in the San Manuel maintenance area. 
The San Manuel maintenance area includes portions of Pima and Pinal counties. ADEQ relied on the EPA's 2014 NEIv1 data as the basis for estimating 2014 SO2 emissions for nonpoint sectors and the EPA's Motor Vehicle Emission Simulator (MOVES) model for estimating mobile source SO2 emissions in Pima and Pinal counties. County level data were allocated to the San Manuel Maintenance Area by applying population, land area, airport activity, and rail line length allocation factors (see Appendix A San Manuel SO2 Emission Inventory Technical Support Document Section A3.2). Future year emissions were estimated by applying growth factors to 2014 base year emissions. 
During the EPA's review of Arizona's submittal, we identified several inconsistencies in the 2014 population, land area, and interim emission year values reported. First, Tables A-3 and A-25 indicate that data for 2024 are presented, whereas elsewhere in the submittal, ADEQ indicates that 2023 was used as the interim year. ADEQ confirmed that references to 2024 are typos and that values presented are for a 2023 interim year.
In Section A1.4 Population, the submittal indicates that the 2014 populations for Pima and Pinal counties were 980,263 and 375,770 persons, respectively (see Table 1). However, Table A-9: Population indicates that the 2014 populations for Pima and Pinal counties were 1,007,162 and 396,237, respectively. Values in Table A-9 were used to allocate county-level emissions to the maintenance area. The EPA reviewed the Arizona Department of Administration's (ADOA) "July 1, 2014 Population Estimates for Arizona's Counties, Incorporated Places and Unincorporated Balance of Counties" and found that the values in Table A-9 and used in ADEQ's analysis are consistent with those reported by ADOA. Therefore, the incorrect values presented in Section A1.4 did not affect ADEQ's analysis results.
Table 1. 2014 County-Level Population

                            Section A1.4 Population
                            Table A-9: Population*
 Pima County
                                    980,263
                                   1,007,162
 Pinal County
                                    375,770
                                    396,237
* Values in Table A-9: Population are consistent with the July 1, 2014 Population Estimates for Arizona's Counties, Incorporated Places and Unincorporated Balance of Counties (https://population.az.gov/sites/default/files/documents/files/pop-estimates2014-04pla.pdf) released by the Arizona Department of Administration.
Another inconsistency among values reported in Arizona's submittal was for the 2014 population for the Pinal County portion of the maintenance area. Table 1.3 Population Distribution, Selected Years, 2010-2028 and Table A-2: Geographic Location and Population indicate that the 2014 population of the Pinal County portion of the maintenance area was 9,981, whereas Table A-9 Population indicates that the population was 9,480. The EPA calculated nonpoint emissions for the relevant emission sectors (i.e., emissions sectors for which ADEQ used population to allocate emissions from the county level to the maintenance area) using each of the population estimates and found that ADEQ's nonpoint emission inventory (see Table A-6) was derived using the higher, more conservative population estimate of 9,981. Furthermore, the EPA found that the difference between nonpoint emissions for 2014 estimated using the two different population estimates is small (~0.1 tpy SO2). Given that ADEQ's analysis used the higher population estimate for the Pinal County portion of the maintenance area and that the relative impact on the emissions estimates is small, the EPA finds that the inconsistency in values reported has a negligible effect on ADEQ's analysis results for the purposes of demonstrating continued attainment of the 1971 SO2 NAAQS in the San Manuel maintenance area.
Finally, the EPA identified some inconsistencies in Arizona's submittal related to the land area values reported for the Pima and Pinal County portions of the San Manuel maintenance area (see Table 2). Table A-2: Geographic Location and Population indicates that the area of the Pinal County portion of the maintenance area is 494.5 mi[2], which is larger than the area reported in the same table for the entire maintenance area of 382 mi[2]. Furthermore, the area of 494.5 mi[2] for the Pinal County portion of the maintenance area is inconsistent with the area reported in Table A-11: Land Area of 347 mi[2]. Both Tables A-2 and A-11 report that the area for the Pima County portion of the maintenance area is 102 mi[2]. ADEQ used the values reported in Table A-11 to calculate land area allocation factors. Using the 1971 SO2 nonattainment area shapefile from the EPA's Green Book (https://www.epa.gov/green-book/green-book-gis-download), the EPA estimates that the area of the San Manuel maintenance area is 382 mi[2], the area of the Pima County portion of the maintenance area is 36 mi[2], and the area of the Pinal County portion of the maintenance area is 347 mi[2]. Using these values, the land area allocation factor for Pima County would be lower than the value used by Arizona to estimate 2014 fire emissions. ADEQ assumed the 2014 wildfire emissions remained constant when projecting emissions into the future. Thus, fire emissions are expected to be lower in 2014 and in future years than estimated by Arizona. 
Table 2. Land area in square miles. 

                 Table A-2: Geographic Location and Population
                             Table A-11: Land Area
                               EPA's Analysis*
San Manuel Maintenance Area (MA)
                                      382
                                       -
                                      382
Pima County portion of the MA
                                     102.0
                                      102
                                      36
Pinal County portion of the MA
                                     494.5
                                      347
                                      347
*EPA Green Book shapefiles (https://www.epa.gov/green-book/green-book-gis-download). 

EPA shared the results of these analyses with ADEQ during a teleconference call on July 11, 2017. Following the call, ADEQ recalculated the affected emissions inventory values using the correct land area and population values and confirmed that the numbers presented in the SIP represent an overestimation. A summary of ADEQ's analysis findings was provided in an email from E. Toon to A. Graham on July 26, 2017 and is included in the docket for EPA's action on ADEQ's submittal (Docket EPA-R09-OAR-2017-0377).
In summary, the inconsistencies the EPA identified in the population and land area values presented in Arizona's submittal have only a minor impact, or in some cases no impact, on the emissions estimates derived. In cases where discrepancies did have a minor impact on emissions estimates, the EPA found that they led to an overestimate in SO2 emissions and thus do not alter Arizona's findings that emissions through 2028 are projected to be well below the 2005 attainment inventory emissions and that emissions estimates demonstrate continued attainment of the 1971 SO2 NAAQS in the San Manuel maintenance area.
