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Proposed PM2.5 Measures for the Austin -Round Rock -Georgetown MSA Regional Air Quality Plan <br />bethe most important step that the region can take towards reducing the highest annual PM2.s concentrations, <br />which are located in the urban core. <br />It's important to note that while these represent the best estimates available, they are characterized by o high <br />degree ofcertainty, especially compared tosome ofthe largest sources ofozone-forming emissions. EPA has <br />developed these estimates based on broad national clatasets and emissions factors, and circumstances may vary <br />significantly location to location. For example, EPA assumes that 12.5% of all PMjo emissions from all mines and <br />quarries is PM2.5, but EPA's estimates for the region do not reflect any unique circumstances that may be <br />present at any specific mine or quarry within the region. <br />How Do the Sources Of PM2.5 Emissions Compare to Sources of Ozone -Forming Emissions? <br />The list of the mains sources Of PM2.5 emissions in the region is very different than the list of main sources of <br />emissions contributing to peak O3formation, which is dominated by mobile sources and point sources. While <br />measures toreduce Ds-fomoinQemissions from mobile sources and point sources will also help reduce PK82.5, <br />those sources do not contribute nearly as much to the region's PM2.5 concentrations as they do to peak 03 <br />formation, and the main sources Of PM2.5 emissions require distinct control measures in order to significantly <br />affect regional PM2.5 concentrations. <br />How DDthe Region's PMzsConcentrations Compare tONAADS? <br />When comparing the region's PM2.5 concentrations to the NAAQS, the concentrations are referred to as "design <br />values," and are based on 3 years' worth of data, (i.e., 2018-2020): <br />a Annual design value: 9.7 micrograms per cubic meter (�tg/ml) — 81% of the maximum allowed <br />o The annual NAA8Sis12.Ouu/m3 <br />o EPA staff had proposed consideration ofoNAAQ3aslow axO.O[t8/m' <br />l4-hourdesign value for 20I8'2O2O:22uo/m^-6396ofthe maximum allowed <br />Page 5 of 10 <br />