Response to US EPA comment #524-20, "While the Draft EIS relied on [a dispersion] analysis contained in the Resolution Copper Project NEPA Air Quality Impacts Analyses, we are concerned that it may have underestimated PM10, as well as PM2.5, emissions. The dispersion analysis used the Arizona Statewide average for silt content of 3.0% in calculations for fugitive emissions from unpaved road and other sources. For unpaved roads, EPA's AP-42, Chapter 13.2.2, outlines that site-specific silt content information should be used when available, but, when they cannot be obtained, the sector-specific averages available in Table 13.2.2-1 should be used. In the absence of site-specific silt content for unpaved road emissions from the project, we recommend the sector-specific averages as the closest analogue and average silt content to use. In either case, it does not appear that modelled air quality concentrations would exceed the PM10 or PM2.5 NAAQS; however, this should be verified by the USFS. We acknowledge the revised dispersion analysis is unlikely to show the preferred alternative will cause or contribute to an increase in the severity or frequency of NAAQS violations […]