Region 5 conducted atmospheric dispersion modeling of the SO2 emissions from the Red Wing facility and found that the emissions, primarily from the mineral wool cupolas, resulted in ambient SO2 concentrations in excess of the 1-hour SO2 NAAQS of 75 ppb. Minnesota SIP rule R 7009.0020 requires that no person shall emit any pollutant in such an amount to cause or contribute to a violation of any ambient air quality standard. On January 7, 2016, Region 5 issued an NOV to USG for exceeding the SO2 NAAQS and, consequently, the Minnesota SIP.
This ACO requires USG to comply with both an hourly and 30-day rolling average SO2 emission limit at the Red Wing facility. Compliance with these emission limits ensures compliance with the 1-hour average SO2 NAAQS. On-going compliance with the NAAQS will be determined by a requirement to conduct annual SO2 stack tests and daily measurements of the materials charged to the facility?s two mineral wool cupolas. This ACO will result in an annual SO2 emission reduction of 311 tons per year and addresses the violations set forth in the NOV.