For those projects that may require modeling, DNR updated three guidance documents and Form MD. The updates remove references to the 24-hour and annual SO2 National Ambient Air Quality Standards (NAAQS), which have been revoked. In Iowa, these standards were revoked on April 9, 2019, or one year after the effective date of the area's initial designation for the 2010 1-hour standard. The 24-hour and annual SO2 NAAQS no longer need to be considered in modeling projects.
The 24-hour and annual SO2 Prevention of Significant Deterioration (PSD) Increments, Significant Impact Levels (SIL), and Significant Monitoring Concentration (SMC) are all still in effect and should continue to be used in PSD applications when necessary.
The affected documents include Form MD, the guidance on rounding modeled concentrations, and both the PSD Modeling Guidelines and Non-PSD Modeling Guidelines. In addition, the 24-hour and annual averaging periods for SO2 will no longer be included on Availability of Air Resources (AAR) Summaries provided by DNR. The Form MD in the Iowa Environmental Application System for Air (EASY Air) has also been updated to remove the 24-hour and annual averaging periods for SO2.