Due to the lack of additional guidance from USEPA on refined dispersion modeling for implementation of the 1-hour SO2 standard, and the short timeframe for states to submit attainment plans (i.e., due by June 2013 for attainment/unclassified areas and February 2014 for nonattainment areas), the Lake Michigan Air Directors Consortium (LADCO) (i.e., state agencies in IL, IN, MI, MN, OH, and WI) has proposed a modeling protocol for the development of SO2 attainment plans. Pursuant to earlier guidance from USEPA, the LADCO proposal follows the Guideline on Air Quality Models, 40 CFR Part 51, Appendix W.

The LADCO proposal calls for use of data collected by monitoring stations as well as emission modeling using AERMOD for analysis of included sources, with the option of using alternate models in rare situations where local conditions, e.g. terrain, may be difficult to model with AERMOD. USEPA's original guidance called for modeling of sources greater than 100 tpy LADCO has proposed a two-step approach for sources to include in modeling analyses: as over 99 percent of the Region 5 SO2 emission inventory is made up of sources emitting 80 tpy or more SO2, any source with 80 tpy actual emissions or more will be included, using federally enforceable permit limits or potential to emit where permit limits do not exist. In accordance with Appendix W, LADCO proposed using five years of National Weather Service Data or one year of onsite meteorological data in the modeling analysis. As no background SO2 concentration was proposed by USEPA, LADCO has proposed a background concentration of 8 parts per billion (ppb) based on data collected from rural monitoring sites.

For additional information the letter mailed on July 25, 2011 to USEPA from LADC is available here.