The U.S. EPA finalized the new 1-hour primary National Ambient Air Quality Standard (NAAQS) for SO2 on August 23, 2010.  In Illinois, the new rule requires all new or modified major sources, subject to Prevention of Significant Deterioration (PSD) permitting, to demonstrate compliance with the new 1-hour SO2 NAAQS as part of federal permitting projects through an air quality modeling analysis.  Addtional sources in Illinois will also be modeled during the attainment designation process.

Given U.S. EPA's innovative approach for the new 1-hour SO2 NAAQS, as outlined in the preamble to the final rule (U.S. EPA says they will "not consider ambient air quality monitoring alone to be the most appropriate means of determining whether all areas are attaining a short term NAAQS"), the "superior utility that modeling offers for assessing SO2 concentrations" will result in IEPA modeling many more areas of Illinois in order to assess attainment of the 1-hour SO2 NAAQS.  As a result, The Illinois Environmental Protection Agency (IEPA) sent approximately 700 letters to SO2 emitting sources requesting specific data that IEPA will be using to conduct dispersion modeling analyses in support of attainment designations with respect to the 1-hour SO2 NAAQS.  For the majority of the SO2 sources in the state, IEPA determined that it was lacking modeling information in three key areas:

Ambient air determination.  The 1-hour SO2 NAAQS is an "ambient air" standard.  But considering individual sources of SO2 emissions, sources that have the necessary barriers to prevent the general public access, may eliminate an area from consideration as "ambient air".  IEPA thus determined that it needed to request information from sources that show property boundary locations and the means, if any, by which the general public is precluded access to plant property.

Plume downwash.  Buildings and structures at a source can interact with streamlines and stack locations to "downwash" the SO2 exhaust plume.  Thus, IEPA determined that it needed to request information on the location, designation, and dimensions of all buildings and structures at a source.

Stack locations and configurations.  The location of exhaust stacks with respecy to building location is important based on the plume downwash potential noted above.  Likewise, the orientation of an exhaust stack will influence the dispersion of the plume.  IEPA determined that it needed to request information on the stack orientation in which SO2 emissions are discharged to the atmosphere.

The IEPA has not requested any information regarding SO2 exhaust stack parameters (e.g., height, temperature exit flow/velocity) or SO2 emission rate because IEPA already has this information in a state database.  This IEPA database of SO2 exhaust stack parameters and SO2 emission rates comes not only from IEPA issued permits, but also from past stack exhaust parameter data submitted to the IEPA in annual emission reports and permit applications.  It is a worthwhile exercise to review this SO2 exhaust stack parameter information as part of the IEPA response effort.

Path Forward

While awaiting additional SO2 modeling guidance upcoming from U.S. EPA, IEPA has indicated that it plans on completing some initial pilot modeling of SO2 sources in Wabash County  and Tazewell County. This initial SO2 modeling will give IEPA a better understanding of the level of effort needed to model an entire county, as well as computing demands the effort requires.  Following this effort, Sangamon, LaSalle, Madison and Cook County may be IEPA's next counties targeted for SO2 modeling.  IEPA is still determining what the process will be if modeling results show that a particular source, or set of sources, is causing a modeled SO2 "issue" with the 1-hour NAAQS, but it is sure to involve discussions with the impacting sources.