On April 15, 2011 the Wisconsin Department of Natural Resources (WDNR) Modeling Team Leader John Roth published a memorandum entitled Revised Approach to Dispersion Modeling for Permits. This memorandum serves as guidance for air dispersion modeling in light of USEPA promulgated 24-hour and annual National Ambient Air Quality Standards (NAAQS) for particulate matter with an aerodynamic diameter of 2.5 micrometers or less (PM2.5) and 1-hour nitrogen dioxide (NO2) and sulfur dioxide (SO2) standards. The memorandum outlines several types of permitting projects and the expected dispersion modeling required for those permitting actions. Of note, if draft permits are public noticed on or after April 15, 2011, WDNR expects PM2.5 to be modeled. However, 1-hour NO2 and SO2 modeling is only anticipated for major source PSD permitting at this time; for all other permitting actions, the memorandum states that "considerations for 1-hour NO2 and SO2 standards will be applicable upon effective date of the air quality standards within the Wisconsin Administrative Code".

Permit engineers are anticipated to have some level of discretion when determining what modeling may be required for permitting actions. Furthermore, it is understood that this memorandum is solely a guidance document: facilities undergoing permitting projects requiring modeling should initiate a dialogue with the WDNR Permit Engineer and Modeling Team to learn the latest guidance as well as to address case-by-case circumstances.