The WDNR Air Program is proposing to update PM2.5 background concentration used in dispersion modeling for permits. These proposed updates are motivated by the EPA April 2014 re-designation of the Milwaukee PM2.5 nonattainment area to attainment due to lower measured PM2.5 ambient concentrations and the May 2014 published PM2.5 modeling guidance, which contained recommendations for backgrounds. The WDNR Guidance on this subject is now available here.

Additionally, the WDNR reviewed the 2010 Census data and revised which areas shall use High and LowWI proposed concentrations background concentrations. Generally, cities and surrounding villages and suburbs with a population greater than 25,000 residents will use the high value for background concentration. Areas with populations less than 25,000 residents use the low value. Cities (and additional incorporated areas) using the high value include: Superior, Eau Claire, Wausau, Stevens Point, La Crosse, Green Bay, Appleton, Manitowoc, Oshkosh, Fond du Lac, Sheboygan, West Bend, Madison, Janesville, Beloit, Kenosha, Racine, and Milwaukee.

The table at right shows the proposed Wisconsin background concentrations with the old PM2.5 concentrations crossed out.

While it may be possible to use an alternative background concentration, the WDNR will not consider requests for alternate background concentrations unless the facility has an ambient monitor appropriately installed and has at least two full years of monitoring data.

For questions about the proposed background concentrations or regarding dispersion modeling in Wisconsin, please contact Jon Frost at 651-275-9900 or