On March 6, 2012, Andy Stewart, of the Wisconsin Department of Natural Resources (WDNR), released a policy memorandum regarding the exclusion of intermittent operating units in air dispersion modeling analyses. The memo clarifies that intermittent units which operate no more than 200 hours per year, or are defined as emergency generators, do not have to be included in air dispersion modeling for the 1 hour SO2 or NO2 National Ambient Air Quality Standards. The memorandum references the US EPA March 1, 2011 clarification memo, which similarly discusses the exclusion of intermittent sources from dispersion modeling for the 1 hour SO2 and NO2 standards.

It's important to note, however, that this memorandum includes the following language regarding intermittent units:

"intermittent units do not have a set operating schedule and operate for short periods of time during the year."

While at first glance this memo seems to exempt emergency engines form dispersion modeling completely, the fact that an intermittent unit cannot have a set operating schedule implies that any emergency engine which undergoes regular testing (e.g., daily/weekly) may not actually be considered an intermittent source, and would have to be included in dispersion modeling. Before removing any units from your dispersion modeling analysis, you may want to check with your WDNR modeling contact.

If you have any questions or would like more information related to the modeling of intermittent units, please contact Tony Colombari at 651-275-9900.