Updated SO2 and NO2 Backgrounds



The Wisconsin Department of Natural Resources (WDNR) is proposing to update the existing guidance on background concentrations used in air dispersion modeling for permits.  The focus of the updates is the one-hour (1-hr) sulfur dioxide (SO2) and 1-hr nitrogen dioxide (NO2) concentrations.  This draft guidance is driven by the recent incorporation of the 1-hr SO2 and NO2 ambient air quality standards, discussed in a previous Wisconsin article.  The draft WDNR guidance proposes removing the 24-hour and annual SO2 background concentrations and implementing a temporally varying background for 1- and 3-hour SO2 and 1-hr and annual NO2 that varies every hour of the year, averaged for individual months.  This is a new approach to background concentrations in Wisconsin as the previous background values were a single value and the new approach applies a different background concentration for each hour of the day by month.

The WDNR implements a high value and low value for background concentrations based on regional population, which will remain unchanged as a result of the proposed guidance.  Cities and surrounding villages and suburbs with a population greater than 25,000 residents, based on 2010 Census data, use the high value and those areas with populations less than 25,000 residents use the low value.

The table below shows the existing and proposed Wisconsin background concentration changes.  See the tables in the draft WDNR guidance for the proposed SO2 and NO2 values.

Wisconsin Background Concentrations

PollutantAveraging PeriodHigh Value (μg/m3)Low Value (μg/m3)
PM2.524 Hour23.619.8
Annual9.47.3
PM1024 Hour47.029.4
PbQuarterly0.020.01
SO21 Hour

  

See Tables in Guidance

3 Hour
3 Hour43.211.8
24 Hour30.511.2
Annual8.65.4
NO21 Hour

 

See Tables in Guidance

Annual
Annual24.18.0
CO1 Hour1,362.7950.5
8 Hour1,191.2904.7

See the table below for a comparison of the existing static background and the maximum and annual value of the proposed hourly background.  The values are listed here are not those that will be used in future modeling analyses, but are a snapshot of the proposed background, as it varies on an hourly basis for every month of the year.

 

 

Pollutant

Background Concentrations (μg/m3)

  

 Value

 

 Existing*

Proposed
Maximum AnnualAverage Annual
SO2High28.823.38.1
Low16.514.14.9
NO2High83.581.749.1
Low27.821.910.4
*Existing values shown are interim 1-hour values provided by WDNR to Trinity Consultants, dated 9/22/2016.

Alternate background concentrations can be requested from the WDNR for sources that have installed an ambient monitor at an appropriate location and has at least two full years of monitoring data.

Comments on the proposed guidance will be accepted through October 3, 2016 Comments can be sent to John Roth, Natural Resources Program Coordinator.

As detailed in the proposed guidance, if your facility has a permit already in public comment or waiting to be issued, the WDNR may reconsider modeling on a case-by-case basis if the updated background concentrations might impact emission limits.  For questions about the proposed background concentrations or regarding dispersion modeling in Wisconsin, please contact Jon Frost at (651) 275-9900 x6.