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The Department periodically analyzes the monitored concentrations across the state to determine the background concentrations that should be added to dispersion modeling results for comparison to the National Ambient Air Quality Standards (NAAQS).
The default background concentrations are conservative estimates of the observations across the statewide ambient air monitoring network. These data are intended to provide applicants with pre-approved background concentrations that can be used in any modeling analysis. Applicants may also propose site-specific representative background concentrations. Guidance for doing so can be found in the technical support document provided below.
Background Concentration Value
Urban: 84; Rural: 18
Urban: 15; Rural: 4
23 (Winter: 23; Spring-Fall: 20)
Rolling 3-month average
* Refer to map below for definition of NO2 background areas.
** SO2 concentrations represent natural background. Nearby sources of SO2 should be explicitly modeled
This data is intended for use in 1-hour NO2 modeling analyses when the non-regulatory default Tier 3 Ozone Limiting Method (OLM) or Plume Volume Molar Ration Method (PVMRM) is utilized in the AERMOD dispersion model. OLM and PVMRM are screening methods used to convert NO to NO2. The background ozone concentration is one of the key inputs for use of OLM and PVMRM. The Department has generated a background ozone concentration file for use anywhere in the state for the time period coinciding with the 2010-2014 meteorological dataset. The file provided below is a combination of the ozone monitor data collected from 12 sites across the state. Due to the non-regulatory status of OLM and PVMRM prior approval from DNR is required when either will be used. This ozone data is pre-approved for all such requests that will utilize the Department's 2010-2014 meteorological data.