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Background concentrations in ambient air represent the contributions from natural sources, non-industrial/human activities, and distant industrial facilities. The background concentrations are added to dispersion modeling results for comparison to the National Ambient Air Quality Standards (NAAQS).
Back to topDefault Background Concentrations
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. Default background concentrations are updated every five years (last updated in 2025).
| Pollutant | Averaging Period | Background Concentration Value (micrograms per cubic meter) |
|---|---|---|
| PM10 | 24-hour | 55 |
| PM2.5 | 24-hour | 18 |
| Annual | 7.8 | |
| NO2* | 1-hour | Urban: 58; Rural: 13 |
| Annual | Urban: 11; Rural: 4 | |
| SO2 | 1-hour | 47 |
| 3-hour | 47 | |
| Annual | 1 | |
| CO | 1-hour | 7,600 |
| 8-hour | 3,300 | |
| Pb | Rolling 3-month average | 0 |
* By default, the urban NO2 background should be used for projects in Polk, Pottawattamie, and Scott counties, or refer to the Urban/Rural NO2 map for a more detailed delineation.
Back to topSite-specific Background Concentrations
Applicants may propose site-specific representative background concentrations. Monitor-specific data are provided below, along with guidance on how the data may be used in modeling analyses. Site-specific background concentrations are updated annually (last updated in 2025).
Back to topOther Resources
- Removal of Smoke Events TSD
- Detailed Urban/Rural NO2 Delineation Map
- Establishing NO2 Urban & Rural Delineation Using CAMx
- 2020-2024 Hourly Ozone Background Data for Modeling NO2 (units=ppb)
- Hourly Ozone Data Sensitivity Analysis
- Diurnal & Seasonal Ozone Background Tool for Modeling Direct Ozone Emissions
- Derivation of Diurnal & Seasonal Ozone Pattern