Learn about the DNR's COVID-19 response and how the current health emergency is impacting DNR facilities, services and events.
blue sky with a few wispy clouds
Availability of Air Resources (AAR)

The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) to protect public health and welfare. The NAAQS define the maximum acceptable concentrations of certain air pollutants, and thereby the amount of air resources that must be shared by all industrial facilities and other sources of those pollutants.

When the Iowa Department of Natural Resources (DNR) receives an air quality construction permit application, there may be a determination, through computer-aided air dispersion modeling, of the concentration of air pollutants in the ambient air near the facility. This includes any impact the facility may have as well as impacts from nearby facilities. The DNR compares these impacts to the NAAQS to ensure that public health is protected, and summarizes this information in the form of an Availability of Air Resources (AAR) Summary.

When available, an AAR Summary is provided to permit applicants upon permit issuance. The applicant can then use the AAR Summary for planning purposes and to assist in the preparation of future permit applications. AAR Summaries may also be requested using the contact information on this page.


Frequently Asked Questions

Use the contact information on this web page. Provide your name, email, and the Plant ID of the facility for which you are requesting the AAR Summary. If the Plant ID is not known provide the facility name and city within the comments. The AAR Summary will be sent to the email address provided. Alternatively, you can contact the DNR at 515-725-8200 (ask to speak to a member of the dispersion modeling team).

Yes. The electronic modeling files and full technical reports associated with the information contained in the AAR Summary are available upon request, using the contact information on this page. This information is only available electronically and cannot be sent via mail.

Requests for a single location will be fulfilled at no cost. Representatives of facilities with multiple locations may request an AAR Summary for each of those locations at no cost. Other bulk requests will be referred to the DNR record center and may be subject to a database search fee depending on how long it will take to fulfill the request.

Yes. However, the DNR can only provide this information for areas in the immediate vicinity of industrial sources where data from a comprehensive air quality analysis already exists. Requests for AAR Summaries for areas of the state where no data exists cannot be fulfilled.

When an air pollutant concentration, predicted by computer-aided dispersion modeling, approaches the air quality standards for that pollutant, it becomes more likely that public health and welfare would not be protected if additional emissions of those air pollutants are permitted.

The DNR uses concentration levels referred to as Modeling Determination Thresholds (MDT) as an indicator that an area is approaching the applicable National Ambient Air Quality Standards (NAAQS).

If the facility’s impact is equal to or greater than the corresponding MDT, then the majority of the air resources in the area are already being utilized. For this reason an air dispersion modeling analysis will be required the next time the facility submits a non-Prevention of Significant Deterioration (non-PSD) permit application to increase the emission levels of any pollutant that is currently at or above an MDT. (For PSD projects, an air dispersion modeling analysis is generally required as part of an air quality analysis of ambient impacts associated with the project.)

The modeling analysis is used to predict the future concentration of air pollutants generated by the proposed project, which will help ensure that the project is designed in such a way that the area will continue to meet the NAAQS.

If you have not already done so, request an AAR Summary from the DNR using the contact information on this page. If an AAR Summary is not available then the availability of air resources will not be considered when determining if modeling will be required.

For non-Prevention of Significant Deterioration (non-PSD) projects, the DNR provides Form MD (Non-PSD Modeling Determination Form) to assist applicants in pre-determining when modeling may be required. This form summarizes the two main triggers: project emission levels and availability of air resources.. If emissions exceed the Significant Emission Rate (SER) threshold for any pollutants, then modeling is required for those pollutants. For pollutants that will be increased, if the area is already close to the NAAQS then modeling is required. The AAR Summary can be used to determine if the area is close to the NAAQS. In addition, there may be other considerations on a case-by-case basis, where DNR may determine that modeling is required.

For PSD projects, an air dispersion modeling analysis is generally required as part of an air quality analysis of ambient impacts associated with the project.

The box at the top of the AAR Summary lists all facilities that were the focus of the modeling analyses used to estimate the amount of air resources currently being utilized. All other sources of air emissions are included in the modeling analyses as part of the background concentration.

The amount of available air resources is determined using a computer simulation called a dispersion model. The dispersion model is used to predict the maximum concentration (defined by the form of the standard for the pollutant and its averaging time) caused by emissions from industrial sources in the area. In order to make this prediction, information about the area being evaluated is entered into the dispersion model. This can include: the amount of pollution being released into the air, the height of smoke stacks that the pollution is coming from, and the area’s terrain and weather conditions. The modeled concentration is then added to the background concentration to determine the total predicted pollutant concentration in the area. Subtracting the total concentration from the NAAQS provides an estimate of the amount of air resources that is still available in the area.

Not necessarily. The results from dispersion modeling represent the location(s) of maximum concentration in the area, usually near the largest air emission source(s). The concentration at all other locations in the area will be something lower than this value. It may be possible to design new projects in such a way that minimizes the impact of the additional emissions on the existing point of maximum concentration, or to modify existing emission sources to alleviate the area of high concentration. Dispersion modeling is the appropriate tool to use for this purpose.

There are a few reasons why this might occur. When the concentration is only slightly over the NAAQS, it is most likely due to rounding. The AAR Summary includes the unrounded model concentrations because these are the values that should be used when filling out Form MD as part of the application process. For non-Prevention of Significant Deterioration (PSD) projects, the DNR allows the concentration for some pollutants to be rounded before being compared to the NAAQS. For example, a 24-hour PM10 concentration of 154 rounds down to 150, which meets the NAAQS for that pollutant. However, if the concentration is considerably higher than the NAAQS, it is likely that the analysis included some form of case-by-case considerations that affected the final modeled concentration. These could include non-default background concentrations or a demonstration that the project did not significantly contribute to predicted NAAQS exceedances. Case-by-case considerations from previous projects cannot be assumed for future projects without reevaluating their validity, and are therefore excluded from the data shown on the AAR Summary. For example, default background concentrations are assumed on the AAR Summary. In any case no action is necessary at this time, but future projects will need to be evaluated in a dispersion modeling analysis for the affected pollutant(s).

Knowing the amount of air resources still available in your area can help gauge the impact on air quality permitting projects. Projects are less likely to need additional pollution controls, design modifications, or permit limits in order to meet the NAAQS if there is a majority of the air resources still available. If the NAAQS is not far above the background level for a pollutant, air resources for that pollutant may be limited even though the industrial impacts from that pollutant in the area are low.

The amount of air resources still available will also determine whether a modeling analysis will be required the next time an air permit application is submitted. For projects that involve an increase in emissions, a modeling analysis will be required for pollutants that are at or above one or more of the MDTs (Modeling Determination Thresholds). If there are pollutants that are close to a MDT, it may be beneficial to conduct a dispersion modeling analysis prior to pursuing future projects. Doing so will allow the impact of the facility’s emissions to be considered as part of the site selection and engineering design process.

A Modeling Determination Threshold (MDT) is a concentration level used by the Iowa DNR as an indicator that an area is approaching the applicable National Ambient Air Quality Standards (NAAQS). The MDTs vary based on pollutant and averaging period, and define when a permit application will require a modeling analysis.  Each MDT is equal to the NAAQS for the pollutant of interest minus the applicable Significant Impact Level (SIL).

The National Ambient Air Quality Standards (NAAQS) are concentration levels set by the Environmental Protection Agency (EPA) to protect public health and welfare. The NAAQS define the maximum acceptable concentrations of certain air pollutants, and thereby the amount of air resources that must be shared by all industrial facilities and other sources of those pollutants.

A Significant Impact Level (SIL) is a concentration level for a given pollutant below which emissions of that pollutant will not cause or contribute to a violation of the NAAQS. The SILs are proposed by EPA and vary based on pollutant and averaging period.

Background concentration represents the existing level of pollutants in the air without the influence of nearby industrial facilities. The background includes natural sources, non-industrial human activity, and distant industrial facilities.

Ambient air refers to the air outside the boundary of a facility where its emissions will have an impact on the public.

Request an AAR Summary
  • *
  • *
  • *
  • *

Alternatively, to request an AAR Summary, contact us directly at: aar@dnr.iowa.gov