Frequently Asked Modeling Questions

• Is Air Quality Modeling Necessary After Receiving My Operation Permit?
• What are Monitored Background Concentrations?
• How Many Years of Modeling Are Necessary?
• How Are Nitrogen Oxides Modeled?
• How Are Fugitive Dust Sources Modeled?
• How Are Horizontal Releases Modeled?

Is Air Quality Modeling Necessary After Receiving My Operation Permit?

Unless the new emission source is exempt from a permit, an air quality modeling analysis must be completed to ensure that air quality standards will be attained. DNR asks the applicant to supply updated information for the existing emission sources contained in the operation permit.

How Are Monitered Background Concentrations Used?

Monitored background concentrations are added to the modeled concentration to obtain the total predicted concentration at a receptor point. Wisconsin background concentrations are developed by county for pollutant. Background concentrations for all criteria pollutants other than lead are available as well as separate tables for Particulate Matter (TSP), PM10, NOx, Carbon Monoxide (CO), and sulfur dioxide (SO2). These background tables were last updated March 2004.

How Many Years of Modeling Are Necessary?

All refined modeling is completed over a five year period using five years of sequential meteorological data. All concentrations calculated by the model are based on a one-hour value averaged over the requisite time period. The modeled concentrations are then compared to the applicable standard. The monthly, quarterly, Prevention of Significant Deterioration (PSD) increments, annual standards and all Annual Average Concentrations (AAC) may never be exceeded, so the first highest value is examined for making the comparison. The short-term National Ambient Air Quality Standards (NAAQS) set as 1 hour, 3 hour, 8 hour, and 24 hour increments, may be exceeded once per calendar year. Modeled results are given as the highest second-highest value, which is typically found by examining model output based on 5 years of meteorological data. The second highest modeled impact for each year of data is calculated by the model. The highest of those values is selected as the short term modeled impact.

How Are Nitrogen Oxides Modeled?

The USEPA Guideline on Air Quality Models suggests a multi-tiered screening approach for estimating an annual NO2 value. This approach uses an assumption that only a portion of the NO is converted to NO2.

Tier 1 involves using the appropriate Gaussian model to estimate the maximum annual average concentration. A total conversion of NO to NO2 assumed. If the concentration exceeds the NAAQS and/or PSD increments, then the modeler is to proceed to the second tier. In March of 2002, the EPA also gave permission for the Ambient Ratio Method to be applied during Significant Impact Level analyses.

Tier 2 consists of multiplying the Tier 1 estimate by an empirically derived NO2 / NOx value of 0.75. This value is the national default ratio of NO2 conversion from NO.

If Tier 2 is applied and the facility is found to pass the NO2 standard, no further analysis is necessary.

How Are Fugitive Dust Sources Modeled?

Fugitive dust (particulate matter) from sources such as coal piles and haul trucks is difficult to model for a number of reasons. First of all, it is difficult to establish an emission rate because of the uncertainty of the emission factors. Also, the conditions at each location can vary so widely that it is impossible to categorize sources. Due to the nature of fugitive dust sources, the WDNR does not usually model these sources. The impact of fugitive dust sources can be determined upon request. As improvements are made to the science necessary for understanding fugitive sources, this stance will change.

There are fugitive emissions that should be modeled, however. These include emissions that are created within a facility building that are not vented directly to a stack and small emission units that are written together in the permit. Examples include natural gas heaters with roof vents, foundry furnaces, and dip tanks vented to general building ventilation.

How Are Horizontal Releases Modeled?

Some types of processes commonly vent to stacks that either discharge in a horizontal or downward direction or have some type of raincap. These stack orientations and obstructions affect plume rise and dispersion. In order to account for this, the WDNR assigns a default exit velocity of 0.1 m/s to any stack exhibiting the mentioned characteristics, regardless of the source flow rate.

Air Quality Modeling