Air dispersion modeling – AERA

The Air Emissions Risk Analysis (AERA) process has been designed to be flexible in many areas, including air dispersion modeling. Air dispersion modeling is the process of simulating the movement of air pollutants after they are emitted from a source in order to estimate the concentrations of pollutants at locations around the source. There are multiple ways to use air dispersion modeling in the AERA process. The methods vary from using the conservative Look-up Table in the RASS to using refined modeling, one option for which is the Q/CHI spreadsheet. More refined modeling should follow standard EPA guidance and practices. More information can be found on the Air Dispersion Modeling Web page.

Risk Assessment Screening spreadsheet

The Risk Assessment Screening Spreadsheet (RASS) separates the dispersion modeling from the rest of the assessment, allowing project proposers to input dispersion factors using various levels of conservatism. The following methods may used with the RASS.

1. Default Look-up Table. The RASS contains a generally conservative look-up table developed using the EPA preferred model AERMOD. The look-up table provides default dispersion factors that are used to estimate air concentrations. With the exception of stack height and receptor distance, worst case conditions for other parameters were used to develop the look-up table dispersion factors.
   
   
2. DISPERSE batch program. Rather than, or in addition to, using the default DISPERSE lookup table in the RASS, the user can use dispersion factors generated from the DISPERSE Batch Program, which also uses EPA’s AERMOD dispersion model. The user can provide additional stack parameters and specific data to use in the DISPERSE Batch Program. Appendix B of the AERA Guide provides an overview of how to do this. Detailed technical information on using the Batch program can be found in the AERA Guidance companion document, “Dispersion Information Screening Procedures for Emission Risk Screening Evaluations (DISPERSE) with emphasis on DISPERSE Look-up Table and DISPERSE Batch Programs.” The DISPERSE program can be found on MPCA's Air Dispersion Modeling Web page.
   
   
3. Other Refined Modeling. Project proposers may also use dispersion factors from other site-specific modeling (e.g., PSD modeling or Title V modeling) if available. If using other site-specific modeling, users must follow standard EPA guidance and practices, except they must set up the model to report the maximum (high-first-high, H1H) ambient value, and not other lesser values (e.g., H2H for each year, or H6H over 5 years) as is done in criteria pollutant assessments for comparison with NAAQS or PSD increments. Additionally, the model must be set up to report the highest monthly average so the AERA analysis can account for risks from pollutants that have "subchronic" health effects.

Q/CHI spreadsheet

The Q/CHI spreadsheet uses a different methodology than the RASS. In Q/CHI spreadsheet, the look up table and the DISPERSE program are not used to generate dispersion factors. The Q/CHI spreadsheet does, however, utilize AERMOD as its dispersion tool. In the Q/CHI spreadsheet, emission rates (Q) are divided by inhalation health benchmarks (CHI). The Q/CHI values, in units of g/sec per mg/m³, are summed through pollutants for each respective stack. Each stack-specific Q/CHI sum is then input into AERMOD as an “emission rate.” The Q/CHI sums are then “dispersed” to produce cancer risks or hazard indices at every receptor location. Similar calculations and modeling are completed for PBTs, however in this case a Multi-pathway Screening Factor is multiplied by each Q/CHI value before summation and dispersion modeling. These PBT analyses produce dispersed multi-pathway risks (farmer and resident ingestion). The Q/CHI method eliminates the conservative assumption that the maximum risk is calculated regardless of it occurring at the same time and the same place. If facility risks are still above risk guidelines after the refinement of Q/CHI modeling, additional refinement or mitigation may be necessary. Discussion of refinement options, including dispersion refinements, can be found in the AERA Guide.

Risk Drivers in Q/CHI: Once the Q/CHI dispersion modeling is completed, pollutant-specific risks are calculated in the Q/CHI spreadsheet. The stack-specific risks should be input into each risk driver worksheet (e.g. Q_CHI Acute drivers”, etc.), and the results will be summarized on the “Risk Driver Summary” worksheet.

The Q/CHI spreadsheet can be used in an iterative manner with the RASS. The RASS may be used first, to eliminate pollutants that are not risk drivers for any pathway. The RASS may also be used to eliminate exposure pathways if the summed risks for that exposure pathway are below risk guidelines. This type of screening should be described in the appropriate forms and provided to the MPCA.

To maximize the use of the Q/CHI methodology the user can use the results from AERMOD to generate plot files for use with Geographic Information System (GIS), SURFER, or other graphical tools.

Additional information

• MPCA’s Citizens’ Guide to Air Dispersion Modeling
• Air Dispersion Modeling
• Support Center for Regulatory Air Models

Contacts

Environmental Analysis and Outcomes Division, Risk Evaluation/Air Modeling Unit

• Dennis Becker, 651-757-2217
• Greg Pratt, 651-757-2655
• Ruth Roberson, 651-757-2672
• Melissa Sheffer, 651-757-2718
• Kristie Ellickson, 651-757-2336