sustainable council

October 31, 2022

SCS Engineers Environmental Consulting and Contracting
Air dispersion modeling uses mathematical formulations to characterize the atmospheric processes that disperse a pollutant emitted by specific emission sources. The source(s) can be different than what appears to be an obvious source.


Welcome to Sustainable Council, SCS’s newest blog series focusing on proven solutions to get you down the road to sustainability. Private and public entities are working to become truly sustainable and responsible in their operations and public works, meeting ESG and economic goals, but often identify a gap between their planned goals and accomplishments.  Every project with SCS moves you closer to those goals and can help inform your planning. We’ll publish tools, best practices, and regulatory alerts from our deep bench of experts and proven solutions to support your plans. Let’s kick off Sustainable Council with air monitoring advice from Jeff Leadford; take it away, Jeff.

SCS Engineers performs air dispersion modeling for industries and manufacturing throughout the United States. Still, there continues to be a strong focus on emission sources from the solid waste and energy industries, especially from the legal community. Air dispersion modeling is required by regulators when criteria pollutants’ potential to emit (PTE) reach certain thresholds or when toxic pollutants reach high enough emissions to require health risk assessments (HRA), among other more specific reasons. Common emission sources that are modeled at landfills, for example, include:

  • Fugitive emissions from the landfill surface
  • Landfill gas flares
  • Landfill gas-to-energy (LFGTE) engines
  • Particulate emissions from material handling and roads

Submitting air dispersion modeling that shows compliance with criteria pollutant ambient air quality standards (AAQS) or HRA thresholds is essential for these operations to remain compliant.

Emission Sources Case Study

A large landfill in the San Diego area recently submitted its annual toxic pollutant emissions to the San Diego Air Pollution and Control District (SDAPCD), and this emission level triggered HRA reporting. SCS obtained all needed site information from the landfill on emission source parameters and ran the EPA-preferred air dispersion model “AERMOD” to understand where these toxic pollutants were landing outside the property boundary and at what concentrations. AERMOD takes in source parameters such as exhaust velocity and stack height from a flare and combines that with the local terrain and meteorological data to determine these concentrations at ground level. Meteorological data from a recent 1-5 year period is generally required. Next, the output of AERMOD was run in “HARP2”, the Air Dispersion Modeling and Risk Tool created by the California Air Resource Board. HARP2 can take these ground-level concentrations and turn them into a number that describes the health risk (cancer, noncancer chronic, and acute) to a population living and working in the region.

All modeling was performed and submitted to SDAPCD, focusing on specific residences, businesses, and sensitive receptors such as schools and hospitals close to the landfill. The results showed that the landfill operations demonstrated negligible risk on the closest residences, businesses, and sensitive receptors. All required modeling files were submitted to SDAPCD, and a full HRA report with figures. The report was given SDAPCD approval after the SCS team satisfied all questions and correspondence with the agency.

The experience SCS has in air dispersion modeling, health risk assessment, and industry helps simplify this process for our landfill client. SCS performs numerous assessments of emission sources and, with our in-the-field experience, has streamlined the process to be as efficient as possible, saving time and money for our clients. For additional information, please reach out to Jeff Leadford, PE, or one of SCS’s nationwide offices.

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Posted by Diane Samuels at 6:00 am