On October 23, 2023, the US EPA proposed changes to the “Guideline on Air Quality Models” and the US EPA model AERMOD. The agency additionally proposed a new guidance document (a separate document) entitled “Guidance on Developing Background Concentrations for use in Modeling Demonstrations”. This Guidance has implications for permitting, particularly for sources of PM2.5 (particulate matter less than 2.5 microns in size) and precursors of PM2.5 (sulfur oxides (SOx or SO2) and nitrogen oxides (NOx)) and sources of nitrogen oxides alone.
Importance of Background Concentrations
US EPA is preparing to reduce the National Ambient Air Quality Standard (NAAQS) for PM2.5 from 12 micrograms per cubic meter to as low as 8 or 9 micrograms per cubic meter. Since background concentrations make up the majority of the PM2.5 currently measured in the ambient air, many more areas of the country will be designated as non-attainment, thus subjecting new and modified sources in those areas to much more difficult permitting issues.
For sources in attainment areas, the determination of background concentrations is crucial because current measured background concentrations will be much closer to the NAAQS allowable levels. This will lead to much smaller or non-existent room for new sources of PM2.5 (either direct emissions or emissions from chemical reactions of SOx or NOx after release to the atmosphere). This means that new facilities and modifications to existing facilities will be much more difficult to permit.
Current Use of Monitoring Data
If there is a monitor in a given location which can be deemed representative of background concentrations, current practice is to use that data to represent the background concentration. The measured background concentration is added to the modeled results for the new source to determine compliance with the NAAQS. In most case the new source contributes only a small percentage (1 or 2 micrograms per cubic meter) and the background is much greater than the source contribution.
What the New Guidance Requires
The proposed Guidance requires a four-step process to set a background concentration:
- Define the scope of the study including the NAAQS, the averaging period, the modeling domain and the dispersion environment.
- Find the relevant ambient monitoring data, the available emissions data for all sources which might affect the background concentrations and previous modeling in the area for the pollutant in question.
- Determine the representativeness of ambient monitoring data if available. The Guidance requires a visual comparison of locations of the monitor regarding emitting sources, the proposed source, terrain and meteorology. This requires a qualified individual to conduct an on-site evaluation of the project area and the monitoring site. The result of this requirement may force many sources to conduct their own site-specific ambient monitoring prior to permitting. This might require up to one years’ worth of monitoring.
- Evaluate emission sources that might not be represented in the monitored data (for instance newer or permitted but not built sources), perform additional quantitative assessment to assess the impact of other sources and, finally, fully document justification for any decisions made. The implication is that many situations will require that all identifiable sources will need to be modeled resulting in a lot more effort.
Implications for Project Permitting
As can be imagined, these changes will add significant effort to the front end of any permitting and/or modeling project. The procedure also leads to the expectation that many more sources will have to be included in the model than is currently the case.
Methods that are not condoned by the Guidance but have been used previously are:
- Use of the gradient (fall off with distance) of a source’s concentrations to eliminate it from the modeling for background.
- Use of the hourly monitored data from a monitor designated to be the background concentration for that hour or day.
EPA states that the motivation for this Guidance is due to inconsistency in previous PSD demonstrations and the Guideline lacked definition. No examples or rigorous evaluation of that claim was given, but it is clear that modeling for PM2.5 and the 1-hour average NAAQS for SO2 and NO2, will be significantly more complicated and expensive under this new Guidance.
Next Steps
The comment period on the Guidelines and Guidance closed on December 23, 2023. Promulgation expected by summer.
TRC is available to support your air compliance and permitting needs by offering expert in-house resources to perform:
- Ambient Monitor Siting Evaluation
- Ambient Monitor Deployment, Operation and Maintenance
- Air Dispersion Modeling
- Permitting
- Performance Testing
- Expert Testimony and Agency Negotiation
Gale F Hoffnagle
Gale F. Hoffnagle, CCM, QEP is a Senior Vice President and technical Director in the Air Quality Consulting Practice. He has 52 years of consulting experience and 38 years of service to TRC. He advised clients on the Clean Air Act amendments of 1977 and 1990. Contact Gale at GHoffnagle@trccompanies.com.