Since the 2017 update to the Guideline, several developmental topics have been a priority as a result of the needs expressed by stakeholders and related research that has been documented. The EPA is on a development pathway to propose new updates to the regulatory scientific formulation of the AERMOD Modeling System in 2023 with a final revision to the Guideline in 2024. In the sections that follow, we identify our higher development priorities, what we generally anticipate will be included in the 2023 proposed updates to the Guideline specific to AERMOD, and our prospective timeline.
Mobile Source Modeling with RLINE
The EPA introduced the new RLINE source type into AERMOD version 19191 as a beta option (non-regulatory model option that meets the Guideline’s criteria to be considered an alternative model with appropriate approval) for modeling mobile source emissions on roadways. This new source type was based on the Research LINE-source dispersion model (R-LINE) originally developed by EPA’s Office of Research and Development (ORD). The EPA is currently evaluating RLINE in collaboration with and funding from an Interagency Agreement with the Federal Highway Administration with the goal of promulgating it as a regulatory option in AERMOD. Since the introduction of RLINE, further development continues, including an urban source option (introduced in AERMOD version 22112) and terrain effects (anticipated in an upcoming AERMOD release). Additionally, the RLINE Extended (RLINEXT) alpha option (non-regulatory model option that is experimental and not intended for regulatory usage) can be used to simulate the effects on dispersion resulting from near-road noise barriers and roadway depressions below grade.
Improvements to PRIME Building Downwash
Past analyses1,2 have shown AERMOD to both overpredict and underpredict ground-level concentrations within building wakes. Structures in the path of air flow create a turbulent wake region on the leeward side of the building where emissions are temporarily trapped in a recirculating cavity, leading to higher ground-level concentrations than if the structure was not present. This downwash effect is accounted for in AERMOD by the Plume Rise Model Enhancements (PRIME) algorithm. There have been recent research and development efforts led by ORD to improve the treatment of building downwash in AERMOD. EPA has also collaborated with the Air & Waste Management Association to further the research on building downwash. As a result of these efforts, several new alpha options related to downwash have been added to AERMOD over the last few years to enable further refinement and the evaluation for potential upgrades to improve the performance of AERMOD in downwash cases. EPA is currently evaluating these options, individually and in combination, to determine which options demonstrate improved AERMOD performance.
New and Improved NO-to-NO2 Conversion Methods
AERMOD currently has several techniques to model NO2 concentrations. Per the Guideline, the EPA recommends that NO2 modeling should be done as a three-tiered screening approach, where each tier increases in complexity and decreases in conservativeness. The first tier is total conversion, so all emitted NOX is immediately converted to NO2. The second tier is the Ambient Ratio Method 2 (ARM2), which adjusts the modeled NOX concentrations based on an empirical relationship between ambient NOX and ambient NO2 concentrations. The third tier consists of the Ozone Limiting Method (OLM) and the Plume Volume Molar Ratio Method (PVMRM), both of which explicitly use the amount of available ambient ozone to convert NO-to-NO2. In AERMOD 22112, the Generic Reaction Set Method (GRSM) was introduced as a beta option and potential new Tier 3 option. GRSM3 calculates plume entrainment, similarly to PVMRM, but adds a “reaction rate” based on solar radiation and travel time from source to receptor, based on the generic reaction set (GRS) chemistry scheme4,5 AERMOD 22112 also included the addition of the Travel Time Reaction Method 2 (TTRM2) as an alpha option, which estimated NO conversion based on the reaction rate between NO and ambient ozone, limiting NO conversion based on the travel time between the source and receptor. TTRM2 modifies the NO2 computed by ARM2, OLM, and PVMRM to account for this aspect of the chemical conversion of NOx, potentially improving the NO2 estimate from these existing NO2 options.
Overwater Meteorology for Offshore Sources
The EPA is working toward the eventual goal of the replacement of the Offshore and Coastal Dispersion Model (OCD) with AERMOD. To achieve that goal, one primary developmental need is the ability to properly parameterize the marine boundary layer (MBL) which influences dispersion of emissions over the ocean and at the shoreline from offshore sources. A first step in MBL parametrization was accomplished with the release of AERMET version 22112, which included the specific capability to process prognostic data extracted for overwater grid cells by the Mesoscale Model Interface Program (MMIF). Currently, the formulation of AERMET to process observed meteorological inputs to characterize the boundary layer is based on atmospheric dynamics influenced by terrestrial land cover. Therefore, the parameterizations of the boundary layer performed by AERMET from observed meteorological data such as from offshore buoys is not appropriate. To close this gap in the parameterization of the MBL for offshore sources, the Coupled Ocean Atmosphere Response Experiment (COARE) algorithm6 is being incorporated into AERMET to provide the capability to use observed meteorology collected at offshore buoys to model offshore sources.
Area Source Plume Meander
AERMOD accounts for plume meander (i.e., the slow lateral back and forth shifting of the plume from low frequency, non-diffusing eddies) as the plume travels downwind from the source. This is one of many formulation enhancements to dispersion over AERMOD’s predecessor, the Industrial Source Complex (ISC) model. Meander decreases the likelihood of observing a coherent plume after long travel times and results in a greater plume spread and increased dispersion downwind. Currently, plume meander is only applied to point and volume source types within AERMOD and is not applied to area sources, though an area source plume is expected to exhibit similar behavior downwind of the source. Following AERMOD’s promulgation in 2005, adding plume meander to the area source type was one of the remaining formulation needs for the model by the AMS/EPA Regulatory Model Improvement Committee (AERMIC). EPA is working to improve this formulation capability.
Aircraft Plume Rise
While AERMOD was originally designed for industrial, stationary sources, its use has expanded significantly to determine near-source impacts from a wide variety of sources, including aircraft activity near airports. AERMOD is incorporated in the Federal Aviation Administration’s (FAA) Aviation Environmental Design Tool (AEDT) to determine air quality impacts from aircraft activities. The EPA and FAA are currently collaborating on addressing model improvements specific to airport applications. The focus of these efforts is the addition of algorithms to model plume rise from aircraft exhaust. The algorithms will consider aircraft engine dynamics to determine buoyant plume rise and will be applicable to both area and volume sources in AERMOD. The algorithms are currently being evaluated against available field data to evaluate improvements in model performance.
Timeline for Proposed and Final Updates to the Guideline
Changes to the formulation of the regulatory version of the AERMOD Modeling System (including the AERMOD, AERMET, and AERMAP programs) require revisions to the Guideline and must be promulgated through a formal rulemaking process. The rulemaking process requires a publication of the proposed revisions to the Guideline in the Federal Register followed by a public hearing and public comment period. To meet the requirements for a public hearing for the proposed Guideline revisions, the EPA plans to host the 13th Conference on Air Quality Models in the Fall of 2023. The EPA will subsequently respond to the public comments and finalize revisions to the Guideline in 2024. Figure 1 presents our anticipated timeline for proposed and final updates to the AERMOD Modeling System and the Guideline.
Figure 1.
Anticipated Timeline for Proposed and Final Updates to AERMOD and the Guideline.
Appendix
Appendix W to CFR 40 Part 51, also known as U.S. Environmental Protection Agency’s (EPA) Guideline on Air Quality Models (Guideline), establishes the American Meteorological Society (AMS)/EPA Regulatory Model (AERMOD) as the preferred near-field modeling system. The Guideline provides national consistency in air quality analyses for regulatory activities under various sections of 40 CFR (Code of Federal Regulations). The EPA is committed to ensuring that the Guideline and associated modeling guidance reflect the most up-to-date science and relies heavily on the stakeholder community to identify concerns and emerging science.
The Guideline was last revised in 2017. Major updates included:
Implementation of a two-tiered demonstration approach for addressing single-source impacts on ozone and secondary PM2.5;
Science improvements with new regulatory options for the treatment of low wind conditions, the conversion of NOX to NO2, and plume rise for horizontal and capped stacks;
Replacement of CAlifornia LINE Source Dispersion Model (CALINE3) with AERMOD as the preferred model for modeling of mobile sources;
Screening approach for long-range transport for NAAQS and PSD increments; and
Incorporation of prognostic meteorological data.
Footnotes
Clint Tillerson, George Bridgers, James Thurman, Alyssa Piliero, Chris Misenis, and Matthew Porter are all scientists with the EPA’s Office of Air Quality Planning and Standards. R. Chris Owen and David Heist are scientists with EPA’s Office of Research and Development. Clint Tillerson is the Model Development Team lead, and George Bridgers is the Model Clearinghouse Director.
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