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On January 17, 2017, the U.S. Environmental Protection Agency (EPA) published final revisions to the Guideline on Air Quality Models (Guideline). For regulatory applications of air dispersion models (except for transportation conformity modeling), a one-year transition period was provided before new guidance found in the Guideline must be applied. This transition period ended January 17, 2018; therefore, any new regulatory air quality modeling assessments should conform to the guidance found in the 2017 version of the Guideline.

For modeling for transportation conformity purposes, a three-year transition period applies, which will end on January 17, 2020. Transportation conformity modeling includes modeling required under Clean Air Act Section 176(c) for federally funded transportation projects in nonattainment and maintenance areas. During this transition period, EPA is continuing to work with the Federal Highway Administration (FHWA) and Department of Transportation (DOT) to ensure that adequate training and guidance on AERMOD is made available for transportation modeling purposes. Additional studies comparing the CALINE models (no longer supported in the Guideline) and AERMOD are being conducted to better understand the transition from the CALINE models to AERMOD and to facilitate AERMOD's use by state and local DOTs given the varying levels of users' experience with the model.

Coinciding with the finalization of the Guideline revisions, EPA released an updated version of AERMOD in December 2016 (Version 16216). Shortly after the release of that model, EPA found some inconsistencies between the AERMOD code, the previous version of AERMOD, and the final Guideline language, which prompted a modified release (Version 16216r). The new regulatory version of AERMOD included many of the enhancements and updates that were discussed in the revised Guideline, including the low wind speed turbulence adjustment (ADJ_U*), an update to the Plume Volume Molar Ratio Method (PVMRM) for NOx to NO2 conversion, and removal of the Beta option designation for Tier 2 and Tier 3 NO2 modeling methods (these are no longer considered Beta and may be used). However, EPA's work on AERMOD is not complete. As outlined in the Introduction section of EPA's AERMOD Model Formulation and Evaluation document, one of the design goals of AERMOD was the ability to “accommodate modifications with ease as the science evolves.” With this goal in mind, EPA began work in 2017 on an AERMOD Modeling System Development Plan. The initial elements of this plan were outlined in a series of white papers released by EPA on September 19, 2017. EPA has requested public comment on these white papers and intends to consider these comments for an initial release of the Development Plan sometime in 2018.

This article provides an overview of the white papers, their topics, and a summary of the information in the white paper areas on which the initial AERMOD Development Plan is expected to focus. Additional science updates that are expected in AERMOD in the next few years are also addressed. For each topic, EPA provides an overview of the topic, current implementation into AERMOD, a summary of current literature or research, and considerations for updates in AERMOD.

2018EQWinter-AERMOD_Figure 1LOWWIND Options

In the “proposed” July 2015 revisions to the Guideline published by EPA, modifications to the treatment of atmospheric dispersion under low wind speed conditions were provided. These included the adjusted friction velocity (ADJ_U*) and adjustments to turbulence under low wind speed conditions (so-called LOWWIND1, LOWWIND2, and LOWWIND3 options). In addition to the finalized ADJ_U* function, EPA proposed changes that were focused on the minimum value of lateral turbulence intensity and the treatment of plume meander. The function and keyword within AERMOD to implement this change, known as LOWWIND3, however, were not finalized in the 2017 version of the Guideline. Instead, EPA has deferred the finalization of this option and is proposing to continue research that could refine this option and improve its performance across a range of meteorological conditions and emission source types. EPA's white paper on this topic summarized research that has been conducted to date on plume meander and minimum value assessment for lateral turbulence. The white paper also recommended further study into these phenomena as part of the AERMOD Development Plan.

Saturated Plumes

The AERMOD modeling system is designed to simulate dry, neutrally buoyant plumes; for certain types of sources of air pollutant emissions, however, this assumption is not representative of the effluent conditions. One type of plume for which this assumption is inaccurate is wet or moist plumes, such as those from wet cooling towers or exhaust from stacks controlled by wet scrubbers. For warm, moist exhaust, plume rise characteristics may be enhanced as water vapor in the exhaust warms due to the heat of condensation, which releases heat even as the plume cools in the atmosphere. Even with this heating/cooling due to moisture taking place, a net gain in plume rise results. This phenomenon can have considerable impact on ground-level air pollutant concentrations due to emissions from this type of source because the plume rise will be effectively different than that of a dry gas release.

A preprocessor program (AERMOIST) has been developed using the peer reviewed IBJpluris1 model. Based on hourly plume moisture information, the AERMOIST program adjusts plume temperature on an hourly varying basis for use in AERMOD to account for this enhanced plume rise. EPA's evaluation of the AERMOIST preprocessor concluded that, although it works well in certain cases, there are certain situations in which it may overestimate the plume temperature adjustment and likewise overestimate the enhanced plume rise resulting from the moisture in the plume. In the related white paper, EPA recommends additional research into this technique and consideration of AERMOD formulation enhancements to account for moist plumes directly in AERMOD rather than through a pre-processor program.

Downwash Algorithms

Building downwash occurs near and downwind of an obstruction to air flow (such as a building) in the atmosphere. In situations where a plume is caught in a region of building downwash, the plume can be pulled toward ground level quickly or even be temporarily trapped in a recirculating cavity. Both of these situations can result in higher ground-level concentrations than would be observed if the obstruction to flow were not present, especially in the influence zone of the building or structure.

AERMOD accounts for building downwash using the Plume Rise Model Enhancements (PRIME) model. The PRIME algorithm has not been modified since it was originally incorporated into AERMOD in 2005, and several issues concerning building shapes, sizes, and dimensions have since surfaced. Additionally, more recent research has indicated that PRIME is deficient for certain types of building/stack configurations-most notably for elongated buildings, buildings that are angled rather than perpendicular to the wind, and buildings with stacks located near a building corner.

Continuing research is underway and summarized by EPA in the white paper. This research focuses on wind tunnel experiments and large eddy simulation (LES) model studies. A PRIME2 Advisory Committee has also been formed by the Atmospheric Modeling and Meteorology subcommittee of the Air & Waste Management Association to compile and add to available information and to provide a technical forum for suggesting improvements to the building downwash algorithm in AERMOD. EPA has committed in the white paper to continue to work with various stakeholders to incorporate the latest scientific information on building downwash into the AERMOD model.

NO2 Modeling Techniques

While AERMOD does not generally simulate atmospheric chemical reactions, one exception is the ability added through two tiers of AERMOD refinements (beyond the simple Tier 1 full conversion) to estimate NOx to NO2 conversion via available ozone. AERMOD allows the user to enable one of three tiers of varying conservatism and complexity to simulate this conversion. Tier 1 assumes 100% conversion of NOx to NO2; Tier 2 uses historical ambient monitoring data to develop a relationship between ambient NOx concentration and NO2 to NOx ratios (also referred to as ARM2); and Tier 3 uses more advanced techniques (i.e., the Ozone Limiting Method [OLM] and Plume Volume Molar Ratio Method [PVMRM]) to estimate NOx to NO2 conversion based on background ozone concentration.

Although the Tier 2 and 3 methods were incorporated into the regulatory version of AERMOD as part of the 2017 Guideline revisions, their application must be approved by each applicable regulatory authority, with particular consideration given to choice of Tier 3 inputs. Given the relatively short amount of time for the modeling community to gain experience in the use of these techniques, comprehensive databases of information used as inputs have not been established, particularly for the Tier 3 methods. For example, while EPA has been attempting to collect information on in-stack ratios of NO2 to NOx for various source types, additional work is necessary to compile reliable and robust inventories for use on a routine basis in regulatory modeling applications.

Additionally, alternative NO2 chemistry algorithms have been identified that are in use outside the U.S., such as the NO2 chemistry algorithm in the Atmospheric Dispersion Modeling System (ADMS) used in the U.K. EPA is considering incorporating this formulation into AERMOD as an alternative Tier 3 methodology and is continuing to research additional methods that might simulate NO2 chemistry more accurately than current Tier 3 methods in AERMOD.


Mobile Source Modeling

The 2017 Guideline revisions included the replacement of the CALINE3 model with AERMOD as the preferred dispersion model for refined applications for roadway projects. As noted previously, this revision has a three-year transition period, ending January 17, 2020, before the use of AERMOD will be required. In the meantime, research comparing the CALINE3 model with AERMOD is ongoing.

EPA acknowledges that AERMOD does not include a true non-buoyant line source type, which is the source type included in CALINE3 that is typically used to model emissions from roadways. Rather, in AERMOD, EPA recommends use of an area source or a set of volume sources to represent elongated non-buoyant sources, such as roadways. Other models are also available for roadway modeling applications, such as the Research LINE (R-LINE) model, which is under development by EPA's Office of Research and Development. Incorporation of the R-LINE model into AERMOD may allow for use of a true line source model within the AERMOD framework, as will be required by the Guideline as of the end of the transition period in 2020. Other algorithms under development include options to account for the impact of noise barriers and depressed roadways on plume dispersion.

Overwater Modeling

EPA's preferred model for overwater emission sources, including both deepwater and shoreline applications, is the Offshore and Coastal Dispersion Model (OCD). This preferred model was included in the previous version of the Guideline (2005) and is one of only three remaining preferred models in Appendix A to the 2017 Guideline. In the white paper, EPA states that OCD remains a preferred model because it contains certain algorithms that represent overwater dispersion better than AERMOD, although AERMOD is a newer model and incorporates more recent science as a whole.

For AERMOD to replace OCD in the future as the preferred model for overwater emission sources, EPA has identified the following areas in which improvements to AERMOD are necessary: 1) platform downwash, 2) shoreline/coastal fumigation, and 3) marine boundary layer representation. Work on platform downwash is being conducted in conjunction with the PRIME research discussed previously. Additionally, EPA is currently working on research and development on AERCOARE, which is an alternative meteorological data processor for overwater applications to AERMET (the standard meteorological data processor for AERMOD). The AERCOARE program is not part of the regulatory defaults in AERMOD, but may be a useful tool to overcome AERMOD's current limitations in representing the marine boundary layer. EPA acknowledges that the use of AERCOARE does not address the issue of shoreline and coastal fumigation, which will require additional research and updates to AERMOD for it to fully replace the functions of the OCD model, which is the direction in which EPA is currently moving.

Alpha and Beta Options

For several years, EPA has followed the practice of designating certain options in AERMOD as “Beta” options, meaning that they are available for testing and use but must be approved as an alternative model under Guideline Section 3.2 for use in regulatory applications. As part of its announcement of the Modeling System Development Plan, EPA proposed to further subcategorize these “not ready for prime time” options into Alpha and Beta options. Alpha options will be defined as experimental options that EPA is releasing to the public but that are not yet available for regulatory use, even as alternative models under Section 3.2 of the Guideline. Beta options will be defined as peer-reviewed options that are potentially ready for consideration as alternative models on a case-by-case basis. EPA's intent with this change is to make more options available for research and testing by the modeling community, while also clearly identifying which options are further along in development and/or backed by peer-reviewed research.

Other Topics for Future Research

Finally, in the cover memo to the white papers, EPA identified a wide variety of other areas under consideration for future research and development, but that require further evaluation before being added to the initial Development Plan. These include the following:

  • Formulation Science Issues
    • Theta* calculation method and pass through from AERMET Penetrated plumes
    • Tall stacks in urban areas (boundary layer characterization)
    • Underprediction for tall stacks in flat terrain during stable hours
    • Complex terrain characteristics and influences
  • Industrial Heat Island Effects
    • Heat islands that are not captured by population data
  • Buoyant Line Sources
    • Scientific update to buoyant line source treatment reflecting AERMOD's scientific model formulation
  • Deposition
    • Particle deposition/depletion
    • Gas deposition/depletion
    • SO2 half-life


Based on EPA's model formulation goals for AERMOD, research is continuing into the science of atmospheric dispersion and proven, new science is being incorporated into AERMOD when appropriate. To inform the modeling community of its areas of further research, EPA has proposed to develop and release an AERMOD Modeling System Development Plan. EPA has been careful to note that this plan will not contain a list of updates planned for the next version of AERMOD, but rather will consist of a formal statement of EPA's planned future science updates for AERMOD. As can be seen from the list of topics being considered for the initial Development Plan and the list of additional topics for future research, EPA and the modeling community will be busy continuing to improve the science behind AERMOD. In the September 19, 2017 white paper memorandum, EPA states that an AERMOD Development Plan will be released in early 2018. As of this printing, the plan has not yet been released however, Trinity is actively monitoring and will advise once it is published.


1 Janicke, U., and Janicke L. 2001. A three dimensional plume rise model for dry and wet plumes. Atm. Env. 35(2001):877-890