The long-awaited AERMOD update is finally here! On July 9, 2026, the U.S. Environmental Protection Agency (EPA) released Version 26135 of the AERMOD Modeling System. The release updates AERMOD and AERMOD’s meteorological preprocessor (AERMET) from Version 24142. The land cover preprocessor (AERSURFACE), the one-minute wind data preprocessor (AERMINUTE), and the visualization program (AERPLOT) were also updated to Version 26135. However, the terrain preprocessor (AERMAP) has not been updated as part of this release. Version 24142 continues to remain the regulatory version of AERMAP. EPA’s release memorandum provides an official overview of the package.
For facilities with an active or upcoming air permit application, the practical takeaway is that the modeling basis should be checked before a project moves too far. Unless given explicit guidance or project-specific direction from the state agency, it is very likely that any state agency is going to expect the new model version to be used for air permit applications. This includes air toxics modeling analyses. A short check at the beginning of the project can prevent an unpleasant surprise towards the end of the permitting process, or once modeling has already been completed.
What Changed in Version 26135?
EPA has classified the new release as mostly bug fixes along with a few enhancements. Instead of simply being an AERMOD executable swap, EPA’s release package includes updated versions of the dispersion model and several supporting programs used to prepare meteorological and land-cover inputs or review results. For instance, the new version (26135) of AERMOD comes with updates to aircraft plume rise as well as additional options for mobile sources when using the RLINEXT source type. For both AERMET and AERMINUTE, capability to process surface meteorology from the National Centers for Environmental Information in the Global Historical Climatology Network hourly (GHCNh) format was added. The latest version of AERSURFACE has an additional user option for determining surface roughness and the new version of AERPLOT has additional capability to display a 3D receptor grid based on FLAGPOLE heights.
Despite this version containing mostly bug fixes, the effect of a software update depends on the model options and data used for a particular type of analysis. The safest, recommended approach is to compare the project with the new versions rather than assuming the update will not affect your current application. EPA’s AERMOD model page houses the current code and documentation, while the meteorological processors page provides the current AERMET materials.
Scheduling Issues due to the Meteorological Data Update
For many projects, the most consequential change is the update to the meteorological-data workflow. AERMET 26135 can process the GHCNh format. EPA’s release presentation explained that the older Integrated Surface Hourly Data (ISHD) archive was not completed for 2025, and that archived years were converted to the new format. This allows agencies to prepare newer five-year meteorological datasets, covering 2021 through 2025, with the new raw data format.
State environmental agencies have been waiting for this new AERMET version to create 2021-2025 meteorological datasets. However, this does not mean that every permitting agency has already published a new dataset, or that every active project needs to switch to the new meteorological data. Facilities should evaluate whether state agencies may require newer meteorological datasets before submittal. In some cases, transitioning to updated datasets could affect project schedules or require additional model reruns. Additionally, a facility preparing a new modeling analysis should always confirm the expectations of their state agency.
What Facilities Should Do Now
- Confirm the state agency’s current expectations before beginning a new regulatory modeling analysis.
- Review EPA’s Version 26135 release memorandum and the applicable change bulletins against the model options used in the project.
- Check whether the state agency has published or requested a newer meteorological dataset.
- Document the software versions, data sources and directions provided by the state agency in the modeling protocol or project narrative.
How Can Trinity Help?
Trinity Consultants can help facilities assess how Version 26135 affects a planned or active air permitting project, confirm the expected modeling platforms with the state agency, review meteorological-data options, and provide guidance on the impact of the updated model. If your facility is preparing an air permit application or you need to understand how this new AERMOD Modeling System update may affect project schedule, contact Justin Fickas, or Trinity’s air quality consulting team to talk through the project-specific considerations. Trinity has over 40 years of expertise in air dispersion modeling and offers a variety of services from regulatory compliance to risk assessment and emergency planning.
