U.S. EPA Releases Updated Version (21112) of the AERMOD Modeling System

On May 11, 2021, the United States Environmental Protection Agency (U.S. EPA) released an updated version of the American Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) Modeling System.  The update, dated 21112, includes documentation, executables, and source code for the modified components of the modeling system, which includes the regulatory AERMOD and AERMET data processors and the non-regulatory AERSCREEN screening tool.  The updated AERMOD modeling system is the current, preferred near-field dispersion modeling system for regulatory applications, as promulgated in the “Guideline on Air Quality Models” (Appendix W to 40 CFR Part 51).

AERMOD is a steady-state plume model that incorporates planetary boundary layer characteristics, terrain, building downwash, and source characterization to calculate pollutant concentrations and deposition.   AERMOD contains default modeling options intended for regulatory analyses, BETA options that have been reviewed by the scientific community and intended for future promulgation, but are currently alternative methodologies, and ALPHA options that are considered research or development options that have not been fully evaluated and are not approved for regulatory purposes.

Version 21112 of AERMOD does not introduce any new regulatory options.  In addition to new error messages and multiple bug fixes, version 21112 of AERMOD introduces new modeling options, primarily for research and development purposes, and associated guidance, including:

• Urban options NOTURB and NOTURBST
• Buoyant line groups
• RLINE and RLINEXT algorithm enhancements and dual-barrier capabilities
• NO to NO₂ conversion options GRSM and TTRM
• Low-wind options SWMIN and BIGT

The AERMOD urban modeling option (URBANOPT) applies algorithms to enhance turbulence during nighttime stable conditions for the urban heat-island effect.  For modeling evaluations that incorporate site-specific turbulence measurements in a locale that is subject to the urban heat-island effect, use of the URBANOPT option serves as a duplication of the turbulence enhancement.  Version 21112 adds new NOTURB and NOTURBST options to ignore site-specific turbulence measurements.

The BUOYLINE source type was introduced in a previous AERMOD version (15181) to characterize emissions typically dispersed from buoyant, low-level, parallel line sources, such as in aluminum reduction facilities.  Version 21112 expands the functionality of the BUOYLINE source type, allowing the user to specify multiple groups of individual buoyant lines.  The introduction of multiple buoyant line groups includes the new BLPGrpID keyword that the user must understand for proper characterization.  Version 21112 also adds flagpole receptor and parallel verification capabilities to the BUOYLINE source type.

The RLINE and RLINEXT source types were introduced as non-regulatory options in the previous AERMOD version (19191) to characterize emissions from roadway sources.  The RLINE source type, currently considered a BETA option, and the RLINEXT source type, currently considered an ALPHA option, have additional capabilities in AERMOD version 21112, and have received modifications to improve processing speed and fatal error generation.  Both source types included horizontal meander and numerical integration algorithms previously, with the RLINEXT source type allowing single-barrier and depressed roadway characterization capabilities.  Version 21112 of AERMOD updates the single-barrier algorithm and adds a two-barrier algorithm to the RLINEXT source type to characterize barriers on both sides of a roadway.

Version 21112 of AERMOD introduces two options that have been added to compute the conversion of nitric oxide (NO) to nitrogen dioxide (NO₂) for POINT, VOLUME, or AREA source types.  The Generic Reaction Set Method (GRSM), which requires the user to provide background NO₂ concentrations, calculates the NO to NO₂ conversion based on equilibrium chemistry between NO and NO₂ and the reaction with ozone, whereas the Travel Time Reaction Method (TTRM) calculates the NO to NO₂ conversion with consideration of travel time from source to receptor.  Both options are non-regulatory ALPHA options and, similar to the established regulatory Tier 3 Ozone Limited Method (OLM) and Plume Volume Molar Ratio Method (PVMRM) conversion options, require the user to specify ozone background values as a single value, temporally-varying values, or as hourly values.

Low-wind modeling options were introduced in a previous AERMOD version (18081) for evaluation of model performance in low wind-speed conditions.  Version 21112 incorporates two new ALPHA options to allow the user to modify parameters that may impact low wind-speed model performance.  The SWMIN option allows the user to override the AERMOD default, minimum sigma-w value of 0.02 meters per second (m/s) to a value between 0.0 m/s and 3.0 m/s.  The BIGT option allows the user to override the AERMOD time-scale default value of 24.0 to a value between 0.5 and 48.0 hours.

A notable bug fix removed the calculation of sigma-v from sigma-theta turbulence data when wind speed data is missing.  The code modification only impacts meteorological data generated with site-specific turbulence measurements.

AERMET processes hourly meteorological surface observations, typically from airports, upper-air National Weather Service (NWS) sounding data, and site-specific meteorological measurements to generate meteorological inputs for the AERMOD dispersion model.  Processed meteorological files provide calculated boundary layer parameters, such as surface friction velocity and mixing heights, and observed wind and temperature data at multiple vertical levels.  Version 21112 of AERMET introduces multiple bug fixes, updates information for several airports, and includes refined reporting that does not impact data output.   The updated AERMET version does include changes that impact results when site-specific meteorological data is included, however, including the calculation of mixing heights only when not provided by site-specific meteorological measurements and heat flux interpolation and missing value checks for determination of convective hours.

AERSCREEN is a screening-level version of AERMOD that calculates pollutant concentrations based on the plume centerline from a single source and a matrix of meteorological conditions.  AERSCREEN generally represents a conservative, worst-case approach for estimating concentrations for several averaging periods.  Version 21112 of AERSCREEN introduces three changes; two of which provide additional output information for users to refine desired model options, and one to align inversion fumigation calculations with those used in the SCREEN3 screening model.

Although the release of AERMOD version 21112 primarily provides bug fixes, code enhancements, and new ALPHA options not intended for regulatory modeling applications, understanding of the modified urban and turbulence guidelines and buoyant line source capabilities facilitates accurate source characterization for modeling evaluations.

Contact Ryan Cleary at rcleary@all4inc.com for more information.