Air Quality Dispersion Modeling - Preferred and Recommended Models

These refined dispersion models are listed in the Guideline on Air Quality Models - "Appendix W" to 40 CFR Part 51 (PDF)(42 pp, 531 KB, 11-29-2024) and are required to be used for State Implementation Plan (SIP) revisions for existing sources and for New Source Review (NSR) and Prevention of Significant Deterioration (PSD) programs. The models in this section include the following:

AERMOD Modeling System - A steady-state plume model that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.

CTDMPLUS - A refined point source gaussian air quality model for use in all stability conditions for complex terrain.

OCD - A straight line Gaussian model developed to determine the impact of offshore emissions from point, area or line sources on the air quality of coastal regions.

AERMOD Modeling System

The American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee (AERMIC) was formed to introduce state-of-the-art modeling concepts into the EPA's air quality models. Through AERMIC, a modeling system, AERMOD, was introduced that incorporated air dispersion based on planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain. As of December 9, 2006, AERMOD is fully promulgated as a replacement to ISC3, in accordance with Appendix W (PDF)(42 pp, 531 KB, 11-29-2024).

There are two input data processors that are regulatory components of the AERMOD modeling system: AERMET, a meteorological data preprocessor that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts, and AERMAP, a terrain data preprocessor that incorporates complex terrain using USGS Digital Elevation Data. Other non-regulatory components of this system include: AERSCREEN, a screening version of AERMOD; AERSURFACE, a surface characteristics preprocessor, and BPIPPRIM, a multi-building dimensions program incorporating the GEP technical procedures for PRIME applications.

At this time, AERMOD does not calculate design values for the lead NAAQS (rolling 3-month averages). A post-processing tool, LEADPOST (ZIP)(65 MB, 09-19-2013), is available to calculate design values from monthly AERMOD output. This tool calculates and outputs the rolling cumulative (all sources) 3-month average concentration at each modeled receptor with source group contributions and the maximum cumulative (all sources) rolling 3-month average concentration by receptor.

Below is the model code and documentation for AERMOD Version 24142. The model code and supporting documents are not static but evolve to accommodate the best available science. Please check this website often for updates to model code and associated documents.

AERMOD Modeling System Code and Documentation
AERMOD Implementation Guide

AERMOD Implementation Guide (PDF) (42 pp, 380 KB, 11-20-2024, 454-B-24-009) - Provides information on the recommended use of AERMOD for particular applications and is an evolving document.
AERMOD Modeling System Development
Modeling System Development Website and White Papers
Model Code
Executable (v24142) (64-bit) (zip) (1.5 MB, 12-04-2024) - 64-bit Operating Systems
Executable (v24142) (32-bit) (zip) (1.4 MB, 11-20-2024) - 32-bit Operating Systems
Source Code (v24142) (ZIP) (625 KB, 11-20-2024)
Model Documentation

AERMOD Modeling System Transmittal Memorandum (v24142) (PDF) (5 pp, 266 KB, 11-20-2024)

AERMOD 64-bit Executable Refresh Memorandum (1pp, 236 KB, 12-04-2024)

AERMOD Quick Reference Guide (PDF) (25 pp, 356 KB, 11-20-2024)
User's Guide for the AMS/EPA Regulatory Model (AERMOD) (PDF) (303 pp, 2.3 MB,  11-20-2024, 454-B-24-007)

AERMOD System Bugs, Errata, and Related Guidance (PDF) (4 pp, 198 KB, 01-07-2025)

Model Change Bulletin #18 - Version Date 24142 (PDF) (2 pp, 212 KB, 11-20-2024)
Model Change Bulletins Archive (2024) (zip) (1.2 MB, 11-20-2024)

AERMOD Model Formulation Document (PDF) (179 pp, 2.1 MB, 11-20-2024, 454-B-24-010)
AERMOD Model Evaluation Document (PDF) (58 pp, 3.8 MB, 11-20-2024, 454-B-24-006)
AERMOD System Data Resources

AERMOD Modeling System Data Resources (PDF) (7 pp, 187 KB, 11-20-2024)
Test Cases

AERMOD Test Cases (ZIP) (234 MB, 12-04-2024)
Installation Guide (Sample Run)

Read Me (TXT) (1 KB, 10-12-2023) - please read this file first
Sample Run Instructions (PDF) (33 pp, 1.7 MB, 11-21-2024) - detailed installation and execution instructions
Sample Run (ZIP) (5.4 MB, 11-21-2024) - sample test case

AERPLOT Sample Run Instructions (PDF) (14 pp, 939 KB, 11-20-2024) - detailed installation and execution instructions
AERPLOT Sample Run (ZIP) (4.3 MB, 11-20-2024) - sample test for AERPLOT
Model Supporting Documents

2024 Appendix W Final Rule Supporting Information and Docket Website

Model Evaluation Paper (PDF) (41 pp, 1 MB, 06-01-2003, 454-R-03-003)
Bulk Richardson Number Evaluation Report (PDF)(34 pp, 329 KB, 09-01-2004)
Comparison of Regulatory Design Concentrations: AERMOD vs ISCST3, CTDMPLUS, ISC-PRIME (PDF)(89 pp, 31 KB, 06-01-2003, 454-R-03-002)
Development and Evaluation of the PRIME Plume Rise and Building Downwash Model (1995) (pdf) (5 pp, 31.3 KB, 1995)
Project PRIME: Evaluation of Building Downwash Models Using Field and Wind Tunnel Data (PDF)(4 pp, 32 KB, 1998)
Development and Evaluation of the PRIME Plume Rise and Building Downwash Model (1999) (pdf) (34 pp, 588 KB, 10-05-1999)
Evaluation of Bias in AERMOD-PVMRM (PDF)(33 pp, 236 KB, 06-01-2005)
PVMRM and OLM Sensitivity Analysis (PDF)(67 pp, 522 KB, 09-01-2004)
Ambient Ratio Method Version 2 (ARM2) for use with AERMOD for 1-hr NO2 Modeling - Development and Evaluation Report (PDF)(95 pp, 2.2 MB, 09-20-2013)
Guidance on R-LINE Additions to AERMOD for Refined Transportation Project Analyses
Incorporation and Evaluation of the RLINE Source Type in AERMOD For Mobile Source Applications (PDF) (55 pp, 5.2 MB, 10-12-2023, 454-R-23-011)
Evaluation of Addition of Terrain Treatment to the RLINE Source Type in AERMOD (PDF) (33 pp, 1.7 MB, 10-12-2023, 454-R-23-012)
AERMOD Deposition Science Document (PDF)(22 pp, 196 KB, 03-19-2004)
AERMOD Deposition Parameterizations Document (ZIP)(338 KB, 10-28-2003)
Draft Peer Review Document (ZIP)(797 KB, 08-27-2001) - For the AERMOD Deposition Parameterizations Document (above)
Technical Support Document (TSD) for AERMOD/BLP Development and Testing (43 pp, 1.0 MB, 12-01-2016, 454-B-16-009)
Technical support document (TSD) for NO2-related AERMOD modifications (32 pp, 1.6 MB, 12-01-2015, 454-B-15-004)
Technical Support Document (TSD) for Adoption of the Generic Reaction Set Method (GRSM) as a Regulatory Non-Default Tier-3 NO2 Screening Option (PDF) (43 pp, 2.9 MB, 11-20-2024, 454-R-24-005)

NO2/NOx In-Stack Ratio (ISR) Database
Model Evaluation Databases - 2005 Promulgation

2005 Promulgation README (TXT)(2 KB, 01-22-2004) - Document that explains the databases below that contain input and output data for the model evaluation
AGA (ZIP)(2 MB, 01-22-2004) - Input/output data for AGA: Flat, Rural, Downwash, Independent
Alaska (ZIP)(661 KB, 01-22-2004) - Input/output data for Alaska: Flat, Rural Downwash, Developmental
Baldwin (ZIP)(4.5 MB, 01-22-2004) - Input/output data for Bladwin: Flat, Rural, Non-downwash, Independent
Bowline (ZIP)(2 MB, 01-22-2004) - Input/output data for Bowline: Flat, Rural, Downwash, Developmental/Independent
Clifty Creek (ZIP)(3.5 MB, 01-22-2004) - Input/output data for Clifty Creek: Flat, Rural , Non-downwash, Independent
DAEC (ZIP)(1 MB, 01-22-2004) - Input/output data for DAEC: Flat, Rural, Downwash, Developmental
EOCR (ZIP)(4 MB, 01-22-2004) - Input/output data for EOCR: Flat, Rural, Downwash, Independent
Indianapolis (ZIP)(1 MB, 01-22-2004) - Input/output data for Indianapolis: Flat, Urban, Non-downwash, Developmental
Kincaid SF6 (ZIP)(3 MB, 01-22-2004) - Input/output data for Kincaid SF6: Flat, Rural, Non-downwash, Developmental
Kincaid SO2 (ZIP)(5 MB, 01-22-2004) - Input/output data for Kincaid SO2: Flat, Rural, Non-downwash, Developmental
Lee Wind Tunnel (ZIP)(13 MB, 01-22-2004) - Input/output data for Lee Wind Tunnel: Flat, Rural, Downwash, Independent
Lovett (ZIP)(9 MB, 01-22-2004) - Input/output data for Lovett: Terrain, Rural, Non-downwash, Developmental
Martin's Creek (ZIP)(11.5 MB, 01-22-2004) - Input/output data for Martin's Creek: Terrain, Rural, Non-downwash, Independent
Millstone (ZIP)(660 KB, 01-22-2004) - Input/output data for Millstone: Flat, Rural, Downwash, Developmental
Prairie Grass (ZIP)(342 KB, 01-22-2004) - Input/output data for Prairie Grass: Flat, Rural, Non-downwash, Developmental
Tracy (ZIP)(2.5 MB, 01-22-2004) - Input/output data for Tracy: Terrain, Rural, Non-downwash, Independent
Westvaco (ZIP)(10 MB, 01-22-2004) - Input/output data for Westvaco: Terrain, Rural, Non-downwash, Independent
Model Evaluation Databases - NO2

NO2 README (TXT)(1 KB, 10-31-2023) - Document that explains the NO2 databases below that contain input and output data for the model evaluation
Balko (ZIP)(12 MB, 10-31-2023) - Input/output data for Balko; Terrain, Rural, Downwash, Independent, 4 NO2 monitors
Denver-Julesburg (Supplemental for 2023 TSD) (ZIP) (2 MB, 10-31-2023) - Input/output data for Denver-Julesburg; Terrain, Rural, Downwash, Independent, 12 NO2 monitors
Denver-Julesburg (ZIP)(108 MB, 08-18-2020) - Input/output data for Denver-Julesburg: Flat, Rural, Non-downwash, Independent, 12 NO2 monitors
Empire Abo (ZIP)(14M, 10-31-2023) - Input/output data for Empire Abo; Flat, Rural, Downwash, Independent, 2 NO2 monitors
Pala'au (ZIP)(6 MB, 10-31-2023) - Input/output data for Pala'au; Flat, Rural, Downwash, Inpendent,1 NO2 monitor

CTDMPLUS

Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations (CTDMPLUS) is a refined point source gaussian air quality model for use in all stability conditions for complex terrain. The model contains, in its entirety, the technology of CTDM for stable and neutral conditions. CTSCREEN is the screening version of CTDMPLUS.

CTDMPLUS Model Code and Documentation
Model Documentation

User's Guide Supplement (1993) (pdf) (5 pp, 60 KB, 03-01-1993)
User's Guide, Volume 1 (PDF)(210 pp, 7 MB, 03-01-1989, 600-8-89-041)
User's Guide, Volume 2 (PDF)(77 pp, 2 MB, 10-01-1990, 600-8-90-087)
User's Guide for Terrain Preprocessor (PDF)(180 pp, 6 MB, 03-01-1988, 600-8-88-003)
User's Guide for Meteorological Preprocessor (PDF)(166 pp, 5 MB, 03-01-1989, 600-8-88-004)
Final Report (PDF)(484 pp, 16 MB, 12-01-1987)

Model Change Bulletin #5 - Version Date 93228 (TXT)(5 KB, 08-19-1993)
Model Change Bulletins Archive (1993) (zip) (11 KB, 08-19-1993)

OCD

Offshore and Coastal Dispersion Model Version 5 (OCD) is a straight line Gaussian model developed to determine the impact of offshore emissions from point, area or line sources on the air quality of coastal regions. OCD incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorological data are needed from both offshore and onshore locations.

OCD Model Code and Documentation
Model Code
Code/Executable (ZIP)(9 MB, 01-06-2000)
Model Documentation
User's Guide (PDF)(160pp, 1.7 MB, 11-06-1997)
User's Guide (ZIP)(Original WP6 files, 369 KB, 11-06-1997)
User's Guide Supplement (1989) (pdf) (204 pp, 3 MB, 11-01-1989)

Model Change Bulletin #3 - Version Date 00006 (TXT)(6 KB, 01-06-2000)
Model Change Bulletins Archive (2000) (zip) (4 KB, 01-06-2000)

Support Center for Regulatory Atmospheric Modeling (SCRAM)

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Last updated on May 1, 2025