CALPUFF Model, History and Current Status – Part 1

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CALPUFF 2003 2019

   Dispersion modelling is a key element of most odour assessments around the world. There are 2 or 3 well known dispersion models developed for regulatory assessments of which the most well-known are, AERMOD and CALPUFF. Because both models have been US EPA guideline models until CALPUFF was recently removed in 2017, they are routinely used in most air quality and odour assessments.

   This article highlights the history and background behind CALPUFF and, the reason for its removal as a US EPA guideline model. Concerns with AERMOD for odour assessments will be presented in a separate document, Part 2.

Jennifer Barclay

Atmospheric Science Global (ASG). Auckland, New Zealand


   Competing interests: The author has declared that no competing interests exist.

   Academic editor: Carlos N. Diaz

   Content quality: This paper has been peer reviewed by at least two odour experts.

   Citation: Barclay, J. (2019): CALPUFF Model, History and Current Status – Part 1.

   Copyright: 2019 Open Content Creative Commons license. It is allowed to download, reuse, reprint, modify, distribute, and/or copy articles in website, as long as the original authors and source are cited. No permission is required from the authors or the publishers.

   Keywords: Dispersion modelling, odours, CALPUFF, AERMOD, US EPA, guideline model, State Implementation Plans, New Source Review, Prevention of Significant Deterioration, regional haze implementation,

   Acronyms: AQMG (Air Quality Management Group), BART (Best Available Retrofit Technology)



   Dispersion modelling is a key element of most odour assessments around the world. There are two or three well known dispersion models developed for regulatory assessments of which the most well-known are, AERMOD and CALPUFF. Because both models have been US EPA guideline models until CALPUFF was recently removed in 2017, they are routinely used in most air quality and odour assessments. With CALPUFF’s removal as a guideline model there is the perception amongst the modelling community that the model is no longer acceptable for odour modelling. At the same time AERMOD is increasingly being used for odour assessments in complicated environments. This article highlights the history and background behind CALPUFF and, the reason for its removal as a US EPA guideline model. Concerns with AERMOD for odour assessments will be presented in a separate document, Part II.

   Before explaining the current US EPA guideline status it is useful to point out that in the United States, odour assessments are not limited by the requirements of 40 CFR 51, Appendix W rules and regulations (explained below). These guidelines only apply to criteria air pollutants (air pollutants with established air quality standards). Odours do not have federally enforceable air quality standards and are not regulated through the preparation of State Implementation Plans, New Source Review or Prevention of Significant Deterioration permit requirements. However, despite the fact that odour assessments are not limited to the current US EPA model guidelines AERMOD’s status as a guideline model means that most odour assessments are undertaken using AERMOD, regardless of whether it is suitable or not. Outside the US, many countries regulate odours where dispersion modelling is often a requirement. Many of these countries look to the US for regulatory models’ and guidance.


Becoming a US Guideline Model

   CALPUFF was designated a US EPA Appendix A Guideline model in 2003 1 . Prior to the model promulgation to an Appendix A guideline model, CALPUFF, like the ISCST3 2 model before it underwent rigorous testing, model evaluations and multiple peer reviews over a period of more than a decade. This lengthy, dedicated, state-of-science and transparent process occurred under the scrutiny of the then Air Quality Management Group (AQMG) within the US EPA. The amount of resources invested by the US EPA AQMG to persist with a model to guideline status was large and was an effort beyond the means of modelling communities in most, if not all, other countries. In order to reach guideline status in 2003, CALPUFF underwent;

  • An extensive, multi-year model assessment and evaluation proces
  • numerous evaluations of model performance relative to observations
  • requirements on the CALPUFF model documentation, model access and model codes
  • open public review process at public hearings in Washington, DC
  • formal peer review committees created by the U.S. EPA, and professional organizations such as the AWMA and private industry groups such as API and EPRI

   In short the process to become an Appendix guideline model was an extremely difficult and long process. Today, CALPUFF remains one of the most documented peer reviewed and rigorously tested models of its time, next only to ISCST3. Further, because of this lengthy and thorough process to become a guideline model, the inherent weaknesses of AERMOD, and, the lack of any alternate US EPA model, it is likely that CALPUFF will continue to be used for regulatory applications involving near field non-steady state, odour assessments and, complex flow situations around the world.


The Removal of CALPUFF as a Guideline Model

   In January 2017 in the US EPA Appendix W, Federal Register, Volume 82, No. 10 under Rules and Regulations 3 , the CALPUFF dispersion modelling system was removed from the US EPA Appendix A as the preferred long range transport model with no replacement.

   In the US, AERMOD is now the only dispersion model with guideline status and is the recommended US EPA dispersion model for use for all near field applications out to 50km 4 . The US EPA approved version of CALPUFF 5 , Version 5.85 of the model (equivalent to the 2008 version of the model with bug fixes) can be still be found on the ‘Alternative Models’ web page. It is essential to point out here that the Developers recommend the use of CALPUFF, Version 7 (2017) 6 over the US EPA recommended Version 5.85 for all applications outside of the US. Version 7 of the model contains significant model code changes and improvements as well as bug fixes, plus the inclusion of the sub hourly time step (Version 6).

   The wording of the original 2013 Federal Register 7 was that CALPUFF be allowed to be used for all regulatory applications involving long range transport (> 50 km) and, more importantly, on a case-by-case basis in situations involving near-field complex flows or, non-steady-state situations such as in a coastal area, fumigation event, stagnation and complex terrain.

   The new wording in Appendix W (2017) 8 points out that removing CALPUFF as an Appendix A model does not affect its use under the Federal Land Managers guidance regarding Air Quality assessments in National Parks, nor any previous use of the model as part of regulatory applications requiring Civil Aviation Authority. Similarly, this does not affect the EPA’s recommendation that states use of CALPUFF to determine the applicability and level of BART (Best Available Retrofit Technology) in regional haze implementation. Appendix W (2017) also states that the use of CALPUFF in the near field as an alternative model for situations involving complex terrain and complex winds has not changed by the removal of CALPUFF as a preferred model in Appendix A.

   In this same document, the EPA further points out that it recognises that “AERMOD is limited” and, that CALPUFF or another Lagrangian model may be more suitable in complex environments, therefore they have continued to provide the flexibility to use it. This last point is important as the EPA recognise that AERMOD has its limitations in complex non steady state environments. This is especially important for odour assessments which are often located in complex meteorological environments, i.e., close to water bodies, such as WWTPs and in complex terrain environments such as Pulp and Paper Mills.


The US EPA Version 5.85 vs the latest recommended Version 7

   It may be helpful for readers to understand that updates to CALPUFF by the US EPA were persistently resisted despite the ‘agreement’ made between the Developers and the EPA AQMD. The deal was that the Developers would continuously work to update CALPUFF with technical enhancements and address bugs, and, would present such enhanced versions to EPA as soon as they become available, at no cost. EPA in return would expeditiously review the updates and approve them for release to be used by the ‘modelling community’. It is unfortunate that this did not happen despite the frequent requests for these updates by the ‘modelling community’ at the 10th modelling conference 9 . It is also relevant to point out that all US EPA evaluation studies conducted to-date and, in the future that incorporate CALPUFF will be using the US EPA approved Version 5 of the model, not the latest recommended model. An example of a significant model enhancement worthy of evaluation by the EPA but not endorsed by them in accordance with the agreement was the inclusion of ISORROPIA chemistry and the linkages to WRF model output of liquid water content in Version 6.42 for full cloud aqueous phase conversion of SO2. These enhancements were initiated by Atmospheric and Environmental Research 10 and paid for by the American Petroleum Institute.



   This article has attempted to shed some light on history and current status of CALPUFF for odour assessments for those countries which are regulated to use dispersion models for odour applications. The designation of CALPUFF as a US EPA guideline model in 2003 and its removal in 2017 have been explained. It has been pointed out that similar to the ISCST3 model before it, CALPUFF has undergone years of rigorous testing and model evaluation studies. The amount of resources invested by the EPA to persist with a model to guideline status was large and an effort beyond the means of modelling communities outside the US. It is for this reason that CALPUFF will continue to be used in air quality applications around the world despite its removal as an Appendix W model.



1 Federal Register, Volume 68, No 72, April 15, 2003. Rules and Regulations

2 User’s Guide for the Industrial Source Complex Dispersion Models, Volume 1 User Instructions. US EPA -454/B-95-003a. September 1995

3 Federal Register, Volume 82, No. 10, January 2017. Rules and Regulations,

4 Revision to the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and incorporation of Approaches to Address Ozone and Fine Particulate Matter. A proposed Rule by the EPA on 07/29/2015.

7 Federal Register. Part III. EPA. 40 CFR Part 51. Revision to the Guideline on Air Quality Models: Adoption of a Preferred Long Range Transport Model and Other Revisions, Final Rule. Volume 68. Number 72. Tuesday, April 15, 2003. Rules and Regulations.

8 Federal Register, Volume 82, No. 10, January 2017. Rules and Regulations,

9 Tenth Conference on Air Quality Modelling, March 13-15, 2012. Research Triangle Park, USA

10 AER, 2008. CALPUFF Chemistry upgrade. Document CP277-07-01. Prepared for the American Petroleum Institute. February 2008. 72 pp.


Jennifer Barclay   Jennifer Barclay has more than 26 years’ experience in advanced meteorological studies and atmospheric dispersion modeling on all types of steady-state, non-steady state, particle, Eulerian and prognostic models. She has been involved in many large international meteorological and air quality related modeling jobs from the planning stages, quality control, preparation of data, dispersion modeling, analyses of the data and report writing. Jenny’s clients have included the transport industry, mining industry, aluminium industries, electric industries, manufacturing, chemical facilities, as well as agricultural industries such as fertilizer, compost, waste water treatment plants and dairy facilities all over the world.

   She has instructed at more than 68 +, three to five day Introductory and Advanced meteorological, dispersion modeling and odour training courses for various consultants and industry including Professional Organizations – AWMA, CASANZ, NACA, the US-EPA, Universities, consultants and industry all around the world. Jenny was considered the ‘Odour Expert’ of the Atmospheric Studies Group (Developers of the CALPUFF model), where she was a key member for more than 15 years. Jenny is the author of many published peer reviewed reports, papers and scientific articles and she regularly presents at international scientific conferences. She has been involved in developing model guidance documentation for both Australia and New Zealand.


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