From April 2003 until January 2017, CALPUFF was the U.S. Environmental Protection Agency (EPA) preferred model for long-range transport for the purposes of assessing National Ambient Air Quality Standards (NAAQS) and/or Prevention of Significant Deterioration (PSD) increments. With the 2017 revisions to the Guideline on Air Quality Models (Appendix W to 40 CFR Part 51), the EPA has established in Section 4.2(c)(ii) a screening approach for long-range transport assessments for NAAQS and PSD increments. This screening approach will streamline the time and resources necessary to conduct such analyses and provides a technically credible and appropriately flexible way to use CALPUFF or other Lagrangian models as a screening technique. With the establishment of the screening approach for long-range transport, CALPUFF was delisted as an EPA preferred model in the 2017 revised Guideline. Should a cumulative impact analysis for NAAQS and/or PSD increments be necessary beyond 50 km, the selection and use of an alternative model shall occur in agreement with the appropriate reviewing authority and approval by the EPA Regional Office based on the requirements of Appendix W, Section 3.2.
As stated in Section 6 of the Final Rulemaking Notice for the January 2017 revision of the U.S. EPA Guideline on Air Quality Models, (82 FR 5196) “EPA’s final action to remove CALPUFF as a preferred appendix A model in this Guideline does not affect its use under the FLM’s [Federal Land Managers] guidance regarding AQRV [Air Quality Related Values] assessments (FLAG 2010) nor any previous use of this model as part of regulatory modeling applications required under the CAA [Clean Air Act]. Similarly, this final action does not affect the EPA’s recommendation that states use CALPUFF to determine the applicability and level of best available retrofit technology in regional haze implementation plans. It is also important to note that the use of CALPUFF in the near-field as an alternative model for situations involving complex terrain and complex winds is not changed by removal of CALPUFF as a preferred model in appendix A. The EPA recognizes that AERMOD, as a Gaussian plume dispersion model, may be limited in its ability to appropriately address such situations, and that CALPUFF or other Lagrangian model may be more suitable, so we continue to provide the flexibility of alternative model approvals (as has been in place since the 2003 revisions to the Guideline).”
The CALPUFF modeling system is recommended by the Federal Land Managers’ Air Quality Related Values Workgroup (FLAG) for assessing the effects of distant and multi-source plumes on visibility and pollutant wet/dry deposition fluxes. The CALPOST processor implements the algorithms recommended by FLAG for assessing the change in plume extinction due to a modeled source or group of sources. CALPUFF postprocessors allow the calculation of pollutant deposition fluxes of nitrogen and sulfur as described by the FLAG guidance. Download the October 2010 FLAG Phase I Report Revised for more details.